Blood congeals when a part of the body is wounded. If it failed to congeal the injured person would die from loss of blood.
The congealing, or coagulation of the blood, is the first step towards healing a wound. It closes the wound and builds a scaffold for new tissue by means of a chemical process in the plasma, the fluid part of the blood. In this process the platelets (small cellular bodies in the blood) produce thromboplastin. This changes fibronigin, a protein in the blood, into fibrin. Finally a spongy network of fibrin connects the edge of the wound and prevents the loss of any more blood cells. Often, a scab is formed over the wound as a protection.
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