VASCULAR ACTIVITY

Vascular activity consists of constriction of muscles
within the walls of the blood vessels in response to
vascular damage. This vasoconstriction narrows the
path through which the blood flows and may some-
times entirely halt blood flow. The vascular phase of
hemostasis affects only arterioles and their depend-
ent capillaries; large vessels cannot constrict suffi-
ciently to prevent blood loss. Even in small vessels,
vasoconstriction provides only a brief hemostasis.

PLATELET FUNCTION
Platelets serve two main functions: (1) to protect
intact blood vessels from endothelial damage
provoked by the countless microtraumas of day-to-
day existence and (2) to initiate repair through the
formation of platelet plugs when blood vessel walls
are damaged.
When overt trauma or microtrauma damages
blood vessels, platelets adhere to the altered surface.
Adherence requires the presence of ionized calcium
(coagulation factor IV), fibrinogen (coagulation
factor I), and a protein associated with coagulation
factor VIII, called von Willebrand’s factor (vWF).
The process of adherence involves reversible changes
in platelet shape and, usually, the release of adeno-
sine diphosphate (ADP), adenosine triphosphate
(ATP), calcium, and serotonin. With a strong
enough stimulus, the next phase of platelet activity,
platelet aggregation, occurs and results in the
formation of a loose plug in the damaged endothe-
lium. The platelet plug aids in controlling bleeding
until a blood clot has had time to form.