Question

Explain the steps of hemostasis and the coagulation cascade

Answer 1

The term hemostasis means prevention of blood loss, it is happening in four steps

(1) Vascular constriction.

(2) Formation of a platelet plug.

(3) Formation of a blood clot as a result of blood coagulation.

(4) Growth of fibrous tissue into the blood clot.

VASCULAR CONSTRICTION

When a blood vessel has been cut or ruptured, it causes smooth muscle in the wall to contract; this will reduce the flow of blood from the ruptured vessel.

FORMATION OF THE PLATELET PLUG

If the cut in the blood vessel is very small it is often sealed by a platelet plug rather than by a blood clot. When platelets come with collagen fibers in the vascular wall, they begin to swell, they form numerous pseudopods protruding from their surfaces, their contractile proteins contract forcefully and cause the release of granules that contain multiple active factors, they become sticky so that they adhere to collagen in the tissues and to a protein called von Willebrand factor that leaks into the traumatized tissue from the plasma, they secrete large quantities of ADP, and their enzymes form thromboxane A2. The ADP and thromboxane in turn act on nearby platelets to activate them as well, and the stickiness of these additional platelets causes them to adhere to the original activated platelets. Thus the damaged vascular wall activates increasing numbers of platelets that attract more and more additional platelets, thus forming a platelet plug.

BLOOD COAGULATION IN THE RUPTURED VESSEL

The clot begins to develop in 15 to 20 seconds. Clotting takes place in three steps:

1. In response to rupture of the vessel a complex cascade of chemical reactions occurs in the blood involving more than a dozen blood coagulation factors resulting in the formation of prothrombin activator.

2. The prothrombin activator catalyzes conversion of prothrombin into thrombin with the help of ionic calcium.

3. The thrombin acts as an enzyme to convert fibrinogen into fibrin fibers that help to form the clot.

FIBROUS ORGANIZATION OR DISSOLUTION OF THE BLOOD CLOT

Once a blood clot has formed, (1) It can become invaded by fibroblasts, which subsequently form connective tissue all through the clot, or (2) it can dissolve.