Category Archives: Biology


Darwin’s Theory

1. Overproduction – Organism reproduce at massive rates, leading to exponential increase in population size.

2. Constancy of Numbers – The capacity of an area is limited, thus the population tends to remain stable and thus, only a fraction of the population would survive, mature and reproduce

3. Struggle for survival – Members of the population struggles to survive, competing for finite resources.

4. Variation – Individuals differ in minute ways in terms of physical, physiological and behavioral characteristics.

5. Survival of the fittest – Certain traits confer upon them an advantage in procuring scarce resources while others will lead to them being disadvantages, resulting in their survival rates differing.

6. Like produce Like – Those that survive and mature would breed and produce off-springs taking after themselves.

7. Formation of a new species – The unequal ability of individuals in the population to survive and reproduce will lead to a shift in the composition of the population over time as with each succeeding generation, the proportion of individuals carrying the advantageous trait increases.


Speciation is the process when one or more species arises from a previously existing species as the gene flow in the population is interrupted, as the separated populations cannot interbreed for geographical or behavioral reasons. As biological species are defined in terms of reproductive compatibility, the formation of a new species will require reproductive isolation to eventually form a biological factor preventing interbreeding. These can be in the form of pre or post zygotic barriers.

Prezygotic Barrier:

-        Habitat Isolation : Species occupy different habitats and do not come into contact

-        Temporal Isolation : Species reproduce at different seasons or different times of a day

-        Behavioral Isolation  : Courtship signals differ

-        Mechanical Isolation : Genitalia unsuitable

-        Gamete Isolation : Sperm is unable to fuse with egg

Postzygotic barrier

-        Zygote Mortality : Fertilization occurs but zygote does not survive

-        Hybrid sterility : Hybrid is sterile and is not reproductively viable

-        Fitness : The fitness of the offspring is compromised and survival rates drop

Agents of Evolution

Natural Selection

Differential rates of survival and reproduction alters the allele frequency in a population as favorable allele will be more likely to be passed down as it confers a competitive advantage upon the parent, leading to its higher survival and reproduction likelihood, which in turn results in the allele being passed down to the next generation at a higher frequency. Natural selection first acts on expressed phenotypes before altering genotypes through its effect on allele frequency.

-        Directional selection occurs when the population encounters a different environmental condition favoring a certain extreme phenotype.

-        Disruptive Selection occurs when the gene flow is interrupted and the population may undergo speciation to give separate species.

-        Stabilizing Selection occurs when the environment remains stable over time, promoting phenotypic stability as the species is already optimally suited for the environment.

Evidence of Evolution

-           Anatomical Homology refers to common morphological structures derived from a common ancestor. A basic organism, through descent with modification, would give rise to different populations via natural selection. It can also be reflected in vestigial structures. When the selection pressures that would keep the structure in functional condition are removed, the structure loses its purpose and over generations, would degenerate due to accumulations of mutations that limit its size and shape.

-           Embryological homology refers to development of structures by the embryo. For example, human embryos go through a stage with gill ridges.

-           Molecular Homology can be identifies when cells of the organisms are analyzed at a molecular level, showing similarities in the form of similar genes and amino acid sequences derived from a common ancestor. Closely related species would have less difference in their nucleotide sequence.

-           Fossil records show the succession of organisms and how homologous structures have been modifies through time.

Continental Drift theory shows that identical fossil plants and animals have been discovered on opposite sides of the Atlantic Ocean. This drifting apart of land masses splits organisms by the development of oceanic barriers. Isolating descendent populations and thereby resulting in allopatric speciation

Cell Division


Many steps of meiosis closely resemble corresponding steps in mitosis. Meiosis, like mitosis, is preceded by the replication of chromosomes. However, this single replication is followed by two consecutive cell divisions, called Meiosis 1 and Meiosis 2. These divisions result in 4 daughter cells, each with half as many chromosomes as the parent cell.

Stages of Meiosis


Interphase -        Chromosomes replicate during S phase but remain uncondensed

-        The centrosome replicates, forming two centromere

Prophase 1 -        Chromosome begins to condense

-        Homologous chromosomes pair and cross over

Metaphase 1 -        Tetrads line up

-        Pairs of homologous chromosomes arranges on metaphase plate

Anaphase 1 -        Chromosomes move towards the pole, guided by spindle apparatus

-        Homologous chromosomes split, but sister chromatids remain attached at the centromere

Telophase 1 -        In Telophase, cell plates or cleavage furrow forms
Cytokinesis -        In cytokinesis, cell splits
Prophase 2 -        Same as Prophase 1
Metaphase 2 -        Spindle fibres align chromosomes along the equator
Anaphase 2 -        Same as Anaphase 1 but centromeres split
Telophase 2 -        Same as Telophase 1


Stage of Mitosis


Interphase -        Chromosomes replicate during S phase but remain uncondensed

-        The centrosome replicates, forming two centromere

Prophase -        Chromosome begins to condense

-        Centrioles migrate to the poles of the cell and nuclear membrane disintegrates

Metaphase -        Chromosomes line up

-        Contraction of spindle fibres pull chromatids slightly apart

Anaphase -        Centromere splits and spindle fibre shortens

-        Chromosomes move towards the pole, guided by spindle apparatus

Telophase  -        In Telophase, cell plates or cleavage furrow forms and nuclear membrane forms around the chromatids
Cytokinesis -        In cytokinesis, cell splits







Mitosis vs. Meiosis




DNA Replication Occurs during interphase before mitosis begins Occurs during interphase before Meiosis 1
Number of Division One Two
Synapsis of homologous chromosomes Does not occur Occurs during prophase 1, forming tetrads, is associated with crossing over between non-sister chromatids
No. Daughter Cell 2 4
Genetic Composition Genetically identical to the parent cell Containing half as any chromosomes as the parent cell, genetically different from parent
Role in the animal body Enables multicellular adult to arise from zygote. Produces cells for growth and repair Produces gametes, reduces number of chromosomes by half. Genetic variability


The key differences between Meiosis and Mitosis are that meiosis reduces the number of chromosome sets from two to one, whereas mitosis conserves the number of chromosome sets. Therefore, mitosis produces daughter cells that are identical to the parent cell and each other whereas meiosis produces cells that differ genetically from their parent cell and from each other.

1. Synapsis and crossing over. During prophase 1, duplicated homologous chromosomes line up and become physically connected along their lengths by a zipper-like protein structure, the synaptonemal complex, this process is called synapsis. Genetic rearrangement between non-sister chromatids, known as crossing over also occurs during Prophase 1. Following disassembly of the synaptonemal complex in late prophase, the four chromatids of a homologous pair are visible in the light microscope as a tetrad. Each tetrad normally contains at least one X-shaped region called a chiasma, the physical manifestation of crossing over. Synapsis and crossing over do not occur during mitosis.

2. Tetrads on the metaphase plate. At metaphase 1 of meiosis, paired homologous chromosomes are positioned on the metaphase place, rather than individual replicated chromosomes, as in mitosis.

3. Separation of homologues. At anaphase 1 of meiosis the duplicate chromosomes of each homologous pair more towards opposite poles, but the sister chromatids of each duplicated chromosome remain attached. In mitosis, sister chromatids separate.

Meiosis 1 is called the reductional division because it halves the number of chromosomes sets per cell – a reduction from 2 sets to one set. The sister chromatids then separate during the second meiotic division, meiosis 2, producing haploid daughter cells. The mechanism for separating sister chromatids is virtually identical in meiosis 2 and mitosis.



Cancer is the result of uncontrolled cell division as the population of cancer cells multiply without regulation. It arises from the transformation of normal cells to become immortal via mutations. If these cells are not destroyed, they undergo rapid controlled mitosis to form neoplastic growths or tumors which may be malignant (spread) or benign (localized). Spreading of the cancerous growth is a process known as metastasis.

Cancer can be triggered by various things as listed below.

-        Carcinogens – leading to mutations

-        Oncogenes – cause uncontrolled mitosis

-        Retroviral oncogenes

-        Genetic predisposition – inherited oncogenes

Characteristics of cancer

-        Acquisition of self-sufficiency in growth signals, leading to unchecked growth

-        Loss of sensitivity to anti-growth signals, leading to unchecked growth

-        Loss of capacity for apoptosis

-        Loss of capacity of senescence, leading to limitless replicative potential possibility due to the presence of telomerase, allowing for cell growth beyond he Hayflick’s limit

-        Acquisition of sustained angiogenesis, allowing tumor to grow beyond the limitations of passive nutrient diffusion

-        Acquisition of ability to invade neighboring tissues, with no contact inhibition, causing cell to continue growing even when it comes into contact with neighboring cells.

-        Loss of capacity to repair genetic errors, leading to an increased mutation rate (genomic instability), thus accelerating all the other changes



The theory of epigenetics is that non-mutational changes to DNA can lead to alterations in gene expression. Normally oncogenes are silent, for example, because of DNA methylation. Loss of the methylation can induce aberrant expression of oncogenes, leading to cancer pathogenesis. Known mechanism of epigenetic changes include DNA methylation, and methylation or acetylation of histone proteins bound to chromosomal DNA at specific locations.


Oncogenes promote cell growth through a variety of ways. Mutation in pronto-oncogenes, which are the normally quiescent counterparts of oncogenes, can modify their expression and function, increasing the amount or activity of the product protein. When this happens, the proto-oncogenes become oncogenes and this upsets the normal cell cycle regulations, making uncontrolled growths possible.

Tumor suppressor genes

Many tumor suppressor genes effect signal transduction pathways regulate apoptosis. Tumor suppressor genes code for anti-proliferation signals and proteins that suppress mitosis and cell growth. Generally, tumor suppressors arrest the progression of the cell in order to carry out DNA repair, preventing mutations from being passed on to the daughter cells. The p53 protein, one of the most important and thoroughly studied tumor suppressor genes, is a transcription factor activated by many cellular stressors including hypoxia and UV damage. However, a mutation can damage the tumor suppressor gene itself, or the signal pathway which activates it, thus ‘switching it off’. The invariable consequence of this is that DNA repair is hindered or inhibited: DNA damage accumulates without repair, inevitably leading to cancer.





Immune System

Innate Immunity

The innate immunity is present before any prior exposure to pathogens and is effective from the time of birth. These are largely unspecific and are quick to recognize and respond to a broad range of microbes regardless of their precise identity.

