1.11—Blood clotting and cardiovascular disease

Damage to a blood vessel releases thromboplastin. In the presence of calcium ions, it helps convert soluble prothrombin into thrombin; thrombin converts soluble fibrinogen into insoluble fibrin.

Thrombin activates more clotting reactions and fibrin fibres form a mesh across the wound. Platelets become trapped, the mesh contracts and a clot reduces blood loss while tissue repair begins.

A cut exposes tissue and activates the cascade locally. The resulting fibrin network stays near the damaged vessel rather than turning all circulating fibrinogen into a body-wide clot.

The cascade amplifies a small trigger, but inhibitors and the intact endothelium limit clotting to the injury. If a clot forms inside an artery it can obstruct blood flow and cause thrombosis.

Prothrombin and fibrinogen are soluble plasma proteins; thrombin and fibrin are the active enzyme/product in the sequence. Do not reverse these pairs.