Stopping a bleed takes three overlapping stages, not one simple clot
Within moments of a vessel being cut, blood begins turning from liquid to gel. Platelets pile into a temporary plug while a cascade of clotting factors spins fibrin threads to reinforce it. Later the clot shrinks and is dissolved, so blood can flow freely once the damage has healed.
Coagulation is the body's way of halting blood loss, a result known as hemostasis. It starts almost instantly when the inner lining of a vessel, the endothelium, is breached. Blood meets tissue beneath that lining, which does two things at once: it alters platelets, and it brings a protein called tissue factor into contact with clotting factor VII, the first step toward a mesh of cross-linked fibrin. The injured vessel also narrows, as the lining releases constricting substances such as endothelin and thromboxane that squeeze the muscle in the vessel wall and reduce flow.
The platelet plug is called primary hemostasis. Exposed collagen, normally hidden, grabs passing platelets through receptors on their surface. Von Willebrand factor, released by both the vessel lining and platelets, adds extra tethers, and signals from collagen switch on platelet integrins so the cells lock tightly onto the wound. Activators such as thromboxane A2 recruit still more platelets.
Secondary hemostasis runs in parallel. A chain of enzymes, conventionally described using 13 traditional clotting factors, culminates when thrombin converts fibrinogen into fibrin. Factor XIIIa then cross-links those fibrin strands, turning a loose plug into a durable seal. The third stage, sometimes called tertiary hemostasis, follows: platelets tug on internal actin and myosin fibres to contract the clot, and tissue plasminogen activator generates plasmin, which breaks the fibrin down and reopens the vessel.
Clotting in some form is shared across biology, and all mammals rely on both platelets and protein clotting factors, though the human system is the most thoroughly studied. When any part of it misfires, the results run in opposite directions: excessive bleeding and bruising on one side, unwanted clots, or thrombosis, on the other.
Source: Coagulation