Innate Immunity [Rapid response]

Acquired Immunity

[Slower response]

External Defenses

Internal Defenses

-        Skin

-        Mucous membrane

-        Secretions

-        Phagocytic cells

-        Antimicrobial proteins

-        Inflammatory Response

-        Natural Killer Cell

-        Humoral Response

-        Cell-mediated response

External defense

Intact skin is a barrier that is normally impenetrable by virus or bacteria, but even tiny abrasions might allow their passage. Likewise, the mucous membranes lining the digestive, respiratory tract bars entry of potentially harmful microbes. Certain cells of these mucous membranes also produce mucus, a viscous fluid that traps microbes and other particles. In the trachea for example ciliated epithelial cells sweep mucus and entrapped microbes upwards, preventing microbes from entering the lungs.

Beyond the physical role of inhibiting microbe entry, secretions also provide an environment that is hostile to microbes. Take for example secretions from sebaceous glands and sweat glands that give the skin a pH ranging from 3 to 5 which is acidic enough to inhibit microbe colonization. Secretions from the skin and mucous membranes also contain antimicrobial proteins like lysozyme which digests the cell walls of most bacteria.

Internal Defense

Microbes that penetrate the body’s external defense such as those that enter via a break in the skin would face the body’s internal innate defense. These defenses rely mainly upon phagocytosis, the ingestion of invading microorganisms by certain types of blood cells. Generically referred to as phagocytes, these cells produce certain antimicrobial proteins that help initiate inflammation which can limit the spread of microbes in the body. Non-phagocytic white blood cells, called natural killer cells also play a key role in innate defense. These various non-specific mechanisms help to limit the spread of microbes before the body can mount acquitted specific immune response.


Phagocytic cells

Phagocytes attach to their prey via surface receptors that bind to structures found on many micro-organisms. Among the structures bound by these receptors are certain polysaccharides on the surface of bacteria. After attaching to one or more microbes, a phagocyte engulfs the microbes, forming a vacuole that fuses with a lysosome. First, nitric oxide and other forms of oxygen contained in the lysosomes may poison the engulfed microbes. Second, lysozymes and other enzymes degrade microbial components. Some microorganisms have adaptations that enable them to evade destruction by phagocytic cells, by for example hiding their surface polysaccharides, thus preventing binding of phagocytes. Other bacteria like the one that causes tuberculosis are resistant to destruction within the lysosomes.

There are four different types of white blood cells which are phagocytic and they differ in abundance, average life span and phagocytic ability.

By far, the most abundant are neutrophils, which constitute about 60% to 70% of all white blood cells. Neutrophils are attracted to and then enter infected tissue, engulfing and destroying the microbes there. However, neutrophils tend to self-destruct in the process of phagocytosis, and their average life span is only a few days. Macrophages are more effective and they develop from monocytes which constitutes about 5% of the circulating white blood cells. New monocytes circulate the blood for a few hours before being transformed into macrophages. The other two forms of phagocytes are less abundant and play a more limited role in innate defense. Eosinophils have low phagocytic activity but are crucial to defense against multicellular parasitic invaders, such as blood fluke. Rather than engulfing the parasite, they position themselves against the parasite’s body and then discharge destructive enzymes that damage the invader. The last form of phagocyte is dendritic cells that ingest microbes.

Antimicrobial Proteins

Numerous proteins function in innate defense by attacking microbes directly attacking the microbes or by impeding their reproduction. Apart from lysozyme, there are about 30 other serum proteins that make up the complement system. In the absence of an infection, these proteins are inactive. Substances on the surface of many microbes, however, can trigger a cascade of steps that activate the complement, leading to lysis of invading cells.

Two types of interferon provide innate defense against viral infections. These proteins are secreted by virus-infected body cells and induce neighboring uninfected cells to produce other substances that inhibit viral production, thus limiting the cell-to-cell spread of virus in the body, helping to control viral infections such as colds and influenza. This innate defense mechanism is not virus specific and interferon produced in response to one virus might also confer short term resistance to unrelated viruses.

Inflammatory Response

Damage to tissue by physical injuries or the entry of pathogens leads to release of numerous chemical signals that trigger a localized inflammatory response. One of the most active chemical is histamine which is stored in mast cells found in connective tissues. When injured, mast cells release histamine that triggers dilation and increased permeability of nearby capillaries. These result in increased local blood supply, causing redness and heat typical of inflammation. The blood-engorged capillaries leak fluid into neighboring tissues, causing swelling. These vascular changes help deliver antimicrobial protein and clotting elements to the injured location. Blood clotting begins repair process and helps block the spread of microbes to other parts of the body. Increased blood flow and vessel permeability would also allow more neutrophils and monocyte macrophages to move from the blood into injured tissue. A minor injury causes a local inflammation, but the body may also mount a systemic (widespread) response to severe tissue damage or infection. In a severe infection such as meningitis or appendicitis the number of white blood cells would rapidly increase. Another systemic response is fever which may occur when certain toxins produced by pathogens and substances released by actuated macrophages set the body’s thermostat at a higher slightly higher temperature. A very high fever is dangerous, but a moderate fever can facilitate phagocytosis and, by speeding up body reactions, hasten the repair of tissues. Certain bacterial infection can induce an overwhelming systemic inflammatory response, leading to a condition called septic shock which is characterized by high fevers and low blood pressure.

Natural Killer Cells

Natural killer cells patrol the body and attack virus-infected body cells and cancer cells. Surface receptors on natural killer cells recognize general features on the surface of its targets. Once it is attached to a virus infected cell or cancer cell, the natural killer cell releases chemicals that lead to the death of the stricken cell by apoptosis, or programmed cell death.

Invertebrate Immune Mechanism

Invertebrates also have highly effective innate defense, for example, sea stars possess amoeboid cells that ingest foreign matter via phagocytosis and secrete molecules that enhance the animal’s defensive response. The insects’ equivalent to blood, hemolymph, contains circulating cells called hemocytes. Some of them ingest bacteria and foreign substances while others would form a cellular capsule around large parasites.


Acquired Immunity

Pathogens would inevitably come into contact with lymphocytes while under the assault by one’s innate defense. Lymphocytes are the key cells of acquired immunity – the body’s second major form of defense. Direct contact with microbes and signals from active innate defenses will cause lymphocytes to join the battle. Any foreign molecule that is specifically recognized by lymphocytes and elicits response from them is called an antigen. Most antigens are large molecules, either polysaccharide. Some antigens, such as toxins secreted by bacteria, are dissolved in extracellular fluid, but many protrude from the surface of pathogens or transplanted cells. A lymphocyte actually recognizes and binds to just a small, accessible portion of an antigen, called an epitope (antigenic determinant). A single antigen usually has several epitopes, each capable of inducing a response from lymphocytes that recognize that epitope. Antibodies which are secreted by certain lymphocytes in response to antigens, likewise bind to specific epitopes.

Antigen recognition by Lymphocytes

The body is populated by two main types of lymphocytes – T cells and B cells.  Both types are circulated through the blood and lymph and are concentrated in the spleen, lymph nodes and other lymphoid tissues. B cells and T cells recognize antigens by means of antigen-specific receptors embedded in their plasma membranes, a single B or T cell bears about 100,000 of these antigen receptors, and all the receptors on a single cell are identical – that is, they all recognize the same epitope. Each lymphocyte displays specificity for a particular epitope on an antigen and defends against that antigen or a small set of closely related antigens. A great diversity of T cells and B cells are present, however, only a tiny fraction of the lymphocytes would ever be used.

B cell Receptors for Antigens

Each B cell receptor for an antigen is a Y-shaped molecule consisting of four pol-peptide chains: two identical heavy chains and two identical light chains linked by disulfide bridges. A region in the tail portion of the molecules, the trans-membrane region anchors the receptor in the cell’s plasma membrane and a short region at the end of the tail extends into the cytoplasm. At the tips of the Y are the light and heavy chain variables which vary extensively from one B cell to another. The remainder of the molecule is the constant regions whose amino acids vary little from cell to cell. The unique contour of each binding site is formed from part of a light-chain V region. The interaction between an antigen binding site and its corresponding antigen is stabilized by multiple non-covalent bonds between chemical groups on the respective molecules. The antigens bound by the B cell receptors in this way include molecules that are on the surface of, or are released from, all types of infectious agents, thus meaning that it can recognize an intact antigen in its native state.


T cell receptors for antigens and role of the MHC

Each T cell receptor for an antigen consists of two different polypeptide chains, a  cell and a  chain, linked by a disulfide bridge. Near the base of the molecule is a trans-membrane region that anchors the molecule in the cell’s plasma membrane. At the outer tip of the molecule, the  cell and a  chain variable regions form a single antigen binding site. The remainder of the molecule is made up of constant regions.

T cell receptors recognize and bind with antigens just as specifically as a B cell receptors. However, while the receptors on B cells recognize intact antigens, the receptors on T cells recognize small fragments of antigens that are bound to normal cell surface proteins called major histocompatibility complex (MHC) molecules. As a newly synthesizes MHC molecule is transported towards the plasma membrane, it bind with a fragment of protein antigen within the cell and brings it to the cell surface via a process called antigen presentation. A nearby T cell can detect the antigen fragment thus displayed on the cell surface. There are two ways in which foreign antigens can end up inside cells of the body. Depending on their source, these peptide antigens are handled by a different class of MHC molecule and recognized by a particular subgroup of T cells.

-        Class 1 MHC molecules, found on almost all nucleated cells of the body bind peptides derived from foreign antigens that have been synthesized within the cell. Any body cell that becomes infected or cancerous can display such peptide antigens by virtue of its Class 1 MHC molecules. Class 1 molecules displaying bound peptide antigens are recognized by a subgroup of T cells, called cytotoxic T cells.

-        Class 2 MHC molecules are made by just a few cell types, mainly dendritic cells, macrophages and B cells. In these cells, class 2 MHC molecules bind peptides derived from foreign materials that have been internalized and fragmented through phagocytosis or endocytosis. Dendritic cells, macrophages and B cells are known as antigen presenting cells because of their key role in displaying such internalized antigens to another subgroup of T cells called T helper cells.

Each vertebrate species possesses numerous different alleles for each class 1 and 2 MHC gene. Because of the large number of different MHC alleles in the human population, most of us are heterozygous for every one of our MHC genes and produce a broad array of MHC molecules. Collectively these molecules are capable of binding to and presenting a large number of peptide antigens. Thus MHC produces a biochemical fingerprint unique to virtually every individual. That marks body cells as ‘self’.

