Blood Functions. Hemostasis
The blood is contained under pressure in a vascular
system that includes vast areas of thin and delicate
capillary membranes. Even the bumps and knocks of everyday
life are sufficient to disrupt some of these fragile
vessels, and serious injury can be much more damaging.
Loss of blood would be a constant threat to survival
if it were not for protective mechanisms to prevent
and to control bleeding. The platelets contribute to
the resistance of capillaries, possibly because they
actually fill chinks in vessel walls. In the absence
of platelets capillaries become more fragile, permitting
spontaneous loss of blood and increasing the tendency
to form bruises after minor injury. Platelets immediately
aggregate at the site of injury of a blood vessel, tending
to seal the aperture. A blood clot, forming in the vessel
around the clump of adherent platelets, further occludes
the bleeding point. The coagulation mechanism involves
a series of chemical reactions in which specific proteins
and other constituents of the blood, including the platelets,
play a part. Plasma also is provided with a mechanism
for dissolving clots after they have been formed. Plasmin
is a proteolytic enzyme—a substance that causes breakdown
of proteins—derived from an inert plasma precursor known
as plasminogen. When clots are formed within blood vessels,
activation of plasminogen to plasmin may lead to their
removal. (For additional information about the mechanics
and significance of hemostasis, see bleeding and blood
clotting.
