Blood Clotting

Robin Gray

Blood Clotting AKA Coagulation

• Cardiovascular System• Not necessarily a “balancing” effect• It prevents massive blood loss

• Hemostatis – the cessation of blood loss from a damaged vessel

Organs and Cells Involved

• Veins and arteries • Vessel sides• Red blood cells• Platelets• Fibrinogen fibrin fibers (active form)

Causes of Imbalance

• Upward– Cuts– Bruises– Any damage to the endothelium of a vessel and

connective tissue is exposed to the blood

• Downward– When the connective tissue is no longer touching the

blood, the process stops– Local fibrinogen stops converting to fibrin because

thrombin is no longer produced from clotting factors

Maintaining Homeostasis

• Vascular constriction occurs first to restrict blood to the site

• Platelets, activated by thrombin, form a temporary platelet plug

• A fibrin mesh (the clot) forms and entraps the plug– If it’s only platelets white thrombus– With red blood cells red thrombus

• The clot is dissolved through plasmin

Maintaining Homeostasis• Upon vessel injury, platelets adhere to

macromolecules in the subendothelial tissues and then aggregate to form the primary hemostatic plug.

• The platelets stimulate local activation of plasma coagulation factors, leading to generation of a fibrin clot that reinforces the platelet aggregate.

• Later, as wound healing occurs, the platelet aggregate and fibrin clot are broken down.

• Mechanisms that restrict formation of platelet aggregates and fibrin clots to sites of injury are necessary to maintain the fluidity of the blood.

Maintaining Homeostasis

• Fibrin clot formation in response to tissue injury is the most clinically relevant event of hemostasis under normal physiological conditions. This process is the result of the activation of the extrinsic pathway.

Kaolin

Phospholipids (PL)

Calcium

Thromboplastin

(Tissue Factor + PL)

Calcium

CLOT

Intrinsic Pathway

aPTT prolonged

Extrinsic Pathway

PT prolonged

Common Pathway

aPTT and PT prolonged

• The formation of a red thrombus or a clot in response to an abnormal vessel wall in the absence of tissue injury is the result of the intrinsic pathway. The intrinsic pathway has low significance.

Negative Feedback Mechanism

• In order for coagulation to start connective tissue must be exposed to the blood

• Platelets adhere to collagen fibers in the connective tissue and release a substance that makes nearby platelets sticky

• Once connective tissue is no longer exposed to the blood the process ends

Positive Feedback Mechanism

• Blood clotting is more true to a positive feedback mechanism in general

• When a blood vessel is injured, platelets start accumulating at the site

• Positive feedback occurs as chemicals released by the platelets attract more platelets and the platelet cluster initiates a sequence of chemical reactions that seals the wound with a clot

Without Homeostasis…

• An inherited defect in any step of the clotting process causes hemophilia, a disease characterized by excessive bleeding from even minor cuts and bruises

• Without coagulation, even a minor cut would cause an individual to bleed to death

Summary

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