Physiology Lecture 7: Blood Groups and Hemostasis

Hemostasis and Blood Coagulation

• Hemostasis is the process of preventing and stopping bleeding, keeping blood within a damaged blood vessel.
• It is the first stage of wound healing, involving coagulation, where blood changes from a liquid to a gel.
• Hemostasis has three major steps: vasoconstriction, temporary blockage by a platelet plug, and blood coagulation (formation of a fibrin clot).

Mechanism of Hemostasis

• Hemostasis occurs when blood is present outside of the body or blood vessels.
• The three steps of hemostasis occur in rapid sequence: vascular spasm, platelet plug formation, and coagulation.
• Vascular spasm is the first response, constricting blood vessels to reduce blood flow and limit blood loss.
• Platelet plug formation involves platelets sticking together to form a temporary seal, which is reinforced by fibrin threads acting as a "molecular glue".

Platelet Plug Formation

• Formation of the platelet plug is regulated through thromboregulation.
• Platelets adhere to damaged endothelium, degranulate, and express glycoprotein receptors that interact with other platelets, producing aggregation and adhesion.
• Platelets alone are responsible for stopping bleeding from unnoticed wear and tear of the skin on a daily basis.

Clot Formation

• Once the platelet plug is formed, clotting factors are activated in a sequence of events known as the coagulation cascade, leading to the formation of fibrin from inactive fibrinogen plasma protein.
• A fibrin mesh is produced around the platelet plug to hold it in place, known as secondary hemostasis.

Mechanism of Blood Coagulation

• Blood coagulation occurs in three steps:
  • Formation of prothrombin activator in response to vessel rupture or blood damage.
  • Conversion of prothrombin to thrombin catalyzed by prothrombin activator.
  • Thrombin acting as an enzyme to convert fibrinogen into fibrin fibers, forming a clot.

Initiation of Coagulation

• Coagulation is initiated by trauma to the vascular wall and adjacent tissues, trauma to the blood, or contact of the blood with damaged endothelial cells or collagen and other tissue elements outside the blood vessel.
• These mechanisms lead to the formation of prothrombin activator, which then causes prothrombin conversion to thrombin and subsequent clotting steps.

Fibrinolysis

• Fibrinolysis is a process that prevents blood clots from growing and becoming problematic.
• Primary fibrinolysis is a normal body process, while secondary fibrinolysis is the breakdown of clots due to a medicine, medical disorder, or other cause.
• In fibrinolysis, a fibrin clot is broken down by the enzyme plasmin, leading to the production of circulating fragments that are cleared by other proteases or by the kidney and liver.