Coagulation Definition (Chemistry and Biology)

Blood starts to coagulate when it is exposed to air.
Blood starts to coagulate when it is exposed to air. BirgitKorber / Getty Images

Coagulation is a gelling or clumping of particles, typically in a colloid. The term typically applies to the thickening of a liquid or sol, usually when protein molecules cross-link.

When coagulation or clotting occurs in blood, it proceeds immediately after blood vessel damage. Two processes occur. Platelets change and the subendothelian tissue factor is exposed to plasma Factor VII, which ultimately forms fibrin. Primary hemostasis occurs when platelets plug the injury. Secondary hemostasis occrs as clotting factors strengthen the platelet plug with fibrin factors.

Also Known As: coagulate, coagulating, clotting

Examples of Coagulation

Milk proteins coagulate to thicken the mixture that forms yogurt. Blood platelets coagulate blood to seal a wound. Pectin gels (coagulates) a jam. Gravy coagulates as it cools.


  • David Lillicrap; Nigel Key; Michael Makris; Denise O'Shaughnessy (2009). Practical Hemostasis and Thrombosis. Wiley-Blackwell. pp. 1–5. ISBN 1-4051-8460-4.
  • Pallister CJ, Watson MS (2010). Haematology. Scion Publishing. pp. 336–347. ISBN 1-904842-39-9.