Chapter 43: Blood Plasma Proteins, Coagulation, and Fibrinolysis

Plasma proteins serve multiple critical functions including transport of lipids, hormones, and medications, regulation of osmotic pressure between blood and interstitial fluid, and immune defense. Albumin, the most abundant plasma protein, maintains colloid osmotic pressure and binds various hydrophobic molecules including fatty acids, bilirubin, and drugs, thereby affecting drug distribution and metabolism. Immunoglobulins provide adaptive immunity while the complement cascade eliminates immune complexes and pathogens. Protease inhibitors like alpha-1-antitrypsin protect tissues from enzymatic degradation; deficiency of this protein leads to early-onset emphysema and progressive lung destruction. The chapter then explores hemostasis, the mechanism that arrests bleeding after vascular injury through coordinated platelet and coagulation responses. Platelets derived from megakaryocytes adhere to exposed collagen via von Willebrand factor, become activated, and aggregate to form the primary platelet plug. Granule release amplifies platelet activation through autocrine and paracrine mechanisms. Genetic or acquired defects in platelet function cause bleeding diatheses including Bernard-Soulier syndrome, immune thrombocytopenia, and von Willebrand disease. The coagulation cascade generates thrombin through extrinsic and intrinsic pathways that converge on common pathway activation. Vitamin K-dependent gamma-carboxylation enables calcium-mediated binding of coagulation factors to membrane surfaces, a process blocked by warfarin therapy. Heparin enhances serpin-mediated inhibition of thrombin and activated factors, while natural anticoagulants including proteins C and S, antithrombin III, and thrombomodulin prevent pathologic thrombosis. Mutations such as factor V Leiden impair regulatory mechanisms and increase thromboembolism risk. Fibrinolysis balances coagulation through plasmin-mediated fibrin degradation initiated by tissue or urinary plasminogen activators, with alpha-2-antiplasmin regulating plasmin activity. Therapeutic fibrinolytic agents reverse acute thrombotic events in coronary and cerebrovascular disease. Clinical disorders including hemophilia A, hemophilia B, heparin-induced thrombocytopenia, and thrombophilic states illustrate the pathophysiology underlying bleeding and clotting disorders and demonstrate how therapeutic interventions target specific molecular mechanisms to restore hemostatic balance.