Lesson 4 of 12 · 6 min

Coagulation of blood

NCERT §15.1.4

Meher grazed her knee on the track as she crossed the line. By the evening a dark reddish-brown crust has covered it. What stopped the bleeding?

The lesson in notes

In short

Blood clots in response to injury or trauma; this stops excessive loss of blood.

The dark reddish-brown scum over a cut is a clot (coagulum): a mesh of threads called fibrin in which dead and damaged formed elements are trapped.

Fibrin is made from inactive fibrinogen in plasma by the enzyme thrombin.

Thrombin itself is made from another inactive plasma substance, prothrombin. This step needs an enzyme complex, thrombokinase.

Thrombokinase forms through a cascade: a chain of linked enzyme reactions involving several factors present in plasma in an inactive state.

Injury stimulates platelets to release factors that set the cascade going; factors released by damaged tissue at the site can also start it.

Calcium ions play a very important role in clotting.

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