Normal Blood Clotting

The clotting of blood when bleeding occurs involves 3 different inter-related mechanisms:

• Blood vessel narrowing at the site of bleeding and diversion of blood away from the site of the bleed.
• Platelets circulating in the blood then "plug" the hole in the blood vessel.
• The platelet plug is then strengthened by deposition of the blood clotting factors on to the platelets.

Blood Vessels

Arteries take the blood from the left side of the heart and distribute it to the body organs. Veins then take this blood back from the organs to the right side of the heart, where it is then pumped to the lungs to be re-oxygenated. It then returns to the left side of the heart to be re-circulated.

Arteries and veins consist of three layers, the endothelium which lines the vessels, a middle muscle / elastic layer and an outer collagen layer called the adventitia. Arteries have a much thicker middle layer as they have to cope with higher blood pressures but this also makes them better equipped to constrict and reduce the flow of blood to a damaged area. Some arteries are closed but these open up if bleeding occurs to divert blood away from the site of damage.

Platelets

Platelets are produced in the bone marrow from giant cells called megakaryocytes. When a blood vessel is damaged, platelets which were circulating in the blood stick to the collagen of the adventitia and surrounding tissues to "plug" the hole - a process called adhesion. With the help of the blood clotting factors - especially fibrinogen and von Willebrands factor - the platelets stick to each other, a process called aggregation.

Blood Clotting Factors

The liver manufactures special blood clotting proteins - called clotting factors - which further helps the blood clot. These circulate within the blood in an in-active form but become activated in a cascading way at the site of bleeding (see below). The final protein produced is fibrin which acts as glue, sticking to platelets and ultimately strengthening the whole blood clot. Several of the clotting factors are derived from vitamin K. Hereditary deficiency of these clotting factors results in excessive bleeding e.g. Factor viii deficiency cause haemophilia A, Factor ix deficiency haemophilia B.

Later when the damage to the blood vessel has been repaired the blood clot is removed by other special blood proteins of which one - tissue plasminogen activator (tpa) - is used therapeutically in life threatening thrombosis.

Clotting Cascade