Process of Blood Clotting

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 blood clotting actually is.

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. The restricted flow induces inflammation and increased pressure. This causes the nearby immune cells, and clotting factors to migrate to the wound via the help of osmotic pressure.

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 plug formation, in this step the blood platelets get activated to form a plug at the site of injury.

• Secondary Hemostasis: There are clotting 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 the conversion of fibrinogen into fibrin strands.

These strands strengthen the platelet plug completing the process of blood clotting.

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. The bound platelets release more substances that enhance their clumping and aid the formation of a platelet plug.

Secondary Hemostasis

It 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 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 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 active 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, also converts fibrinogen into strands of fibrin. These strands form a mesh that provides structural support to the platelet plug formed at the site of injury. This completes the process.

Conversely if it takes too long for blood to clot, then one may suffer from hemorrhage. Hemophilia is another blood clotting disorder in which blood does not clot without the help of medical intervention.

Some common blood clot disorders are pulmonary embolism, deep vein thrombosis and blood clot in the brain. One needs to be aware of the symptoms in order to recognize unwanted and excessive 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. The actual process is far more detailed, and each step is equally crucial for the formation of a blood clot.