Humoral and Cell Mediated Immunity

There are two separate branches of acquired immunity, humoral immune response which involves the activation and clonal selection of B cells, resulting in production of secreted antibodies that circulate in the blood and lymph. The cell mediated immune response involves the activation and clonal selection of cytotoxic T cells, which directly destroy certain target cells. Central to the network of the humoral and cell mediated immune response are the helper T cells which responds to peptide antigens displayed on antigen presenting cells and in turn stimulates the activation of nearby B cells and cytotoxic T cells.

Helper T cells: A response to antigens

When a helper T cell encounters and recognizes a class 2 MHC molecule-antigen complex on an antigen presenting cell, the helper T cell proliferates and differentiate into a clone of activated helper T cells and memory helper T cells. Activated helper T cells secrete several different cytokine that would stimulate other lymphocytes, thereby promoting humoral and cell mediated response. The helper T cell itself is also subject to regulation by cytokines.

Cytotoxic T cells: A response to infected cells and cancer cells

Cytotoxic T cells, the effectors of cell-mediated immunity eliminate body cells infected by viruses or other intracellular pathogens as well as cancer cells and transplanted cells. Fragments of non-self-proteins synthesized in such target cells associate with class 1 MHC molecules and are displayed on the cell surface. When a cytotoxic cell is selected by binding to class 1 MHC molecule-antigen complexes on an infected body cell, the cytotoxic T cell is activated and differentiates into an active killer. Cytokines secreted from nearby helper T cells promote this activation. The activated cytotoxic T cell then secretes proteins that act on the bound infected cell, leading to its destruction. The death of the infected cell not only deprives the pathogen of a place to reproduce but also exposes it to circulating antibodies, which marks it for disposal. After destroying an infected cell, the cytotoxic T cell may move on and kill other cells infected with the same pathogen. In the same way, cytotoxic T cells defend against malignant tumors. Because tumor cells carry distinctive molecules not found on normal body cells, they are identified as foreign by the immune system. Class 1 MHC molecules of a tumor cell display fragments of tumor antigens to cytotoxic T cells.

B Cells: A response to extracellular Pathogens

Antigens that elicit a humoral immune response are typically proteins and polysaccharides present on the surface of bacteria or incompatible transplanted tissue or transfused blood cells. The activation of B cells is aided by cytokines secreted from helper T cells activated by the same antigen. Stimulated by both an antigen and cytokines, the cell proliferates and differentiates into a clone of antibody-secreting plasma cells and a clone of memory B cells. When an antigen first binds to receptors on the surface of memory B cells, the cell takes in a few of the foreign molecules by receptor mediated endocytosis. In process similar to antigen presentation by macrophages and dendritic cells, the B cell then presents antigen fragments to the helper T cells. However, a macrophage or dendritic cell can present peptide fragments from a wide variety of antigens, whereas a B cell internalizes and presents only the antigen to which is specifically binds.

Antigens that induce antibody production only with assistance from helper T cells are known as T-dependent antigens. Some antigens however, can evoke a B cell response without involvement of helper T cells. Such t-independent antigens include polysaccharide of many bacterial capsules and proteins that make up the bacteria flagella. However, this response is generally weaker than the response to T-dependent antigens and through this process; no memory B cells are generated.

Active and Passive Immunization

Immunity conferred by natural exposure to an infectious agent is called active immunity because it depends on the actions of a person’s own lymphocytes and the resulting memory cells specific for the invading pathogen. Active immunity also can develop following immunization, or vaccination. Modern vaccines include inactivated bacterial toxins, killed microbes or parts of microbes, viable but weakened microbes that generally do not cause illness. All these agents would induce an immediate immune response and long lasting immunological memory.

Immunity can also be conferred by transferring antibodies from an individual who is immune to a particular infectious agent to someone who is not. This is known as passive immunity because it does not result from the action of the recipient’s own B and T cells. Instead, the transferred antibodies are poised to immediately help destroy any microbes for which they are specific. Passive immunity provides immediate protection, but it only persists for as long as the transferred antibodies last.


Allergies are exaggerated response to certain antigens called allergens. One hypothesis to explain the origin of allergies is that they are evolutionary remnants of the immune system’s response to parasitic worms. The humoral mechanism that combats worms is similar to the allergic response that causes such disorders.

The most common allergies involve antibodies of the IgE class. Hay fever; for instance, occur when plasma cells secrete IgE antibodies specific for antigens on the surface of pollen grains. Some of the antibodies attach by their tails to mast cells present in the connective tissues. Later when pollen grains enter the body, it induces the mast cell to release histamine and other inflammatory agents from their granules, a process called degranulation. Such vascular changes lead to typical allergy symptoms: sneezing, runny nose, teary eyes or difficulties in breathing. Antihistamines diminish allergy symptoms by blocking receptors for histamine.

An acute allergic response can sometimes lead to anaphylactic shock, a whole body, life threatening reaction that can occur within seconds of exposure to an allergen. Anaphylactic shock develops when widespread mast cell degranulation triggers abrupt dilation of peripheral blood vessels, causing a precarious drop in blood pressure.

DNA Profiling

Blood Typing

  • Very fast and straight forward à Can only be used to exclude suspects
  • Not very specific


  • Restriction Fragment Length Polymorphism
  • Tests regions in the DNA sequence that have Variable Tandem Repeats (VNTR)
    • No. of repeats vary for person to person and coded by alleles
    • Highly variable à Over 50 mutant alleles
    • Each person can only have 2 alleles à Inherited from parents à Large no. of possibilities
    • 14 – 100 base pairs
  • Procedure
    • Restriction enzymes used to cut out the target DNA sequence (VNTR region)
    • The DNA fragments are denatured into single strands
    • The fragments ran through gel electrophoresis
      • The fragments separated by length
    • Blotting
      • Nitro – Cellulose Paper put on top of results
      • The DNA fragments would be drawn up along with the solvent by capillary action
      • A copy of the gel electrophoresis would be on the nitro – cellulose paper
    • The DNA fragments are made back to double strands by adding complimentary primers (attached with radio – nuclides)
    • The radiation emitted by the radio – nuclides are captured on photography paper
  • Advantages
    • High discriminating potential
  • Disadvantages
    • Laborious and not easily automated
    • DNA fragments are of large size and therefore cannot undergo PCR
    • DNA requires to be of reasonable quality
    • Large amounts of DNA required as it cannot undergo PCR
    • Time consuming


  • Short Tandem Repeats
  • Test polymorphic regions in the DNA which contains Short Tandem Repeats
    • No. of repeats vary for person to person and coded by alleles
    • Highly variable à Over 50 mutant alleles
    • Each person can only have 2 alleles à Inherited from parents à Large no. of possibilities
    • Just 2 – 10 base pairs
  • Procedure
    • The polymorphic regions (13 loci) are amplified by PCR
    • The results are run through gel electrophoresis
    • The fragments ran through gel electrophoresis
      • The fragments separated by length
    • Blotting / Staining
      • Nitro – Cellulose Paper put on top of results
      • The DNA fragments would be drawn up along with the solvent by capillary action
      • A copy of the gel electrophoresis would be on the nitro – cellulose paper
      • The DNA fragments are made back to double strands by adding complimentary primers (attached with radio – nuclides)
      • The radiation emitted by the radio – nuclides are captured on photography paper
      • The DNA fragments are stained with dyes that make them visible
  • Advantages
    • High discriminating potential
    • Short DNA fragments required and therefore can undergo PCR
    • PCR can amplify DNA, therefore, minute samples are required
    • Can be carried out speedily and cheaply

Mitochondrial DNA Analysis

  • mtDNA is more stable (occur in a plasmid) than chromosomal DNA
  • More copies of mtDNA are present in every cell, allowing molecular analysis even when material is limited
  • Hypervariable (polymorphic) region allows for discrimination
  • Can be used when DNA of reasonable quality is not available
  • However, this method is not highly discriminating


Genetic Technology

Many different organisms have been genetically modified by humans in order for them to possess the desired traits. Genetic modification has specific applications in the bacterial industry in order to produce drugs. Genetic engineering has many different potential benefits for humans, but there may be unknown hazards which may arise in the future. Genetic engineering began with the discovery of the restrictive endonucleases. These enzymes occur naturally in bacteria where they protect the organism against DNA injected by virus by cutting it into small pieces, thereby inactivating it. Virus DNA otherwise would take over the host cell. Restriction enzymes were named so due to their ability to restrict the multiplication of viruses. Many different restriction enzymes have been discovered and purified, and today they are used widely in genetic engineering experiments.

A bacterium contains two different type of genetic material. One is a single strand of long double strand DNA in the form of a ring while the other is a single circular chromosome – the plasmid. Plasmids are easily isolated from the bacteria and then can be re-introduced to the bacteria cell relatively easily. They can therefore be extremely useful as vectors. In the bacterium, plasmids replicate themselves independently from the bacteria so the gene introduced to the plasmid would also be replicated when the bacteria reproduces.

To prepare plasmids as vectors, a restriction enzyme would have to be used which would form the ‘sticky ends’ which would have to be used to cut open the plasmid. The restriction enzymes would also be required to cut open the DNA and isolate the required gene. The open plasmid and the gene would then be combined in the presence of an enzyme called ligase. Ligase occurs naturally in the nuclei where it will ‘repair damaged’ DNA during the replication. The ligase would therefore catalyze the combination of the complimentary strand of DNA and the plasmid after their sticky ends have paired through a process called ‘annealing’. These plasmids are then reintroduced to the bacterium my mixing bacterium and the plasmids in the presence of Calcium Chloride which would render the cytoplasm permeable to the plasmids

An alternative technique would be to create a genetically modified bacteriophage which contains the gene. The bacteriophage would then inject that strand of DNA into the bacteria and following that, the gene would be incorporated into the bacteria plasmid and it would be expressed in the bacterium’s metabolism.


The vector that is most commonly used as a plasmid that carries two genes for antibiotic resistance is known as the R-plasmid has a gene for ampicillin resistance and a gene for tetracycline resistance. It happens that the tetracycline resistance gene is cut in half by the BamHI restriction enzyme whilst the ampicillin gene is uncut. When a gene is cut, it becomes deactivated; the resistance against tetracycline is therefore lost when the gene is cut. In order to isolate bacteria which are resistant to the respective disease, the bacteria colonies are first placed in the agar plate which kills are bacteria which are not resistant to ampicillin. Following, these bacteria would then be placed on an agar plate with tetracycline and all bacterium with damaged genes would fail to grow. A comparism between the different colonies which were placed in the ampicillin and the tetracycline plate would then be made and a conclusion would be drawn the colony which is found in the ampicillin plate, however, failed to grow in the tetracycline plate would be selected for use while the others would be disposed.

