Object:
To observe the coagulation of blood.
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Requirements:
Test-tube and cotton wool.
Principle and procedure:
Bleeding takes place if the skin is cut, but after a time blood coagulates to close the cut, thus preventing much loss of blood.
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Take blood of a mammal in a large test- tube and close the tube with cotton wool. It will be seen that blood forms a soft jelly and a faint yellow liquid appears.
In a few hours the liquid or serum increases in amount and the solid jelly or clot becomes smaller.
The serum consists of blood plasma without its fibrin and almost no blood corpuscles. The clot is scarlet on the surface and bluish-black within.
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The clot is made of fibrin, an insoluble protein derived from the soluble fibrinogen of the blood and-entangled in the fibrin are almost all the blood corpuscles. The fibrin is a mass of fine interlacing fibres.
The blood contains an inactive enzyme prothrombin which is kept in check by heparin (antiprothrombin) found in blood.
Hence the blood flows freely in blood vessels and does not coagulate. If an injury occurs, blood begins to escape from it, but the injured tissue cells and blood platelets on contact with air release a substance called thromboplastin.
Thromboplastin soon neutralizes heparin, and it combines with calcium ions in the blood to act on prothrombin changing it into thrombin.
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Thrombin is an active enzyme it acts on the fibrinogen of blood converting it into fibrin. The fibrin forms a network of fibres which entangle the corpuscles to form a clot at the site of the injury.