1 Homeostasis & Clot Formation

Hemostasis

Stoppage of blood flow

Goals of hemostasis regulation

1) Maintain blood fluidity, 2) Prevent blood from leaving the vascular compartment

Disorders of hemostasis

1) Thrombosis (inappropriate clotting), 2) Bleeding (failure to clot)

Vascular constriction

Triggered by vessel injury; reduces blood flow

Formation of the platelet plug

Platelets attracted to damaged vessels; involves adhesion and aggregation

Platelet adhesion

Mediated by Von Willebrand Factor

Platelet aggregation

Mediated by adenosine diphosphate (ADP)

Platelet lifespan

8 to 12 days

Blood coagulation

Conversion of plasma protein fibrinogen into fibrin through the coagulation cascade

Coagulation factors

Synthesized in the liver

Requirements for coagulation

Calcium and Vitamin K

Why does a clot form?

When collagen is exposed at the site of injury, damaged cells initiate the coagulation cascade

Intrinsic pathway

Activated in response to injury; slow process (1 to 6 minutes)

Extrinsic pathway

Activated when blood leaks out of a vessel and enters tissue spaces; faster process (15 seconds)

Two pathways of coagulation

Intrinsic pathway and extrinsic pathway; both lead to the same end stage of normal hemostasis

Dissolution of clots

Fibrinolysis or clot removal; starts within 24-48 hours of clot formation and continues until resolved

Purpose of fibrinolysis

Restores circulation so tissue can resume normal function after clot restricts blood flow

Fibrinolysis initiation

Release of tissue plasminogen activator (TPA)

Tissue plasminogen activator (TPA)

Converts plasminogen to plasmin

Plasmin

Digests fibrin strands to restore circulation

Regulation of fibrinolysis

Removes unwanted clots while leaving fibrin in wounds