Bacteria are used because of its fast replication rate and that it is easy to culture and grown. They are also easy to observe due to their relatively simple structure. Ideal conditions for replication like the sufficient amount of nutrients, optimal temperature and pH are required to maximize production.


Pregnancy Prevention Methods

Mechanical Methods


It is a flexible sheath that is designed to cover the man’s penis during sexual intercourse. The condom is to be put on the penis before it touches the vulva. Condoms prevent sperms from entering the woman’s vagina by trapping the semen inside the condom, and not let it go into the vagina. They are easily available in drugstores, family planning clinics, some supermarkets, and from vending machines. (Do not require a prescription). They are lightweight and disposable and can protect user against many sexually transmitted diseases—called STDs for short. It is also easier to use than the other methods. No side effects unless you are allergic to latex. However, condoms may break and cannot be used for people who are allergic to latex. They also dull the sex sensation. Of 100 women whose partners use condoms, about 15 will become pregnant during the first year of typical use. Only two women will become pregnant with perfect use.


A small dome shaped rubber cap that is fitted in the female’s vagina. The diaphragm is placed in the female’s vagina before having sex. This prevents sperms from entering the woman’s vagina, and acts as a barrier. There are no side effects unless you are allergic to its ingredients and they are widely sold and easy to get, and some do not need prescriptions. This method is often not used correctly, thus increasing the chances of accidental pregnancy. It is not as easy to use as condoms, as it is more inconvenient to put in a woman’s vagina rather than a man’s penis. Similarly, it cannot be used for people who are allergic to latex. Sixteen out of 100 women who use the diaphragm will become pregnant during the first year of typical use. Six
will become pregnant with perfect use.

IUD (Intrauterine device)

It is a small, T- shaped plastic object with a fine copper wire around the device and a thread attached to the base. It is to be placed in the uterus to prevent the implantation of a fertilized egg.

The IUD must be inserted in the uterus by a doctor. It prevents sperm from fertilizing the egg, by changing the way it moves. It also changes the lining of the uterus which makes it very hard for the fertilized egg to attach itself to the wall of the uterus. It is the longest lasting method and you need not do anything before sexual intercourse. It is almost 100% effective and you can get pregnant once the IUD is taken out. However, it cannot protect you from STDs and it has to be prescribed by a doctor and it may cause an increase in menstrual flow

Chemical methods


It is a chemical that kills sperms and it is inserted into the vagina, or applied along with other contraceptive methods, such as condoms or diaphragms, so as to kill the sperms and prevent fertilization of the egg. It is easy to use and it is easily purchased with no prescriptions needed. It can be used with other contraceptive methods, improving the effectiveness. It may not be able to kill all the sperms, thus not effective if used on its own and 6 out of 100 might become pregnant for perfect use, while 21 of 100 might become pregnant for typical use


Tubal occlusion (female) is an operation that blocks, seals or cuts the fallopian tubes; this means that your eggs can no longer be fertilized by your partner’s sperm through sexual intercourse. Vasectomy (males) is an operation that blocks, seals or cuts the vas deferens which carries sperm from your testicles to your penis. Although you will still be able to ejaculate, your semen will no longer contain any sperm. It is 100% effective and there are no hormonal side effects. However once sterilized, the couples may no longer give birth and there is a risk of getting infection through the surgery.

Oral Contraceptive Pills

It is a pill that contains hormones that prevent pregnancy and it prevents pregnancy primarily by preventing ovulation. It also has the side-effect of thickening the mucus over the cervix, which can prevent or slow sperm entry into the uterus. The Pill also thins the endometrium. It is simple, safe, and convenient and it does not interfere with having sex. There is also protection against developing cancer of the ovary or the lining of the uterus can last up to 30 years. There are however, side effects like headache, breast tenderness and many conditions to be fit to take the pill like 35 or older, smoke. Consultation at a clinician to tell whether you can take the pill and what dose is right for you. Of 100 women who use the pill, only eight will become pregnant during the first year of typical use. Less than one will become pregnant with perfect use.


Natural Methods

Outer course

It is not having sexual intercourse. It is any kind of sexual activity in which the penis does not enter the vagina, mouth, or anus. It allows a couple more intimate and even have an orgasm with one another without having sexual intercourse. With outer course, no semen, vaginal fluids, or blood is shared between partners. Abstinence is 100% effective at preventing pregnancy and STDs. However both you and your partner must be committed to not having vaginal, oral (involving the mouth), or anal sex with anyone. Abstinence is 100% effective at preventing pregnancy. Outer course is nearly 100% effective at preventing pregnancy. There is a small chance pregnancy could occur if sperm or pre-ejaculate (the fluid that is sometimes released from the penis before orgasm) is ejaculated (or released) close to the opening of the vagina.

Fertility Awareness

It is a way for a woman to find out what days during her menstrual (monthly) cycle she either is or is not likely to get pregnant. The days she is likely to get pregnant are called “fertile” days. It is done by keeping track of the changes that occur in her body during the menstrual cycle-the time between the first day of her period and the last day before her next one. To avoid getting pregnant, a woman should not have sexual intercourse on her fertile days. There are no side effects and they can be used in combination with barrier methods during the fertile time. They often are acceptable to couples who choose not to use other birth control methods for religious, cultural, health, or other reasons. The disadvantages however lead to low rates of effectiveness when not used effectively. If used exactly according to the directions, between 2 and 10 women out of 100 might become pregnant in one year. For typical users, between 12 and 25 women out of 100 might become pregnant when using FAB methods for one year.


Hormones are chemical signals which are produced by most plants and animals. Animal hormones transported and distributed by the circulatory system, and reach other parts of the body where they may communicate their regulatory messages. Hormones are made and secreted by the organs with endocrine glands – in general all organs hat secrete hormones are included in the endocrine system. A hormone can only bind to a target cell with receptors fro that specific hormone. Because hormones reach all parts of the body, the endocrine system is especially important in controlling whole body activities. For example, hormones trigger changes in target cells in different ways, depending on whether the hormone is water soluble or liquid soluble: water soluble hormones trigger responses without entering the cell, whereas the lipid soluble hormones, after entering the cell, the specifics steps would be discussed later.

Water soluble hormones that bind to plasma membrane receptors come in three varieties, all derived from amino acids: amine hormones, which are modified versions of single amino acids; peptide hormones which are short chains of amino acids; and protein hormones, made of polypeptides. Insulin which regulates blood glucose level is a protein hormone.

-        Water soluble hormones cannot pass through the phospholipid layer of the plasma membrane, but they can bring about cellular changes indirectly without entering their target cells. To start, a water soluble hormone binds to a specific receptor protein in the plasma membrane of the target cell.

-        The binding activates the receptor protein which initiates a signal transduction pathway, a series of molecular changes that converts an extracellular chemical signal into a specific intracellular response.

-        The final relay molecule activates a protein that either carries out a response in the cytoplasm or affects gene regulation in the nucleus.

In contrast, lipid soluble hormones pass through the phospholipid bilayer and trigger responses by binding to receptors inside the target cell. Hormones that are steroids, all of which are molecules derived from cholesterol works this way.

-        A hormone enters the cell by diffusing through the plasma membrane.

-        The hormone binds to a receptor protein in the cytoplasm or the nucleus. Rather than triggering a signal transduction pathway, the receptor itself carries the hormones’ signals

-        The hormone receptor complex attaches to specific sites on the cell’s DNA in the nucleus.

-        The bindings to the DNA may stimulate transcription of certain genes into mRNA molecules, which are translated into new proteins, or it may turn transcription off.

Hypothalamus and Pituitary 

The hypothalamus is the main control center of the endocrine system. As part of the brain, the hypothalamus receives information from nerves about the internal condition of the body and about the external environment. It then responds by sending out appropriate nervous or endocrine signals. Its signals directly control the pituitary gland, a pea-sized structure that hangs down from the hypothalamus. In response to signals from the hypothalamus, it would secrete hormones that influence numerous body functions. The hypothalamus exerts master control over the endocrine system, using the pituitary to relay directives to other glands.

One of the more significant and most broad effect hormones is the growth hormone. During childhood and adolescence, high levels of growth hormones promote the development and enlargement of all parts of the body. If too much GH is produced in a young person, it may lead to gigantism. Overproduction of GH in adults on the other hand can lead to acromegaly, characterized by enlarged bones in hands and feet. The underproduction of GH however, can result in dwarfism.

Endorphin is another kind of anterior pituitary hormones – the body’s natural painkillers. Some researchers speculate that the ‘runner’s high’ is resultant from the release of endorphins when stress and pain in the body reaches critical levels. The release of endorphins may also produce pleasant feelings during such diverse activities as meditation, acupuncture and even eating spicy food.

Thyroid and parathyroid glands

The thyroid gland is in our necks, just beneath the larynx. The thyroid produces hormones that increase oxygen consumption and metabolic rate in all the cells of your body. These thyroid hormones also play crucial roles in development and maturation, particularly of the bone and nerve cells. Insufficient levels of the thyroid hormones in the blood or excess levels can result in serious metabolic disorders.

Hyperthyroidism can result from dietary deficiencies or from a defective thyroid gland. To produce thyroxine and triiodothyronine, the thyroid requires iodine. The lack of iodine can result in underproduction, which would interrupt the feedback loop and cause over-stimulating and swelling of the thyroid gland.

A common form of hyperthyroidism is Graves’ disease, characterized by overheating, profuse sweating, high blood pressure and protruding eyeballs.

Embedded within the thyroid are four disk-shaped parathyroid glands. The thyroid and parathyroid glands function in calcium homeostasis, keeping the concentrations of calcium ions within a narrow range. An appropriate level of calcium in the blood and the interstitial fluid is essential for many body functions. Without calcium, nerve signals cannot be transmitted from cell to cell and muscles cannot function properly. Two hormones, calcitonin from the thyroid and parathyroid hormone regulate the blood calcium levels. Calcitonin lowers the calcium level in the blood, whereas PTH raises it. These two are called antagonistic hormones because they have opposite effects. The calcitonin and PTH operates by means of feedback system that keeps the calcium level near the homeostatic set point.

-        A rise in the blood calcium level above the homeostatic set point induces the thyroid gland to secrete calcitonin

-        Calcitonin, in turn has two main effects: It causes more calcium to be deposited in the bones and it makes the kidneys reabsorb less calcium as they form urine

-        The result is a lower calcium level in the body.

-        When Calcium levels drop below the set point, the parathyroid release PTH into the bloodstream

-        PTH stimulates the release of calcium from bones and increase calcium uptake by the kidneys and intestine, raising calcium levels.

