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Blood is a living, fluid, connective tissue. It circulates in the blood vessels constantly because of the pumping action of the heart. The blood consists of a fluid matrix called plasma and three types of living cells
  1. Red blood corpuscles or erythrocytes.
  2. White blood corpuscles or leucocytes, and
  3. Blood platelets or thrombocytes.
The colour of the blood depends on the nature of the haemoglobin, the coloured pigment contained in the red blood cells. As the blood passes through the lungs, the haemoglobin picks up oxygen and is then called oxyhaemoglobin. Oxyhaemoglobin gives blood a bright scarlet colour. If the haemoglobin contains no oxygen (as it happens after death) the blood is dark blue, nearly black. In passing through the living tissues, the blood gives up some of its oxygen. Therefore, the blood which has passed through the tissues is deoxygenated (venous blood) has less of redness and more of blueness than fully oxygenated (arterial) blood.

Plasma forms about two-thirds of the blood. It is a viscous fluid containing 80 per cent water and 20 per cent other substances such as salt, organic substances and plasma proteins.

The volume of blood in the body varies from one individual to another. However, it remains constant for every individual human being. In adults the total volume of blood is about 5 liters. The average volume of blood is calculated on the basis of weight, that is, approximately 70 ml of blood for each kilogram body weight. The fluid volume of blood is not affected even when large volumes of water is consumed. The excess water in the body is excreted in the form of urine. Similarly during haemorrhage (excessive blood loss due to cuts and injuries) the lost amount of blood is soon restored to normal volume. After haemorrhage the fluids from the tissues move into the blood vessels to restore the blood volume.

Excessive loss of blood may take place during surgical operations or due to injury. The body is usually unable to restore the blood volume in case the loss of blood exceeds 40% of the total volume. This will lead to the death of the person unless the lost blood is restored immediately. The intravenous injection of blood plasma or serum with the object of restoring the blood volume is called transfusion. The blood is taken from another healthy individual (donor) and transfused into the patient (recipient).

Human Blood Groups

Blood Groups

It has been found that great care should be exercised in choosing the blood for transfusion. This is because, sometimes, the blood of the donor and the recipient do not match and may result in the clotting of blood and death of the recipient. In such cases the blood of the donor and the recipient are said to be incompatible (blood of the wrong type).

The work of a German biochemist, Karl Landsteiner, and others showed that the entire human population, regardless of colour of the skin or race, can be divided into four major groups according to the reaction of their blood when mixed together. The four groups of blood or blood groups are A, B, AB and O.

The incompatibility of different types of blood is due to red blood corpuscles coming in contact with incompatible substances in the blood plasma. The substances which are found in the red blood cells are called antigens or agglutinogens. The substances found in the plasma are called antibodies or agglutinins. The incompatibility of the blood groups in other words, is due to the antigen-antibody reaction. The letters A, B, AB, and O refer to the antigen found in the red blood cells. Type A has antigen A, B has antigen B, type A, B has both the antigens while type O has none. The human blood plasma contains two types of antibodies, namely A group has (anti B) and B (anti A). Clumping of RBC's occur when antigen A comes in contact with antibody a and when antigen B comes in contact with antibody b. The type A blood has antigen A and antibody b; Type B has no antibody. Type O has both antibodies a and b but no antigen. It is evident then, that the blood group A can be given only to those person who do not possess antibody a that is only to type A and AB; similarly the blood group B can be given only to types B and AB. AB blood can be given only to AB recipient. O type blood can be given to any other type because it does not contain any antigen at all. A person with O blood group is called a universal donor, while AB is called a universal recipient.

Antigens and Antibodies

Rh Factor

Rh factor is a type of blood protein first discovered in the blood of the rhesus monkey. Later it was found that some human beings also possess this factor in their blood. If this factor is present in the blood, the blood is called Rh positive and if absent it is called Rh negative. Rh factor is inheritable and therefore an Rh negative mother and an Rh positive father may have an Rh positive baby. During pregnancy the Rh positive factor goes from the baby's circulation to the circulation of the mother through the placenta and causes formation of antibodies against Rh positive factor. The anti-Rh-positive protein enters the baby's blood causing coagulation of blood. This results in the death of the baby. This disease is called erythroblastosis foetalis.

Blood Coagulation

When any part of the body is injured, blood immediately flows out. After a few minutes the blood flow stops, as it thickens forming a clot. This process is called coagulation or clotting of blood. Blood coagulation is a protective function through which excessive loss of blood from the body is prevented. Clotting is a chemical reaction by which a soluble protein in the plasma called fibrinogen is converted into insoluble fibrin. For this reaction the presence of another protein called thrombin in the plasma is necessary. Thrombin exists in an inactive form called prothrombin. The conversion of prothrombin into thrombin is catalysed by an enzyme, thromboplastin which is released from the damaged blood platelets and injured tissue. Calcium ions play a vital role in the clotting process. A summary of reactions involved in coagulation of blood is shown in the figure. The fibres of the insoluble fibrin form the clot in which the corpuscles are entangled.

Summary of the Mechanism of Blood Coagulation

Blood does not normally coagulate in blood vessels as thrombin does not form in the absence of thromboplastin. A substance called heparin also interferes with conversion of prothrombin into thrombin.

A substance which prevents coagulation of blood is called an anticoagulant. Heparin is one example of an anticoagulant. Hirudin is another anticoagulant released from the salivary glands of leeches. Substances like sodium or potassium oxalates and citrates also prevent coagulation of blood. Sometimes, one of the important plasma proteins, antihaemophilic globulin, needed for coagulation of blood, is absent from the blood, resulting in a very slow coagulation of blood. Such persons suffer from a disease known as haemophilia. They bleed excessively even from a slight injury.

Functions of Blood

Blood has many functions. The most important of these are
  1. Transport of Food
    The digested food from the alimentary canal enters the blood and is distributed by it to all parts of the body.
  2. Transport of Oxygen and Carbon dioxide
    Blood transports oxygen from the respiratory surfaces (lungs) to the tissues and carbon dioxide from the tissues to the lungs, thus assisting respiration.
  3. Transport of Waste Products
    Waste products like urea, uric acid and ammonia are constantly produced in the tissues due to metabolic activities. These materials are collected from the tissues by the blood, which transports them to the excretory organs.
  4. Maintenance of Water Balance
    Blood maintains the water balance at a constant level, by bringing about constant exchange of water between the circulating blood and the tissue fluid.
  5. Transport of Hormones
    Hormones produced by the endocrine glands are distributed to various tissues by the blood. Hormones control the chemical coordination and working of the body.
  6. Protection from Invading Organisms and Foreign Substances
    The leucocytes, particularly neutrophils and monocytes, engulf bacteria protozoa etc. by phagocytosis and destroy them. Lymphocytes participate in forming antibodies which react and destroy invading organisms and their toxins.
  7. Temperature Regulation
    Blood helps to maintain the body temperature at a constant level by distributing heat within the body.
  8. Tissue Support
    Exerting pressure in the arteries, blood helps to support the tissues.
  9. Blood Coagulation
    Because of its property of coagulation, blood prevents excessive loss of blood, when the tissue is injured.

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