Coagulation of Blood

Cutting or otherwise damaging a blood vessel must be dealt with quickly, since an animal with a closed, pressurised circulatory system can lose a dangerous volume of blood in a short time if the leak is not sealed. The body's response is blood coagulation, or clotting — not a single reaction but a cascade, in which one activated clotting factor activates the next in a fixed sequence, so that a small initial trigger is rapidly amplified into the formation of a solid seal across the wound.

The cascade begins at the site of injury itself. Platelets circulating in the blood encounter the damaged vessel wall, where the normally smooth internal lining has been breached to expose the underlying collagen fibres; on contact with this collagen, platelets become sticky, clump together to form a temporary platelet plug across the breach, and in the process disintegrate, releasing a phospholipid-rich substance called platelet thromboplastin. The injured tissue cells of the vessel wall itself release a similar substance, tissue thromboplastin, at the same time. Together with calcium ions and several other clotting factors that circulate continuously in the plasma in an inactive form (there are roughly a dozen or so recognised clotting factors in all), this thromboplastin converts prothrombin — an inactive plasma protein synthesised continuously by the liver, a synthesis that itself depends on an adequate supply of vitamin K — into its active enzyme form, thrombin. …