The Pancreas

The pancreas produces two hormones that play an important role in managing the body’s energy supplies. One of the hormones is insulin, a protein hormone produced by clusters of specialized pancreatic cells called islet cells. Other islet cells secrete another protein hormone called glucagon. Insulin and glucagon help maintain a homeostatic balance between the amounts of glucose stored as polymer glycogen in body cells. These two hormones similarly are antagonistic hormones that counter each other in the feedback cycle.

-        Rising glucose concentrations in the blood occurs shortly after a carbohydrate meal, stimulating the pancreas to secrete more insulin

-        Body cells take up more glucose from the blood while liver and skeletal muscles takes it up to form glycogen

-        As a result blood glucose levels falls to the set pint where the pancreas cells would lose their stimulus to release insulin.

-        When blood glucose level dips below the set point if a meal is skipped for example, more glucagon is released.

-        Glucagon is a fuel mobilizer that make liver cells break glycogen down into glucose and release into the blood

-        When blood glucose level returns to the set point, the pancreas slows its secretion of glucagon



Adrenal Glands

The humans have two adrenal glands sits atop the kidneys. Each adrenal gland is actually two glands in one: a central portion called the adrenal medulla and an outer portion called the adrenal cortex. Though the cells that they contain and the hormones that they release are both different, both adrenal medulla and adrenal cortex secrete hormones that enable your body to respond towards stress. The two hormones that are released in fight or flight situation are epinephrine and norepinephrine. These two hormones ensure a rapid short term response to stress that can be activated in seconds.

-        Stressful stimuli, whether negative or positive would activate the nerve cells in the hypothalamus, sending signals that would stimulate the adrenal medulla to secrete epinephrine and norepinephrine into the blood

-        These two hormones both contributes to the short term stress response by stimulating liver cells to release glucose, making more furl available for cellular work.

The hormones secreted by the adrenal cortex however, can provide a slower, long lasting response to stress that can last for hours or days

-        The hypothalamus secretes a releasing hormone that stimulate the pituitary to secrete a hormone called ACTH (adrenocorticotropic hormone) which would in turn stimulate cells of the adrenal cortex to synthesize and secrete a family a family of steroids called cortisteroids.

Very high levels of glucocorticoids can suppress the body’s defense system, including inflammatory responses that occur at infection sites. Cortisone, for example, can be used to treat arthritis. Some professional athletes often receive cortisone injections into injured joint to mask the pain of the tissue. With this treatment, the pain usually subsides, however, its underlying cause remains, thus with further exercising of the injured joint, the injury might become worse and long lasting. 

Homeostasis and Regulation

Functions of Key Components of Endocrine System

Endocrine gland: ductless glands which secrete hormones into the bloodstream

Hormones: Chemicals that alter specific organ activities which is carried through blood for controlling growth, regulating metabolism and homeostasis. These are destroyed in the liver.

Posterior pituitary gland: Located in the hypothalamus, it produces anti-diuretic hormone which regulates the permeability of distal tubule and collecting duct to water

Pancreas:  β and α cell in the Islets of Langerhans produces insulin and glucagon respectively, which regulates blood sugar levels.

Adrenal glands: Located above the kidneys, it produces epinephrine and norepinephrine for fight or flight responses.

Anti-diuretic hormone:

-        Increase permeability of distal convoluted tubule and collecting duct to water

-        More water can be reabsorbed and the urine becomes more concentrated


-        Metabolic activities and cellular respiration greatly increases for greater energy production

-        High blood sugar due to conversion of glycogen to glucose in liver

-        Heart rate increases to transport more blood to muscles

-        Vasoconstriction of blood vessels leading to skin and less important organs, and vasodilation of blood vessels to skeletal muscles

-        Increase in blood clotting protein

-        Pupils dilate

-        Hair erector muscles contract


-        Increases cell permeability to glucose

-        Increases cell respiration and oxidation of glucose

-        Converts glucose to glycogen which is stored in the liver

-        Converts glucose to lipids which is stored in the adipose tissue


-        Decreases cell permeability to glucose.

-        Decreases cell respiration.

-        Converts glycogen, lipids and amino acids to glucose in the liver.

-        Causes cells to release glucose into bloodstream.

Negative and Positive Feedback Pathways

-          Positive and Negative feedback pathways are the two major ways at which the body is able to regulate processes. Similar to the nervous system, homeostasis involves 3 factors, receptors, messengers and effectors. The receptors detect a stimulus while the messengers coordinate a response and the effectors carry out that response.

  • A negative feedback mechanism would be a process which produce a substance that would in turn inhibit the production of itself when too much of it is present in the system
  • A positive feedback system would be a process that produces a substance which would intensify the process when it is in abundance

-          Thermoregulation:  A rise in temperature stimulates thermoreceptors to send neural impulses via sensory neurons to the hypothalamus which integrates the signals and relays it to effectors via the motor neurons. Sweat glands are stimulated to produce more sweat, which will take away heat during evaporation. Arterioles near the surface of the skin dilate, allowing more blood to flow through, increasing heat loss by radiation, conduction and convection. Hair erector muscles relax, allowing hairs to lie close to skin surface, reducing insulation. The core body temperature returns to 37.0 degrees Celsius.

-          Osmoregulation: When osmotic pressure is high, osmoreceptors in the hypothalamus are stimulated and send a chemical message to the posterior pituitary gland to secrete anti-diuretic hormones. The anti-diuretic hormone travels via the blood stream to receptors in the distal convoluted tubule and collecting duct of kidney nephrons, causing more aquaporin-2 channels to become activated. The distal convoluted tubule and the collecting duct becomes more permeable to water, thus more water is reabsorbed in the kidney nephron. The return of blood plasma to normal water potential is detected by osmoreceptors in the hypothalamus and anti-diuretic hormone will stop being secreted.


-          Blood Sugar Regulation: When blood sugar levels increase above the norm of 90mg/100mL, β cells of the Islets of Langerhans in the pancreas detect the stimulus and start to secrete insulin into the blood stream which binds to tyrosine kinase receptors on the surface of many body cells, making them more permeable to glucose. In the liver and muscle cells, glucose is absorbed and converted into glycogen for storage. In adipose tissue, glucose is absorbed and converted to lipids. As the blood sugar levels drops and returns to the norm, insulin stops being secreted. However, as insulin travels via the blood stream and its effect only ceases when it has been destroyed by the liver, it usually causes blood sugar levels to decrease beyond 90mg/100mL àαcells in the Islets of Langerhans produce glucagon triggering stored glycogen to be released as glucose, raising blood glucose levels.

-          Fever: A situation where one has abnormally high internal temperature is a body wide response that usually indicates an ongoing fight against infection. While immune system cells encounter invading microbes, the cells release chemicals that stimulate the control center to raise the body’s internal temperature, producing a fever. Many people mistakenly believe that the invading microbes themselves causes a fever, however in fact, the cause is the body’s fight against the microbes. A higher temperature would discourage bacterial growth, protecting the body’s internal environment against potentially harmful invaders.


The body functions within a fluid environment; and for such systems to function properly, the relative concentration of water and solute must be maintained within fairly narrow limits. This is the function of the excretory and homeostatic systems. It figures to remove the waste materials from metabolic processes to maintain the homeostatic conditions in the body. Homeostasis therefore refers to the maintenance of a constant internal environment.

The excretory system of mammals centers on the kidneys which are the principle sites of water balance and salt regulation. The kidney is bean shaped and is around 10 cm long. Blood is supplied by a renal artery and drained by a renal vein. The filtrate is then drained off as urine through the ureter into the bladder. During urination, urine is expelled through the urethra. Sphincter muscles near the junction of the urethra and the bladder regulates urination.

Excretory Process

Excretion is the removal of metabolic waste products such as carbon dioxide, urea, uric acid, bile pigments, water and mineral salts. Multicellular organisms contain many cells making up internal structures which are not in direct contact with the environment; hence metabolic waste products cannot be removed by direct diffusion into the environment.

Blood flowing through the renal artery branches into the glomerulus, enclosed by the Bowman’s capsule, which consists of very narrow arterioles that experiences very high blood pressure. Ultrafiltration occurs and water, urea, uric acid, *drug metabolites, *bilirubin, ions, amino acids and glucose are removed. The glomerular filtrate enters the juxtamedullary loop of Henle which is surrounded by the hypertonic renal medulla. Some water is reabsorbed in the descending loop of Henle; and most small molecules and ions except urea and uric acid are reabsorbed in the ascending loop of Henle. This results in a high concentration of ions in the interstitial fluids surrounding the ascending loop of Henle and the collecting duct.Renal arteries wrap around the distal convoluted tubule and selective reabsorption of water and ions takes place. Water can be reabsorbed in the collecting duct into the efferent arteriole by entering the interstitial fluids down the concentration gradient. The efferent artery transports blood free of urea and uric acid back to the heart while urine is excreted into the bladder for storage and then out of the body via the ureter and urethra.

Counter-current system: Blood in the arterioles always flow in the opposite direction to filtrate flowing in the Loop of Henle, proximal and distal convoluted tubules and collecting duct to maximize the diffusion gradient.

Body Fluid (Blood, Coelomic Fluid or Hemolympth) is collected

  1. Filtered through selectively permeable membranes consisting of a single layer of transport epithelium. The membrane retains cells as well as proteins and other large molecules in the body fluid. Hydrostatic pressure forces water and small solutes (salts, sugars, amino acids) and nitrogenous waste into the excretory system.
  2. But the filtration is largely non-specific, so it is crucial that the small molecules are restored to the body fluids – by selective reabsorption. Excretory systems use active transport to reabsorb the valuable solutes and restore them to the body fluid. Selective secretion, also by active transport, also adds waste and toxins not removed by filtration to the filtrate
  3. The processed filtrate is then released as urine

Action of Lungs

Carbon dioxide is produced by cells during respiration. Hydrogen carbonate anions are released into the blood stream and transported to the alveolus where it is excreted as carbon dioxide gas.

Structure, Function & Action of Kidney Nephron

The function of the kidney is to remove nitrogenous waste, excess water and mineral salts thereby regulating blood osmotic pressure; salvage necessary ions such as K+, Na+ and Cl-; remove and reabsorb hydrogen bicarbonate ions thereby regulating blood pH

Mechanism, Effects & Treatment of Diabetes

As (Type I) insulin is not produced by the β cells of the Islets of Langerhans, or (Type 2) body cells do not respond to insulin, body cells are less permeable to glucose and less glucose will be absorbed into body cells for use or storage. As a result, blood glucose level will remain high a long period of time after food consumption, and cells will lack glucose to store as glycogen.

