BLOOD

Transport System

In complex multi-cellular organisms, such as human, numerous cells are situated deep inside the body, far from the external environment. Simple diffusion is not sufficient for providing enough oxygen and food materials to these cells. There needs to be a transport system to carry substances from one part of the body to another.

The main transport system in humans is the circulatory system. The fluid in this system is blood.

Blood is a fluid which has different types of cells suspended in it. For this reason, blood is referred to as a fluid tissue.

Constituents of blood:

- Plasma (55%)

- Red Blood Cells (erythrocytes)

- White Blood Cells (leucocytes)

- Platelets (thrombocytes)

Plasma

- A pale yellowish liquid

- 90% of plasma is water, 10% is a complex mixture of various dissolved substances such as:

Soluble Proteins - Fibrinogen, prothrombin, and antibodies. Fibrinogen and prothrombin are important for blood clotting.
Dissolved mineral salts - Hydrogencarbonates, chlorides, sulfates and phospates of calcium, sodium and potassium. All these occur as ions in plasma. Calcium is essential for blood clotting.
Food substances - Glucose, amino acids, fats and vitamins.
Excretory Products - Urea, uric acid, and creatinine. Carbon dioxide is present as hydrogencarbonate ions.
Hormones - Insulin

- Plasma transports all these substances, alongside blood cells, around the body.

Erythrocytes (RBCs)

- About five million erythrocytes in each cubic millimeter of blood.

- Each mammalian RBC has these features:

It contains the pigment haemoglobin (iron-containing protein). Haemoglobin combines reversibly with oxygen. This enables RBCs to transport oxygen from the lungs to other parts of the body
Circular, biconcave shape - Increased surface area to volume ratio (can absorb and release oxygen faster)
No nucleus - Can carry more haemoglobin due to extra space.
Elastic and can turn bell shaped in order to squeeze through blood vessels smaller than itself in diameter

- RBCs are produced by the bone marrow.

- Lifespan of a RBC is about 3-4 months, worn out RBCs are destroyed in the spleen and the haemoglobin released from these worn out RBCs are brought to the liver and broken down.

- Larger than RBCs but are fewer in number. Ratio of RBCs to WBCs is 700:1.

- Only 5000-10000 WBCs in each cubic millimeter of blood

- Each leucocyte has these feature:

Colorless and does not contain haemoglobin
Irregular in shape and contains a nucleus
It can move, change shape and squeeze through the walls of the thinnest blood capillaries into spaces amongst tissues
Two types of leucocytes:
- LYMPHOCYTES - A large, rounded nucleus and relatively small amount of cytoplasm. Show limited movements. They produce antibodies that help in protecting the body from disease causing micro-organisms.
- PHAGOCYTES - Ingest and digest foreign particles such as bacteria.

- Leucocytes play a vital role in keeping the body healthy by fighting diseases

Leucocytes (WBCs)

Thrombocytes (Platelets)

- Not true cells

- Membrane bound fragments of cytoplasm from certain bone marrow cells

- Play a part in blood clotting

Figure 1. Lymphocyte

Figure 2. Phagocyte ingesting a bacterium

Blood Groups

Large volumes of blood may be lost due to serious injury or surgical operations. To save a person from dying, blood is transfused from one person to another, but this blood must be of a specific group.

Death can be caused by transfusing blood of the wrong group into a patient. This is because RBCs of the donor clump together (agglutinate) in the recipient's body. such clumps block up small blood vessels and obstruct blood flow.

What causes clumping of RBCs?

- The surface of one's RBCs contain antigens.

- Blood plasma of the said person contains natural antibodies which are present in the blood. These natural antibodies do not react with the antigens on RBCs, but they may react with the antigens of RBCs of another person.

When antigens and antibodies react, RBCs clump together. This is why blood is classified into various groups.

Blood Groups

- Blood can be classified according to the types of antigens and antibodies present in one's blood.

- Antigens are represented by capital letters A and B.

- Antibodies AGAINST antigens A and B are represented by small letters and b (This means that antigen A will react with antibody a, and antigen B will react with antibody a).

- The table on the right summarizes the four different blood groups - A, B, AB & O

What happens when different blood groups are mixed?

- The table below summarizes the effects of different blood groups on one another.

- When a transfusion is carried out, only the effect of the recipient's plasma on the donor's RBCs is considered.

- The plasma of the donor's blood is so diluted, that the antibodies present will not react with the antigens of the recipient's antigens (as can be seen when Group O blood is transfused into Group A, B & AB).

FUNCTIONS OF BLOOD

- Transport medium carrying various substances around the body

- Protects the body against disease-carrying organisms (pathogens)

- Blood clotting at wounds prevents excessive loss of blood

BLOOD CLOTTING

- Blood exposed to air will soon clot or form a solid lump. This clot seals the wound, preventing excessive blood loss and the entry of pathogens into the body.

- Haemophillia is a hereditary disease in which the blood-clotting mechanism is greatly impaired.

- In undamaged blood vessels, blood does not clot due to the presence of an anti-clotting substance called heparin. When thrombokinase is released (mentioned below), it neutralizes the actions of heparin.

PROCESS OF BLOOD CLOTTING

1. When blood vessels are damaged, damaged tissues and blood platelets release the enzyme thrombokinase.

2. Thrombokinase in the presence of calcium converts the protein prothrombin into thrombin.

3. Thrombin (an enzyme) converts the protein fibrinogen into insoluble threads of fibrin.

4. Fibrin threads entangle blood cells and the whole mass forms a clot or scab which seals the wound, preventing entry of bacteria.

5. When blood clots a yellowish liquid called serum is left behind.

Antibodies

When pathogens such as bacteria or viruses gain entry into the bloodstream, they stimulate lymphocytes to produce certain chemical substances called antibodies. Antibodies protect our bodies against disease. Antibodies do the following:

- Destroy bacteria by attaching to them and causing the bacterial surface to rupture.

- Cause the bacterial to agglutinate so that they can be easily ingested by phagocytes.

- Neutralize the harmful substances (toxins) produced by bacteria.

Antibodies stay in the blood long after the disease has been overcome. Thus, a person who has recovered from a disease becomes immune to it (or contains the necessary antibodies to fight against it if s/he ever comes into contact with the specific disease).

Immunization

Dead or weakened bacteria are injected into the bodies of certain animals (e.g. horses) to induce the formation of antibodies in the animal's blood. These antibodies are then extracted from the animal's serum and injected into human beings to protect them from the certain disease.

These dead or weakened bacteria can be directly introduced into a human as well to induce the production of antibodies. This process of injecting the dead or weakened pathogen directly into a human is called immunization or vaccination.

Organ Transplant and Tissue Rejection

Sometimes when a person's tissue or organ is damaged or diseased, it can be replaced by a healthy tissue or organ from a donor. This is called tissue or organ transplant. Some common transplants include kidney, liver and heart transplants.

The organ being transplanted must not be rejected by the recipient's immune system. Any organ from another person may be treated as a foreign body by the recipient's immune system. The recipient's lymphocytes may respond by producing antibodies to destroy the transplanted organ.

Tissue rejection will not be a problem if the tissue being transplanted comes from the same person (e.g. skin grafts). Tissue rejection can be prevented till a great extent by:

- Keeping the same blood type

- Donor and recipient are as genetically identical as possible (likely to have similar genes) (brothers, sisters, mothers, fathers)

- Using immunosuppressive drugs (inhibits the responses of the recipient's immune system.

Problems of immunosuppressive drugs:

- The recipient has lower resistance to many kinds of infections

- The recipient has to continue taking the drugs for the rest of his or her life.