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View of blood cells under a microscope.

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Blood is a fluid substance that circulates in the arteries and the veins of the body. Blood is bright red when it has been oxygenated in the lungs and passes into the arteries. It becomes bluish red when it has given up its oxygen to nourish the tissues of the body and is returning to the lungs through the veins and the tiny vessels called capillaries. In lungs, the blood gives up the carbon dioxide wastes it has taken from the tissues and receives a new supply of oxygen and then it begins a new cycle. This movement of blood is brought about by the vital activities of the heart, lungs, and blood vessels.

Composition of Blood

Blood is composed of a yellowish fluid which is called plasma. The plasma is suspended the millions of cells that constitute about 45 per cent by volume of whole blood. It has a characteristic odour. In an average healthy adult, the volume of blood is one-eleventh of the body weight, or between 4.5 and 6 litres.

A great portion of the plasma is composed of water which is a medium that facilitates the circulation of the many indispensable factors of which blood is composed. A cubic millimetre of human blood contains about 5 million red corpuscles called erythrocytes and 5,000 to 10,000 white corpuscles called leucocytes and also 200,000 to 300,000 platelets called thrombocytes. The blood also carries many salts and organic substances in the form of a solution.


The red corpuscles or red blood cells, are round discs which are concave on two sides, and are approximately 7.5 thousandths of a millimetre in diameter. In humans, and most other mammals, the mature red blood cell contains no nucleus while in some vertebrates, it is oval and thus nucleated. Haemoglobin a protein in the red blood cells, is the most prevalent of the special blood pigments that transport oxygen from the lungs to the body cells, where it picks up carbon dioxide for transport back to the lungs to be expired.


The white blood cells are of two principal types: the granular, which have multi lobed nuclei, and the non granular which have rounded nuclei. The granular leucocytes include neutrophils, which ingest and destroy bacteria , eosinophils are also present which increase and become active in the presence of certain infections and allergies; and basophils, which secrete the anticoagulant heparin and the substance histamine which further stimulates inflammation. The non granular leucocytes are the lymphocytes and the less numerous monocytes are both associated with the immune system. Lymphocytes have an important role in producing antibodies and provide cellular immunity. Monocytes ingest nonbacterial foreign substances.


Blood platelets are small, round, non nucleated bodies with a diameter about one-third that of red blood cells. Thrombocytes adhere to the walls of blood vessels at the site of an injury and so they plug the defect in the vascular wall. While disintegrating , they release coagulating agents that lead to the local formation of thromboplastin, which helps to form a clot this is the first step in the healing of an injury.


Plasma is a complex substance whose its principal component is water. It also contains plasma proteins and inorganic constituents such as sodium, potassium, calcium chloride, carbonate, and Among the component plasma proteins are albumin, the principal agent in maintaining the osmotic pressure of the blood and therefore in controlling its tendency to diffuse through the walls of blood vessels. A dozen or more proteins, including fibrinogen and prothrombin, which participate in clotting


One of the most remarkable properties of blood is its ability to clot, when it is withdrawn from the body. Inside the body, a clot is formed in response to tissue injury, such as a muscle tear. In the blood vessels, the blood remains in a fluid condition even shortly after being withdrawn, it becomes viscid and gelatinous and sets into a firm, jelly-like mass. This mass then separates into two portions: a firm red clot floating free in a transparent and a straw-coloured fluid called serum.

A clot consists almost entirely of red corpuscles entangled in a network of fine fibrils or threads, composed of a substance called fibrin. This substance does not exist as such in blood but is created by the action of thrombin which is an enzyme that promotes the conversion of fibrinogen, one of the plasma proteins, to fibrin in the clotting process. Thrombin is not present in circulating blood; it is formed from prothrombin, by a complex process involving blood platelets and certain calcium salts. If any of these factors is deficient, clot formation is defective. The addition of sodium citrate removes calcium ions from the blood and thus prevents a clot from forming. Lack of vitamin K makes impossible the maintenance of the proper amount of prothrombin in the blood. Certain diseases may lower the concentration of the various clotting proteins or of the platelets of the blood.

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