18.5 Hemostasis

Hemostasis is the body's mechanism for stopping bleeding when a blood vessel is damaged. It involves three overlapping steps: vascular spasm, platelet plug formation, and coagulation. Understanding hemostasis matters because it sits at the intersection of circulatory function and defense. Too little clotting leads to dangerous blood loss; too much leads to clots that can block vessels and cause strokes or heart attacks.

Hemostasis

Mechanisms of hemostasis

Hemostasis unfolds in three stages, each building on the one before it.

1. Vascular spasm happens immediately after injury. Smooth muscle in the damaged vessel wall contracts (vasoconstriction), narrowing the vessel and reducing blood flow to the area. This buys time for the next two steps.

2. Platelet plug formation (primary hemostasis) begins when platelets encounter collagen fibers exposed by the break in the vessel lining. The process works like this:

• Platelets adhere to the exposed collagen, anchored by von Willebrand factor (vWF).
• Adhered platelets activate, changing shape and releasing chemical signals (ADP, thromboxane $A_2$) that recruit more platelets.
• Additional platelets aggregate together, stacking onto the growing plug until it seals the damaged area.

This plug is effective for small injuries but isn't strong enough on its own for larger ones.

3. Coagulation (secondary hemostasis) reinforces the platelet plug with a mesh of fibrin protein strands. A series of clotting factors (numbered with Roman numerals) activate one another in a chain reaction called the coagulation cascade. The end result is a stable, fibrin-reinforced clot.

Extrinsic vs. intrinsic coagulation pathways

The coagulation cascade has two entry points that converge into one shared ending.

Extrinsic pathway (faster)

• Triggered when damaged tissue releases tissue factor (factor III) into the blood.
• Tissue factor binds factor VII, which activates factor X.
• Because it starts outside the blood itself (in the tissue), it's called "extrinsic."
• This is the primary initiator of clotting in most injuries.

Intrinsic pathway (slower)

• Triggered when blood contacts negatively charged surfaces like exposed collagen (or glass in a lab tube).
• Involves factors XII → XI → IX → VIII, activating one after another.
• Called "intrinsic" because all the necessary components are already present within the blood.
• Its main role is to amplify the clotting response started by the extrinsic pathway.

Common pathway (where they meet)

Both pathways converge at the activation of factor X. From there:

1. Factor X (with factor V as a cofactor) converts prothrombin (factor II) into thrombin.
2. Thrombin converts fibrinogen (factor I) into fibrin strands.
3. Fibrin strands weave through the platelet plug, forming a tough mesh that stabilizes the clot.
4. Factor XIII cross-links the fibrin strands, making the clot even stronger.

Regulation of hemostasis

Clotting can't go unchecked. The body has built-in mechanisms to keep clots localized and temporary.

Fibrinolysis is the process of dissolving clots once the vessel has healed. The enzyme plasmin breaks down fibrin strands, clearing the clot and restoring normal blood flow. Without fibrinolysis, old clots would accumulate and obstruct circulation.

Natural anticoagulants prevent clotting from spreading beyond the injury site:

• Antithrombin III inactivates thrombin and several other clotting factors. Its activity is greatly enhanced by heparin, a naturally occurring anticoagulant produced by mast cells and basophils.
• Protein C and Protein S work together to inactivate factors V and VIII, slowing the cascade.

Pharmaceutical anticoagulants are used clinically to manage clotting disorders:

• Heparin (given by injection) boosts antithrombin III activity for rapid anticoagulation.
• Warfarin (taken orally) interferes with the liver's use of vitamin K, which is required to synthesize factors II, VII, IX, and X. This makes it effective but slower-acting than heparin.

Disorders of hemostasis

These conditions illustrate what happens when the hemostatic balance tips too far in either direction.

Hemophilia is a genetic bleeding disorder with two main forms:

• Hemophilia A (most common): deficiency of factor VIII.
• Hemophilia B ("Christmas disease"): deficiency of factor IX.

Both impair the intrinsic pathway, resulting in prolonged clotting times and excessive bleeding, especially into joints and deep tissues. Even minor injuries can cause significant blood loss.

Von Willebrand disease is the most common inherited bleeding disorder. It involves a deficiency or dysfunction of von Willebrand factor (vWF), the protein that anchors platelets to exposed collagen. Without adequate vWF, platelet adhesion is impaired, so the platelet plug forms poorly. Symptoms tend to be milder than hemophilia and often involve mucosal bleeding (nosebleeds, heavy menstrual periods).

Thrombocytopenia is an abnormally low platelet count (below ~150,000 per $\mu L$). Causes include decreased production (bone marrow disorders), increased destruction (autoimmune conditions), or sequestration in an enlarged spleen. With fewer platelets available, plug formation is impaired and the risk of spontaneous bleeding rises.

Disseminated intravascular coagulation (DIC) is a life-threatening condition where the coagulation cascade activates throughout the entire body at once, usually triggered by severe infection (sepsis), major trauma, or certain cancers. Widespread tiny clots form in small vessels, which rapidly consumes clotting factors and platelets. The paradoxical result is both clotting (thrombosis in small vessels) and bleeding (because clotting factors are depleted). DIC is always secondary to another serious condition and requires treating the underlying cause.