Blood Clotting Process

Blood clotting or coagulation is an important process that prevents excess loss of blood from our body when we suffer from an injury. Although it depends on the severity of the injury, the blood usually clots within 2 minutes of a cut and forms a plug at the site of injury that prevents blood from flowing out of the body. Just as important as the blood clotting process is, so is it complex too. Here is a simplified explanation of what is the blood clotting process.

Minimizing Blood Loss

There are two mechanisms by which loss of blood is minimized by the body. The two steps are:

Vascular Spasm
As soon as the blood vessel is broken, the smooth muscles of the vessel contract. This decreases the volume of blood passing through the blood vessel and hence, reduces the amount of blood lost due to the broken vessel.

Blood Clotting
This includes two steps that will be discussed in detail in the succeeding paragraphs. As of now, let us have a basic understanding of these steps:

• Primary Hemostasis: Also known as platelet activation, in this step the blood platelets get activated to form a plug at the site of injury.
• Secondary Hemostasis: There are blood clotting factors or coagulation factors in blood which are actually proteins. When there is an injury to a blood vessel, these factors interact with each other in a cascade of reactions that finally result in fibrinogen converting into fibrin strands. These strands strengthen the platelet plug completing the process of blood clotting.

Blood Clotting Process

Primary Hemostasis
When a blood vessel is injured, sub-endothelium proteins, notably the von Willebrand factor (vWF) is exposed. These factors recruit collagen (main protein of the connective tissue of animals) and other clotting factors. As a result, the platelets circulating in blood adhere to the collagen and set off a series of reactions which result in release of other substances that cause further adhesion of platelets with collagen. The bound platelets release more substances that enhance their clumping and aid the formation of plug of platelets.

Secondary Hemostasis
Secondary hemostasis involves a cascade of reactions, the aim of which is to form fibrin. The initial part of this step can be achieved either by:

• The Tissue Factor Pathway (or the extrinsic pathway)
• The Contact Activation Pathway (or the intrinsic pathway)

The tissue factor pathway is the primary pathway for beginning the blood clotting process. It is also faster than the contact activation pathway. Whichever the pathway, the aim is to produce prothrombin activator that would take the blood clotting process further. Both the pathways are composed of a series of reactions. Each reaction initiates another reaction. The result is a cascade of reactions that proceed with the help of a number of coagulation factors. These factors have specific names, although they are most commonly denoted using Roman numerals with a lowercase a, appended to them to indicate the inactive form.

Common Pathway
After the prothrombin activator has been formed by either the rapid extrinsic or the slower intrinsic pathway, the next step is the formation of thrombin. Prothrombin and fibrinogen are two specific plasma proteins that are synthesized in the liver. Both these proteins are present in the plasma in an inactive form. They need to be converted into their respective active forms to take the blood clotting mechanism further. It is the prothrombin activator that converts prothrombin into thrombin. The thrombin thus produced in turn converts fibrinogen into strands of fibrin. These strands form a mesh that provides structural support to the platelets plug formed at the site of injury. This completes the blood clotting process.

Blood Clotting Disorders

Of the various blood disorders, there are many that are caused due to improper blood clotting. If blood clots very easily or faster than the normal rate, then it leads to a condition called thrombosis. Conversely if it takes long for blood to clot, then one may suffer from haemorrhage. Hemophilia is another blood clotting disorder in which blood does not clot on its own without the help of medical intervention. Some common blood clot disorders are blood clot in lungs, deep vein thrombosis and blood clot in the brain. One needs to be aware of blood clot symptoms in order to recognize unwanted blood clots in the body.

This was an oversimplified version of the blood clotting process. There are a large number of co-factors, clotting factors and regulators that regulate the entire process, primarily the blood clotting cascade of reactions.