Physiology of Hemostasis

The mechanisms by which blood loss in mammals is stopped after vascular disruption are complex. Small vascular injuries are sealed by platelets that adhere to the site of damage, where they attract other circulating platelets, so as to form an occlu­sive aggregate or plug that can close small gaps.

The basic structure of both the occlusive clots that halt blood loss and pathological intravascular clots (or thrombi) is a meshwork of fibrous protein (fibrin) that entraps blood cells. Plato and Aristotle both described the fibers found in shed blood. When the blood vessel wall is disrupted, whether by trauma or disease, a soluble plasma protein, fibrinogen (factor I), is transformed into the insoluble strand of fibrin. Fibrin formation takes place through three steps: First, a plasma proteolytic enzyme, thrombin, cleaves several small peptides from each molecule of fibrinogen. Molecules of the residue, called fibrin monomers, polymerize to form the fibrin strands. Finally, these strands are bonded covalently by a plasma transamidase, fibrin-stabilizing factor (fac­tor XIlI), itself activated by thrombin, increasing their tensile strength.

Thrombin is not found in normal circulating plasma, but evolves after vascular injury from its plasma precursor, prothrombin (factor II), via either or both of two interlocking series of enzymatic events, the extrinsic and intrinsic pathways of thrombin formation. The steps of the extrinsic path­way begin when blood comes into contact with in­jured tissues (such as the disrupted vascular wall).

The intrinsic pathway of thrombin formation is launched when vascular disruption brings plasma into contact with certain negatively charged sub­stances, such as subendothelial structures or the oily sebum layer of skin. Exposure to negative charges changes a plasma protein, Hageman factor (factor XIΓ), to an enzymatic form, activated Hageman fac­tor (factor XIIa), that participates in both clotting and inflammatory reactions. In the latter role, acti­vated Hageman factor converts a plasma proen­zyme, prekallikrein, to kallikrein, an enzyme that releases small peptides from a plasma protein, high molecular weight kininogen. These peptides, notably bradykinin, increase vascular permeability, dilate small blood vessels, and induce pain.

The role of activated Hageman factor in the intrin­sic pathway is to initiate a series of proteolytic reac­tions that lead ultimately to the release of thrombin from prothrombin. These reactions involve the se­quential participation of several plasma proteins, including plasma thromboplastin antecedent (PTA, factor XI), high molecular weight kininogen, plasma prekallikrein, Christmasfactor (factor IX), antihemo­philic factor (factor VIII), Stuart factor (factor X), and proaccelerin (factor V). Of these various pro­teins, PTA, plasma prekallikrein, Christmas factor, and Stuart factor are the precursors of proteolytic enzymes, whereas high molecular weight kininogen, antihemophilic factor, and proaccelerin serve as nonenzymatic cofactors. The ultimate product, acti­vated Stuart factor (factor Xa), releases thrombin from prothrombin through the same steps as those of the extrinsic pathway.

Certain steps of both the extrinsic and intrinsic pathways require the presence of calcium ions and phospholipids, the latter furnished, in the extrinsic pathway, by tissue thromboplastin and, in the intrin­sic pathway, by platelets and by plasma itself.

Antihemophilic factor (factor VIII) is of peculiar interest as it circulates in plasma loosely bound to another plasma protein, υon Willebrand factor (vWf), which fosters hemostasis by promoting adhe­sion of platelets to injured vascular walls. The plasma proteins participating in coagulation are syn­thesized at least in part by the liver, except for von Willebrand factor, which is synthesized in vascular endothelial cells and megakaryocytes.

Synthesis of certain of the plasma clotting factors - namely prothrombin, factor VII, Stuart fac­tor (factor X), and Christmas factor (factor IX) - is completed only in the presence of vitamin K, fur­nished by leafy vegetables and by bacterial flora in the gut.

The clotting process is modulated by inhibitory proteins present in normal plasma. Plasmin, a proteolytic enzyme that can be generated from its plasma precursor, plasminogen, can digest fibrin clots as well as certain other plasma proteins. Antithrombin III inhibits all of the plasma proteases of the clotting mechanism, an action enhanced by heparin, a glycosaminoglycan found in various tis­sues but not in plasma. Heparin cofactor II, a protein distinct from antithrombin III, also inhibits clotting in the presence of heparin. Cl esterase inhibitor (Cl- INH) originally detected as an inhibitor of the acti­vated form of the first component of the immune complement system (Cl), also blocks the activated forms OfHageman factor and PTA as well as plasmin. Alpha-I -antiproteinase (alpha-1 -antitrypsin) is an in­hibitor of activated PTA. Protein C, when activated by thrombin, blocks the coagulant properties of antihemophilic factor (factor VIII) and proaccelerin (factor V), an action enhanced by protein S; proteins C and S both require vitamin K for their synthesis. Activated protein C also enhances the conversion of plasminogen to plasmin. Alpha-2-macroglobulin is an inhibitor of plasma kallikrein, and plasmin can be inhibited by several plasma proteins, notably by alpha-2-plasmin inhibitor.

Human disorders due to the functional deficiency of each of the factors needed for the formation of a clot have been recognized and extensively studied. In some instances, the patient’s plasma appears to be deficient or totally lacking in a specific clotting factor or inhibitor. In others, plasma contains a non­functional variant of the normal plasma protein.