High blood glucose level results in high blood pressure as glucose increases the solute potential of blood plasma, thus increasing water retention. This will result in poor circulation which causes fatigue and increased susceptibility to infections as immunological agents are not efficiently transported to the site of infection; cardiovascular diseases including aneurysms, blindness and leakage at the Bowman’s capsule. Glucose may not be fully reabsorbed in the ascending limb of the loop of Henle, thus reducing the amount of water that can be reabsorbed, therefore producing larger volumes of urine and causing dehydration.

Reduced glucose permeability to body cells results in small amounts or the absence of glycogen reserves, which causes muscle weakness and general fatigue, and also ketone poisoning as fat is oxidized in place of glucose.

The treatment is long-term insulin injections, pancreas transplant, or gene therapy for Type I diabetes; modification of lifestyle, including diet and exercise regime, and gene therapy for Type II diabetes.


Blood is drawn from the artery in arms, and flows through the tube into the dialysis machine. The tube walls are selectively permeable, allowing urea, uric acid, some ions and water to diffuse out but retaining larger molecules and most ions. The tubing is bathed in an isotonic dialysate which minimizes the loss of essential ions. The tubing is coiled and a counter-current system is employed whereby the dialysate is flowing in the opposite direction to blood flow, so as to increase surface area for diffusion and maximize the diffusion gradient, to speed up the process.

Disease for Substance present in Urine


Glucose can be measured with Benedict’s Test. Although glucose is easily filtered in the glomerulus, it is not present in the urine because all of the glucose that is filtered is normally reabsorbed from the renal tubules back into the blood. Presence of glucose in the urine is called glucosuria.


Proteins may be measured with the Albustix Test. Since proteins are very large molecules (macromolecules), they are not normally present in measurable amounts in the glomerular filtrate or in the urine. The detection of protein in urine, called proteinuria may indicate that the permeability of the glomerulus is abnormally increased. This may be caused by renal infections or it may be caused by other diseases that have secondarily affected the kidneys such as diabetes mellitus, jaundice, or hyperthyroidism.


The fixed phagocytic cells of the spleen and bone marrow destroy old red blood cells and convert the heme groups of hemoglobin to the pigment bilirubin. The bilirubin is secreted into the blood and carried to the liver where it is bonded to (conjugated with) glucuronic acid, a derivative of glucose. Some of the conjugated bilirubin is secreted into the blood and the rest is excreted in the bile as bile pigment that passes into the small intestine. The blood normally contains a small amount of free and conjugated bilirubin. An abnormally high level of blood bilirubin may result from: an increased rate of red blood cell destruction, liver damage, as in hepatitis and cirrhosis, and obstruction of the common bile duct as with gallstones. An increase in blood bilirubin results in jaundice, a condition characterized by a brownish yellow pigmentation of the skin and of the sclera of the eye.

Red Blood Cells

May be present as intact RBC which indicates bleeding. Even trace amount of blood is enough to give the entire urine sample a red/pink hue. If the RBCs are of renal or Glomerular origin (due to glomerulonephritis), the RBCs incur mechanical damage during the glomerular passage, and then osmotic damage along the tubules and thus get dysmorphic features. The dysmorphic RBCs in urine which are most characteristic of glomerular origin are called “G1 Cells” which are doughnut shaped rings with protruding round blebs sometimes looking like mickey mouse (with ears).Painless hematuria of non-glomerular origin may be a sign of urinary tract malignancy which may warrant a more thorough cytological investigation.

Central Nervous System

The nervous system forms a communication and coordination system throughout an animal’s body and it is essential to the survival of living things. The nervous system can be broken down into two further sections – the central nervous system and peripheral nervous system. The central nervous system includes the brain and the spinal cord while the peripheral nervous system includes the cranial nerves from the brain, spinal nerves from the spinal cord and the receptors. The peripheral nervous system can be further broken down into different sections- the somatic nervous system and the autonomic nervous system. The autonomic nervous system is further simplified to the sympathetic division and the parasympathetic division.

The method at which the nervous system would work is pretty straight forward.

-              First, the body would detect a change – the stimulus

-              Following via the nerve endings of the sensory neurones, the message is sent to the central nervous system.

-              The CNS would receive and process the information and decide the response

-              Response would be sent via motor neurones to the effector where it would be carried out.


In general, there are three different types of neurones – motor neurones, interneurones and sensory neurones. These different neurones each have different abilities and functions.

-                  The motor neurones have many fine dendrites which carries impulses towards the cell body through a single long axon which carries impulses away from the cell body.

-                  The interneurones have many short fibres and their main function is to relay messages across a long distance.

-                  The sensory neurones lastly have a single long Dendron which brings impulses towards the body and a single long axon which carries the impulses away from the CNS.

Neurones all have very special structure or adaptations which would allow them to complete their job better and be more efficient in terms of conveying messages.

-        The dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body.

-        The axon is a fibre that transmits signals towards other neurons

-        The myelin sheath is an insulator that provides resistance to current flow between the axon membrane and the fluid surrounding the axon – these cells are called support cells or neuroglia cells (Schwann cells).

-        The nodes of Ranvier are breaks in the myelin sheath which help to pass the action potentials faster along the axon by saltatory conduction

The presence of the myelin sheath affects the speed of transmission of the action potential. The junctions in the sheath are spaced at intervals of 1-2 mm. Only at these nodes is the axon membrane exposed. Elsewhere along the fiber, the electrical resistance of the myelin sheath prevents depolarization so the action potentials are forced to jump from node to node. This is called saltatory conduction and it greatly increases the rate of transmission.  However, even with non-myelinated fibres, it is possible to speed up the passage of action potential. Large diameter axons transmit action potentials much more speedily than do the narrow ones. 

There is a disease called multiple sclerosis which is the gradual degradation of the myelin sheath that takes place, rendering the axon improperly myelinated or totally bare. These would result in the interference and slowing down of the rate at which impulses are transported towards and away from the CNS.

Reflex Arc

The neurones within the nervous system transmit impulses along pathways called the reflex arc. A generalized reflex arc is as shown in Fig 2. The reflex arc connects a sense organ with a muscle or gland via the neurones.

-        Firstly, the sense organ detects a stimulus which is a form of energy such as sound, light or mechanical pressure which is converted into an impulse in the nerve fiber of a neurone that serves the sense cell.

-        Once generated, the impulse travels along the fibers of the sequence of neurones of the reflec arc to an effector organ.

-        When it arrives at the effector, the impulse causes a response, for example flexing a muscle.

The simplest form of response in the nervous system is called a reflex action. It is a rapid automatic, but short lived response to a stimulus. It is an involuntary response as it is not generally controlled by the brain’s decision makin centers. In a reflex action, a particular stimulus produces the same automatic response everytime, for example jerking your hand away from scalding water.

In verterbrates, and particularly in mammals, there is a complex nervous system. Within the nervous system there are many reflex arcs and they are all connected to the control center – the brain. The brain is a highly organized mass of interneurones connected to the rest of the nervous system by motor and sensory neurones. With a nervous system of this type, complex patterns of behavior are common, in addition to reflex actions. This is because – impulses that originate in a reflex arc also travel to the brain and the impulses may originate in the brain and be conducted to effector organs. Consequently, much activity is initiated by the brain, rather than merely being a response to an external stimuli. Also, many reflex actions may be overruled by the brain, and the response modified – for example not dropping a hot object because of its value. Therefore we can deduce that the nervous system has two roles.

-        Quick and precise communication between the sense organs that detect stimuli and the muscles or glands that respons with changes.

-        Coordination and ontrol of the body’s responses by the brain.


Impulses are transmitted along nerve fibers, but it is not an electrical current that flows along the wires of the nerves. Rather, the impulse is a momentary reversal in the electrical potential difference in the membrane of the fibers. That is, it is a change in the position of positive and negative charged ions between the inside and outside of the membrane. This reversal travels from one end of the neurone to the other in a fraction of a second. Between impulses the neurone is said to be ‘resting’. Actually, this is far from case as the ‘resting’ intervals between impulses the membrane of a neurone actively creates and maintains an electrical potential difference between its inside and outside.

Two processes together create the resting potential difference across the neurone membrane.

-        There is active transport of potassium ions into the cell and sodium ions out of the cell across the membrane. The ions are transported by potassium and sodium pumps which uses energy from ATP/ So potassium and sodium gradually concentrate on the opposite side of the membrane, however this in itself make no change to the potential difference

-        There is also facilitated diffusion of potassium and sodium ions back in. The important point here is that the membrane is far more permeable to potassium flowing out than to sodium ions returning. This causes the tissue fluid outside the neurones to contain many more positive ions than the cytoplasm inside. As a result, the inside becomes more and more negatively charged as compared with the outside. This resting neurone is said to be polarised. The difference in charge or potential difference is about -70mV. This is known as the resting potential.

The action potential, the next event, sooner or later is the passage of an impulse. An impulse or an action potential is triggered by a stimulus arriving at a receptor cell or sensitive nerve ending. The energy of this stimulus causes a temporary and local reversal of the resting potential. The result is that the membrane is briefly depolarized.

The change in potential across the membrane happens through pores in the membrane, these are called ion channels because they allow ions to pass through. One type of channel is permeable to sodium ions and the other is permeable to potassium ions. These channels are globular proteins that span the entire width of the membrane. They have a central pore, with a ‘gate’ that can open and close. During resting potential all these channels are closed.

-        The energy of the stimulus opens the gates of the sodium channel in the plasma membrane. This allows the sodium to quickly diffuse in, down their electrochemical gradient. So the cytoplasm inside the neurone fibre quickly becomes progressively more positive and with respect to the outside. This charge reversal continues until the potential difference has altered from the outside. This charge reversal continues until the potential difference has altered from -70 mV to +40 mV. At this point, an action potential then travels along the whole length of the neurone fibre. At any point of the fibre, it exists for only two-thousandth of a second, before the membrane starts to re-establish the resting potential; therefore, action potential transmission is exceedingly brief.

Almost immediately after an action potential has passed, the sodium channels close and the potassium channels open so that potassium ions can exit the cell, again down their electrochemical gradient into the tissue fluid outside. This causes the interior of the neurone fibres to become less positive again. Then the potassium ion channels also closes. Finally the resting potential of -70 mV is re-established by the action of the sodium/potassium pump and the process of facilitated diffusion.

