Blood Physiology Flashcards
What is hemostasis?
This is a physiological process which prevents loss of blood following vascular injury.
What is primary hemostasis?
Primary hemostasis is the initial step in hemostasis. It involves:
(a) Platelet Clotting: Platelets in blood stick to damaged tissue and activate. They recruit more platelets to form a temporary “plug” that stops blood loss from the injury site.
(b) Constriction: The damaged blood vessel may also narrow due to substances released by activated platelets.
(c) End Result: The platelet plug acts like a cork, keeping blood in and preventing debris or germs from entering.
What is secondary hemostasis?
Secondary hemostasis, also known as the coagulation cascade, is a series of enzymatic reactions that lead to the conversion of fibrinogen into fibrin monomers.
Discuss the extrinsic pathway of secondary hemostasis.
(1) Initiation: The extrinsic pathway begins when there’s tissue damage (e.g., skin injury). Damaged extravascular cells release a protein called tissue factor/tissue thromboplastin (factor III).
(2) Factor VII Activation: Tissue factor interacts with factor VII/proconvertin, which is present in the blood. Factor VII requires vitamin K for activation.
(3) Factor X Activation: The tissue factor and factor VII complex activates factor X (Stuart-Prower factor).
(4) Common Pathway: Factor X, now activated, enters the common pathway, where it plays a crucial role in converting prothrombin to thrombin.
(5) Thrombin Formation: Thrombin then converts fibrinogen to fibrin, stabilizing the platelet plug and forming a stable blood clot.
[Diagram]
Discuss the intrinsic pathway of secondary hemostasis.
(1) Activation: The intrinsic pathway is called “intrinsic” because all the factors required to activate it are intrinsic, meaning they are found within the blood.
(2) Initiation: It begins when there’s damage to the vascular endothelium (e.g., injury to blood vessel walls). This exposes clotting factors to negatively charged subendothelial surfaces.
(3) Factor XII (Hageman Factor): Factor XII (also known as the Hageman factor) is activated by contact with collagen, kallikrein, and high-molecular-weight kininogen (HMWK).
(4) Cascade: Factor XIIa then activates factor XI into XIa. With a calcium ion, factor XIa further activates factor IX. Factor IXa activates factor VIII.
(5) Complex Formation: Factor IXa, along with factor VIIIa and calcium, forms a complex that activates factor X.
(6) Common Pathway: The intrinsic pathway converges with the extrinsic pathway, leading to the activation of factor X, which ultimately contributes to fibrin formation and clot stabilization.
[Diagram]
Discuss the common pathway of intrinsic hemostasis.
(1) Activation of Factor X: Once factor X has been activated by either the intrinsic or extrinsic pathway, it becomes factor Xa.
(2) Prothrombinase Formation: Factor Xa combines with factor V (factor Va) to form an enzyme complex called prothrombinase.
(3) Conversion of Prothrombin to Thrombin: Prothrombinase then converts factor II (prothrombin) into its active form, thrombin.
(4) Thrombin’s Role: Thrombin plays a central role in the clotting process. It converts fibrinogen (soluble) into fibrin (insoluble), which forms a meshwork to stabilize the platelet plug and seal off the vessel.
List the blood clotting factors whose synthesis depends on Vitamin K.
Factors II [Prothrombin], VII [proconvertin], IX [Christmas factor] and X [Hageman factor].
