There are three types of blood cells or corpuscles. They are
1. Red Blood Corpuscles (RBC) or Erythrocytes
2. White Blood Corpuscles (WBC) or Leucocytes
3. Blood Platelets or Thrombocytes
These are circular, biconcave and non-nucleated cells. Males have about 5.2 million erythrocytes per cubic millimeter of blood (range : 4.2-5.8 million). Females have about 4.5 million/mm3 (range 3.6-5.2 million).
Each disc shaped RBC is about 7.5 mm in diameter. Their main component is a pigmented protein, haemoglobin. It gives red colour to the blood. The haemoglobin transports O2. The oxygenated form of haemoglobin is called oxyhaemoglobin.
Erythrocytes stay in circulation for about 120 days in males and 110 days in females. They are manufactured in the marrow of bones such as ribs and vertebrae. They disintegrate in the spleen and liver.
These are clear cells lacking haemoglobin. They are nucleated cells exhibiting amoeboid movement. They protect the body against invading micro-organisms and remove dead cells from the body. There are five types of leucocytes.
a). Neutrophils - These are the most common type of leucocytes (60-70%) in the blood. Their nuclei can occur in more than one form. Hence they are called polymorphonuclear neutrophils (PMN).
b). Eosinophils (0.5-3.0%) - They are motile cells that leave the circulation to enter the tissues during an inflammatory reaction. During allergy reaction their number increases.
c). Basophils (0.1%) - They play a role in allergic and inflammatory reaction. They contain heparin which inhibits blood clotting.
d). Lymphocytes (20-30%) - These are smallest leucocytes. They are more common in lymphatic tissues namely the lymph nodes, spleen, tonsils and thymus. Lymphocytes, called B-cells can produce proteins called antibodies that can get attached to the bacteria and destroy them. T-cells 49
protect us against viruses by attacking and destroying cells in which viruses are reproducing.
e). Monocytes (1-4%) - These are largest leucocytes. They destroy bacteria, dead cells and cell fragments. During chronic infection their number increases.
These are minute fragments of cells that play a very important role in coagulation of blood. Their life expectancy is 5-9 days.
Clotting of Blood or Haemostasis
When a blood vessel is damaged, it results in coagulation or clotting of blood. A blood clot is a network of thread like protein fibers, called fibrin, that traps blood cells, platelets and fluid.
The clotting depends on several proteins in the plasma. They are called coagulation factors. Normally these factors are in an inactive state. After injury they are activated to produce a clot. The activation can happen in three stages.
Stage 1 - Formation of thrombokinase - Damaged tissues release a mixture of lipoproteins and phospholipids called tissue factor(TF) or thromboplastin. This factor in the presence of certain factors in the blood form a complex called prothrombinase orthrombokinase.
Stage 2 - Formation of thrombin - During this stage soluble plasma protein prothrombin is converted into the enzyme thrombin by prothrombinase. Prothrombin synthesis in liver requires vitamin K.
Stage 3 - The soluble plasma protein fibrinogen is converted to insoluble protein, fibrin by thrombin
The fibrin forms the fibrous network of the clot.
The formation of a thrombus or blood clot within an intact blood vessel is called thrombosis. Clotting is a normal response that prevents bleeding when a blood vessel wall is injured. However thrombus formation is abnormal if it occurs in an intact vessel.
A thrombus within an artery may block the artery preventing blood and oxygen from reaching the organ or tissue supplied by an artery.
A thrombus that forms within one of the coronary arteries supplying heart muscle is known as coronary thrombosis. This is the cause for heart attack .
A thrombus within arteries supplying the brain is known as cerebral thrombosis. It causes stroke. When a portion of a thrombus clot becomes fragmented and enters the circulating blood, it is called embolus. Embolus may block a circulation to vital parts resulting in serious consequences such as stroke.