The Refractory period

For a brief period after the passage of an action potential, the neurone fibre is no longer excitable. This is called the refractory period. It lasts only 5-10 milliseconds in total. During this time, firstly, there is a large excess of sodium ions inside the neurone fibre and it is impossible for more to enter. As the resting potential is progressively restored, however, it becomes more possible for an action potential to be generated again. Because of the refractory period, the maximum frequency of impulses is 500 – 1000 per second.

The all or nothing principle

Obviously the energy carried by various stimuli may be widely different – think of the difference between a light touch and the pain of a finger hit by a hammer. A stimulus must be at or above a minimum intensity, known as the threshold of stimulation, in order to initiate an action potential. Either a stimulus depolarizes the membrane sufficiently to reverse the potential difference in the cytoplasm fully – from -70 m to +40 mV – or it does not. If not, no action potential at all arises and no message is sent along the fiber. With all sub-threshold stimuli, the influx of sodium ion is quickly reversed, and the resting potential is re-established. However, as the intensity of the stimulus increases, the frequency at which the action potential passes along the fibre increases. For example, with a very persistent stimulus, action potentials pass along a fibre at an accelerated rate, up to the maximum possible permitted by the refractory period. This means that the effector recognizes the intensity of a stimulus from the frequency of the action potentials.

Junctions between neurones

The synapse is the point where the ends of two neurones meet. It consists of a swollen up synaptic knob of the axon of one neurone – presynaptic neurone and the dendrite or cell body of the next neurone. At the synapse, the neurones are extremely close, but they are not in direct contact. Between them there is a tiny gap called the synaptic cleft, about 20 nm wide. Because there is a gap here the action potential cannot cross it. Here, another form of transmission must carry the impulse. Transmission across the synaptic cleft is not electrical, but chemical. The impulse is carried from one side of the gap to the other side by specific chemicals known as transmitter substances. These substances are all relatively small, diffusible molecules. They are produced in the Golgi apparatus in the synaptic knob and are held in tiny vesicles prior to use.

Acetylcholine is a commonly occurring transmitter substance; the neurones that release acetylcholine are known as cholinergic neurones. Another common transmitter substance is noradrenalin. The brain uses the transmitter glutamic acid and dopamine among others.

-        At the arrival of an action potential at the synaptic knob opens calcium ion channels in the presynaptic membrane, and calcium ions enter from the synaptic cleft.

-        The calcium ions cause vesicles of transmitter substance to fuse with the presynaptic membrane, releasing the transmitter substance into the synaptic cleft.

-        The transmitter substance diffuses across the synaptic cleft. In the postsynaptic membrane thee are specific receptor sites containing a receptor protein for each transmitter substance. Each of these receptors also acts as a channel in the membrane for a specific ion.

-        As a transmitter molecule binds to its receptor protein, this instantly opens the ion channel, allowing specific ions to pass through.

For instance when a molecule of acetylcholine attaches to its receptor site, a sodium ion channel opens. As the sodium ions rush into the cytoplasm of the post synaptic neurone, depolarization of the postsynaptic membrane occurs. As more and more molecules bind to their receptors, it becomes increasingly likely that this depolarization will reach the threshold level. When it does, an action potential is generated in the postsynaptic neurone. The process of building up to an action potential in postsynaptic membrane is called facilitation.

Meanwhile, the transmitter substance on the receptors is immediately deactivated by enzyme action, causing the ion channel of the receptor protein to close again. The resting potential in the postsynaptic neurone can then be re-established. Meanwhile, the inactivated form of the transmitter diffuses back across the gap, re-enters the presynaptic knob, is resynthesized into transmitter substance, and packaged for reuse.

The Brain

The vertebrate brain develops in the embryo from the anterior ends of a simple tube called the neural tube. This tube enlarges to form three primary structures known as the forebrain, midbrain and hindbrain. The various parts of the mature brain develop from the selective thickening and folding of the walls and the roof. These enlargement processes are most pronounced in mammals and a striking feature is the enormous development of the cerebral hemispheres which are an outgrowth of the forebrain. When tissues inside the brain are examined, the parts where cell bodies are grouped together appear grey, so they are known as ‘grey matter’. Areas where myelinated nerve fibers occur together appear whiter, so they are called white matter. White and grey matters are present in both the brain and the spinal cord. Grey matter makes up the interior of the brain and white the exterior. However, in the cerebral hemisphere and cerebellum, there are additional layers of grey matter.

Blood capillaries are also present throughout the nervous tissue. However, in the brain, the capillary walls form a barrier against many of the dissolved substances in the blood. This means that only essential substances such as oxygen and glucose can pass through. This is called the blood/brain barrier. Other substances dissolved in the plasma that might affect the brain’s neurotransmitters in brain synapses. This barrier is therefore important for maintenance of normal brain functions.

The human brain controls all body functions, apart from those under the control of simple spinal reflexes mentioned earlier. According to current understanding of brain functions, it achieves this by

-        Receiving impulses from sensory receptors

-        Integrating and correlating incoming information in association centres

-        Sending impulses to effector organs causing bodily responses

-        Storing information and building up an accessible memory bank

-        Initiating impulses from its own self-contained activities, ‘personality’ and emotions, and enable us to imagine, create, plan, calculate, predict and abstractly reason.

Within the brain, it is segmented into different parts which would handle different tasks.

-        The hypothalamus is a part of the forebrain which is exceptionally well supplied with blood vessels, monitor and controls body temperatures and the levels of sugar, amino acids and ions. Feeding and drinking reflexes and aggressive and reproductive behaviours are also controlled here. It works with the pituitary gland to control the release of hormones.

-        The cerebral hemisphere is an extension of the forebrain that forms the bulk of the brain and coordinates most of the body’s voluntary actions, together with many involuntary ones. These hemispheres have a vastly extensive surface which is achieved by extensive folding of the surface such that it forms deep grooves.

-        The cerebellum, part of the hind brain has an external surface of grey matter. It is concerned with the control of involuntary muscle movements of posture and balance. Here, precise and voluntary manipulations including hand movements, speech and writing are coordinated.

-        The medulla, the base of the hind brain, houses the regulatory centres concerned with maintaining the right heart rate, ventilation of the lungs and temperature. In the medulla, the ascending and descending pathways of nerve fibres connecting the spinal column and brain cross over. As a consequence, the left side of our body is controlled by the right side of the brain vice versa.

Neurological Disorders

In addition to trauma, various neurological disorders such as depression and Alzheimer’s disease can also affect brain function. Nearly 20 million American adults are affected by depression; about two thirds of them are women. Two broad forms of depressive illness have been identified: major depression and bipolar disorder.

-         People identified with major depression may experience persistent sadness, loss of interest in pleasurable activities, changes in body weight and sleep patterns, loss of energy and suicidal thoughts. While all of us feel sad from time to time, major depression is extreme and more persistent, leaving the sufferer unable to live a normal life. If left untreated, symptoms may become more frequent and severe over time.

-      Bipolar disorder or manic-depressive disorder, involves extreme mood swings. The manic phase is characterized by high self-esteem, increased energy, a flood of thoughts and ideas and extreme talkativeness, as well as behaviours that often court disaster, such as increased risk taking, promiscuity, and reckless spending. In its milder forms, this phase is sometimes associated with great creativity, and some well-known artists, musicians and literary figures with bipolar disorders. The depressive phase is marked by sleep disturbances, feeling of worthlessness, and decreased ability to experience interest and pleasure.

Alzheimer’s disease is a form of mental deterioration or dementia, characterized by confusion, memory loss and a variety of other symptoms. Its incidence is often age related, rising from about 10% at age 65 to about 35% at age 85. This disease is progressive as patients become gradually become less able to function and eventually need to be dressed, bathed and fed by others.

Spinal Cord

The spinal cord is a cylindrical structure with a tiny central canal. The canal contains cerebrospinal fluid and I continuous with the fluid-filled spaces – ventricles – in the center of the brain. The cerebrospinal fluid brings nutrients to the spinal cord and helps to cushion the CNS. The cord consists of an inner area of grey matter surrounded by white matter. The spinal cord is surrounded and protected by the vertebrae of the backbone, with 31 pairs of spinal nerves that emerge at regular intervals along the length of the spinal cord. The spinal nerves bifurcate into two – the ventral and dorsal roots. The central root contains only motor neurones and the cell bodies of these are found in the grey matter of the spinal cord. The dorsal root only contains sensory neurones. Cell bodies of the sensory neurones in the dorsal root aggregate in a small swelling called the dorsal root ganglion.

In the junction between each pair of vertebrae, two spinal nerves leave the cord, one to each side of the body. The role of the spinal cord is to relay action potential between sensory organs and effector organs of the body, and also between them and the brain, so that the reflex action may be overridden, for example the toleration of a hot object.

Peripheral Nervous System

The vertebrate nervous system is divided into two functional components: the somatic nervous system and the autonomic nervous system. Neurones of the somatic system carry signals to and fro the skeletal muscles, mainly in response to external stimuli. When you walk for instance, these neurones carry commands to make your legs move.

-         The somatic nervous system is said to be voluntary because many of its actions are under conscious control. In contrast, the motor neurones of the autonomic nervous system regulates the internal environment by controlling smooth and cardiac muscles and the organs and glands of the digestive, cardiovascular, excretory and endocrine system.

-        The autonomic nervous system contains two sets of neurones with opposing effects on most body organs.

  • One set, called the parasympathetic division, primes the body for activities that gain and conserve energy for the body. These effects include stimulating the digestive organ, decreasing the heart rate and narrowing the bronchi, which correlates with a decreased breathing rate.
  • The other set of neurones called the sympathetic division tends to have the opposite effect, preparing the body for intense energy consuming activities such as fighting, fleeing or competing in a strenuous game. When the division is stimulated, the digestive organs are inhibited, the bronchi dilate so that more air can pass through and the adrenal glands secrete hormones epinephrine and norepinephrine.

Relaxation and the fight-or-flight response are opposite extremes. Your body usually operates somewhere in between, with most of your organs receiving both sympathetic and parasympathetic signals. The opposing signals adjust an organ’s activity to a suitable level. As carriers of command signals, the motor neurones of the parasympathetic and sympathetic systems constitute lower levels of the nervous system’s hierarchy.


