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| Components of Closed Circulatory System in Man - Contd. |
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| Platelets |
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| They are round biconvex cells that do not have a distinct nucleus. They are also called the thrombocytes. They number 250,000 to 400,000 per cubic mm of blood. Their life span is 8 to 14 days. They are involved in blood clotting. They are destroyed in the spleen. |
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| Role in Blood Clotting |
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| The damaged platelets release a substance called thromboplastin. This sets in action, a series of reactions that result in the formation of clots. These clots block the site of the injury, avoiding excess loss of blood. |
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| Role in Repair of Damaged Endothelium |
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| Endothelium is the inner wall of the blood vessels. The platelets stick to the damaged portion of the endothelium and prevent loss of blood. The platelets are not allowed to stick to healthy endothelium by the secretion of a substance called prostacyclin. |
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| Thrombosis is a condition during which a clot is formed in the narrowed portion of the blood vessels. The narrowing occurs due to the deposition of substances like the cholesterol. This also prevents secretion of prostacyclin which allows the platelets to stick to the walls of the vessels. This results in the formation of clots that can cause blockage of the blood vessels. These blockages in the important vessels can also lead to death. |
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| Clot Retraction |
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| The clot formed is made denser and smaller by the action of platelets. |
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| Transport of Nutrients |
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| The food is digested into the simplest of the forms in the digestive system. Blood carries the digested food or the nutrients absorbed from the intestine to the liver and then to other parts of the body. The food transported includes glucose, fatty acids, amino acids, glycerol, etc. |
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| Transport of Respiratory Gases |
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| Oxygen is transported from the lungs to the cells and carbon dioxide is transported from the cells to the lungs. The transport of these gases is chiefly due to the pigment haemoglobin. |
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| Functions of the Circulatory System |
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| Transport of Excretory Wastes |
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| The cells are constantly undergoing metabolic reactions that produce numerous wastes. Some of these wastes are toxic and can be harmful. The wastes have to be constantly removed to the regions where they are either excreted or rendered harmless. Blood transports the wastes to liver, kidney, intestines and skin. |
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| Transport of Hormones |
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| The hormones are synthesised far from their site of action. Thus, blood is necessary for transporting the hormones. |
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| Role in Immune System
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| The leucocytes are phagocytic and engulf the bacteria and other micro-organisms that attack the body cells. The lymphocytes secrete antibodies, that are special protein molecules that act against specific proteins present on the surface of the germs. These proteins are called the antigens. |
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| Maintenance of pH |
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| The proteins act as buffers in regulating the acid and base concentration in blood. |
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| Maintenance of Ionic Balance |
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| Maintenance of Water Content
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| The blood circulates materials between itself, the tissue fluid and the cells. The tissue fluid should have the correct water content. If the fluid has less water, it will dehydrate the cells by drawing water out. If the fluid has more water, the cells are swollen with water as the excess water enters the cell. |
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| The water level in the blood changes the volume of the blood which affects the blood pressure. |
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| Role in Temperature Regulation |
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| Heat is distributed evenly throughout the body as the blood circulates. The heat from the deeper warmer tissues are carried to the surface cooler parts. |
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| Role in Homoeostasis |
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| Homeostasis means maintaining same state. The above aspects pH, ionic concentration, water, blood pressure, temperature, etc. have to be maintained at optimum levels in order to maintain homeostasis. |
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| Role in Clotting |
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| The blood contains platelets that secrete substances called the thromboplastin on being damaged. This substance initiates reactions that result in the formation of blood clots. The blood clots prevent excessive flow of blood. |
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| Excessive loss of blood is prevented from the cut blood vessels by the formation of clots. This process is also called coagulation. The steps involved in clotting of blood are: |
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| Damaged platelets secrete thromboplastin or thrombokinase. |
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| Thrombokinase is an enzyme that changes prothrombin (present in the blood) to thrombin in the presence of calcium ions. |
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| Thrombin combines with soluble fibrinogen present in the blood and forms insoluble fibrin. |
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| Fibrin forms thread-like structures that form a sticky mass. This forms a network of a sticky substance at the damaged portion of the blood vessel. |
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| This network does not allow the corpuscles to pass through. Only the plasma is allowed to pass through and this plasma which lacks the fibrinogen is called the serum. |
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| The mass that is formed at the cut becomes denser and denser and is called the clot. |
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