The 5 Senses

Sensory input is the process of using receptors to sense the environment and send information about it to the CNS (central nervous system). Sensory input is the process of using receptors to sense the environment and send information about it to the CNS to be integrated and acted upon. Sensory receptors, such as the sensory cells in the eyes and taste buds of your tongue are tuned to the condition of both external world and internal organs. Sensory receptors detect stimuli such as chemicals, light, sound, cold, heat and touch. The sensory receptors in the eyes for instance detect light energy. In the process of sensory transduction, for example in a tongue:

-        When sugar molecules first come into contact with the taste-buds, they bind to membrane receptors of the sensory receptor cells

-        This would trigger a signal transduction pathway that causes some ion channels in the membrane to close and others to open, changing the flow of ions and alters the membrane potential.

-        Each receptor cell forms a synapse with a sensory neuron. When there are enough sugar molecules, a strong receptor potential is triggered, making the cell release enough neurotransmitter to increase the rate of action potential generation in the sensory neurone.

Based on the type of signals to which they respond, we can group sensory receptors into five general categories – Pain receptors, thermo-receptors, mechanoreceptor, chemoreceptor and electromagnetic receptors. These five types of receptors work in various combinations to create the five human senses.

-        Pain receptors may respond to excessive heat or pressure or to chemicals released from damaged or inflamed tissues. Prostaglandins are local regulators that increase pain by sensitizing pain receptors. Aspirin and ibuprofen reduce pain by inhibiting prostaglandin synthesis.

-        Thermo-receptors in the skin detect either heat or cold. Other temperature sensors located deep in the body monitor the temperature of the blood. The body’s thermostat is the hypothalamus. Receiving action potentials both from surface sensors and from deep sensors, the hypothalamus keeps a mammal’s or bird’s body temperature within a narrow range.

-        Mechanoreceptors are highly diverse. Different types are stimulated by different forms of mechanical energy such as touch and pressure. All these forces produce their effects by bending or stretching the plasma membrane. When the membrane changes shape, it becomes more permeable to positive ion and the mechanical energy of the stimulus is transduced into a receptor potential.

-        Chemoreceptors include the sensory cells in our nose and taste buds which are attuned to chemicals in the external environment, as well as some internal receptors that detect chemicals in the body’s internal environment. Internal chemoreceptors include sensors in our arteries that monitor our blood, with some sensors detecting changes in pH and other detecting changes in oxygen concentrations.

-        Electromagnetic receptors are sensitive to energy of various wavelengths, which takes such forms as magnetism and light. For example, photoreceptors detect the electromagnetic energy of light, and the nonhuman sense of electroreception and magneto-reception rely on other types of electromagnetic receptors.

Eyes (Vision)

The structure of the human eyeball is very complex.

-        There is first an outer layer of connective tissue called the sclera, at the front of the eye, the sclera becomes the transparent cornea

-        The sclera surrounds a pigmented area called the choroid, and at the front of the eye, the choroid forms the iris

-        The opening in the center of the iris is called the pupil

-        Behind the pupil is the dislike lens which is held in position by ligaments

-        At the back of the eyeball is the retina, a layer just inside the choroid that contains the photoreceptor cells

-        The optic nerve connects the retina with then brain

-        There are two fluid chambers that make up the bulk of the eye, the large chamber behind the lens that is filled with jelly-like vitreous humor and a smaller chamber in front of the lens that contains a thinner liquid called aqueous humor

-        A thin mucous membrane called the conjunctiva helps keep the inner surface of the eyelids moist. The conjunctiva lines the inner surface of the eyelids and then folds back over the white of the eye.

-        A gland above the eye secretes tear, a dilute salt solution that is spread across the eyeball by blinking and that drains into the ducts leading to the nasal cavities. The eyelid helps to spread the moisture over the eyeball so as to prevent it from drying.

The human eye is a remarkable sense organ that is able to detect a whole multitude of colors.

-        The cornea lets light into the eye and also helps focus the light.

-        The muscles of the iris regulate the size of the pupil, controlling the amount of light that enters.

-        After going through the pupil, light passes through the lens. The lens focuses light onto the retina by bending light rays

-        Focussing is done by changing the shape of the lens, the thicker the lens, the more sharply it bends light. The shape of the lens is controlled by muscles attached to the choroid

-        When the eye focuses on a nearby object, these muscles contract, pulling the choroid layer of the eye inwards, towards the pupil, making the ligaments that suspend the lens slacken

-        A blind spot exists for each eye; however it is not possible for a ray of light to fall on both blind spots simultaneously.

The human retina contains two types of photoreceptors, the rods and the cones. Rods are extremely sensitive to light and enable us to see in dim light, though only shades of grey. Cones are stimulated by bright lights and can distinguish color, but they contribute little to night vision. In humans, rods are found in greatest abundance at the outer edges of the retina and are completely absent from the center. In contrast, the retina’s center of focus contains a high concentration of cones.

-        Rods contain a visual pigment called rhodopsin, which is derived from Vitamin A

-        Cones contain photopsins which absorb coloured bright light. We can perceive a great number of colours because light from each of the colours trigger a unique pattern of stimulation among the three different types of cones – red, green, blue.

When staring at something near to the eye, the eye would automatically adjust in order to allow you to see things clearly. Firstly, the ciliary muscles would contract and the suspensory ligament would slacken, puller the lens, causing it to become thicker and more convex. In the case of looking at things that are far away on the other hand, the opposite would occur the ciliary muscle would relax. The light would thus be refracted and be focused on the retina. Three of the most common visual problems are near sightedness and astigmatism. All three are focusing problems that are easily corrected with artificial lenses.

-        People with near sightedness cannot focus well on distant objects, although they can see well at short distances. A near-sighted eyeball is longer than normal. The lens cannot flatten enough to compensate, it focuses distant objects in front of the retina instead of on it.

-        Farsightedness occurs when the eyeball is shorter than normal, causing the lens to focus images behind the retina.

Humans and most predators have two eyes, one located on each side of the face. The image that each will see is slightly different because each eye views an object from a different angle. The slight displacement of the image permits binocular vision, the ability to perceive three-dimensional objects and to sense depth.

Ear (hearing)

The ear is composed of three regions – the outer ear, the middle ear and the inner ear. The outer ear consists of a flap-like pinna – the bendable structure that we commonly refer to as our ear – and the auditory canal. A sheet of tissue called the eardrum separates the outer ear from the middle ear. Both outer ear and middle ear are common sites of childhood infections.

The middle ear contains three small bones: the hammer, the anvil and the stirrup. The stirrup is connected to the inner ear through an opening in the skull bone. The Eustachian tube conducts air between the middle ear and the back of the throat, allowing air pressure to stay equal on either side of the eardrum. This tube is what enables you to move air in and out in order to equalize pressure when changing altitude.

The inner ear consists of fluid-filled channels, the cochlea, and a long coiled tube. Our actual hearing organ, the organ is the Corti which is located within a fluid-filled canal inside cochlea. The organ of Corti consists of an array of hair cells embedded in a structure called the basilar membrane. The hair cells are receptor cells of the ear.

-        The ear functions through the process of picking up sound waves in the surrounding air by which would be channelled to the eardrum by the pinna and the auditory canal.

-        From the eardrums, the vibrations are concentrated as they pass through the hammer, the anvil and the stirrup.

-        The stirrup would transmit the vibrations to the inner ear, producing pressure waves in the fluid within the cochlea.

-        As the pressure waves pass through the cochlea, it makes the basilar membrane vibrate.

-        Vibration of the basilar membrane makes the hair like projections on the hair cells alternately brush against and draw away from the overlying membrane. When a hair cell’s projection is bent, ion channels on its plasma membrane open and positive ions enter the cell, causing the hair cell to develop a receptor potential and release neurotransmitter molecules at its synapses

-        In turn, the sensory neurone sends action potentials through the auditory nerves to the brain

Deafness, the loss of hearing, can be caused by the inability to conduct sounds, resulting from middle ear infections, a ruptured eardrum, or the stiffening of the middle ear bones. Deafness can also result from the damage of the receptor cells or neurones. Few parts of our anatomy are more delicate that the organ of Corti, deafness is often progressive and permanent. Frequent or prolonged exposure to very loud sounds can damage or destroy hair cells of the inner ear.

Muscles (Functioning)

Muscles are basically made up of a hierarchy of smaller and parallel strands. Each muscle fiber is a single cell with any nuclei. Each muscle fiber itself is a bundle of smaller myofibrils. Skeletal muscles are also called striated (striped) muscles because the myofibrils exhibit alternating light and dark bands when viewed under the microscope. A myofibril is a continuous repeat of units called sarcomeres. Structurally, a sarcomere is a region between two dark, narrow lines in the myofibril. Functionally, the sarcomere is a contractile apparatus in a myofibril – the muscle fiber’s fundamental unit of action. During contraction of the muscle, the filaments overlap in the middle of the thick and thin filaments. A muscle can shorten to about one third of its resting length when all its sarcomeres are contracted. The process that makes the thin filaments slide requires energy that is provided by ATP.

-          ATP binds to a myosin head, causing the head to detach from a binding site on actin.

-          The myosin head gains energy from the breakdown of ATP and changes shape, into a high energy position

-          The energizes myosin head binds to an exposed binding site on actin

-          The molecular event that actually causes sliding are the power stroke. The myosin ends back to its low energy position, pulling the thin filament towards the centre of the sarcomere

-          After the power stroke, the whole process repeats.

The sarcomeres of a muscle fiber do not contract on their own and they must be stimulated by motor neurones. A typical motor neurone can stimulate more than one muscle fiber because each neurone has many branches.


The function of the skeletal system is to provide a rigid internal framework and maintain the shape of the body. The skeletal structures also help to protect the important organs in a body from injury. The brain for example is protected by the skull. The bones also provide points of attachment of skeletal muscles which would pull the bones when contracting, producing movement. The bones marrow within the bones is also in charge of the production of red and white blood cells. Altogether there are three different types of joints – immovable, partially moveable and free moveable.

-        The immovable joints are like those present in the cranial bones as the skull bones are fused together at the sutures. These bones are generally in place to protect fragile and important organs of the body.

-        The partially moveable joints are joints which allows for only limited movement. These joints include the gliding joints between bones of the vertebral column.

-        Free moving joints finally are joints which has the greatest flexibility. It allows for a 360 degrees movement in all planes. Joints like this include the hip joint and the shoulder joint.  These joints are also called the synovial joints. This is due to the synovial fluid which is present in the joints. The fluid helps to lubricate the joints, reducing friction when they move against each other, and provide nutrients for the cartilage of at the ends of the bones.

Skeletal muscles are usually a voluntary muscle which is under conscious control; however, it can be involuntary in reflex actions.










Voluntary and Involuntary




Attached to bone

Forms bulk of heart wall

Walls of internal organs

Number of nuclei

Many per cell

One per cell

One per cell