How Does Blood Clot?


Blood clotting, also called blood coagulation, occurs in reaction to a wound to halt the flow of blood. This happens so that blood loss will stop and also so that the wound will be closed and no bacteria or other harmful organisms can enter the body and start an infection.


The process of blood clotting begins when platelets encounter a wound and send a message to the blood vessels to constrict and slow the flow of blood in the area, a process called vascular constriction. More platelets in the blood then travel to the site of the wound and gather there to plug the hole temporarily until another compound called fibrin is created by the enzyme thrombin to form into threads that block the wound more effectively.

Platelet Changes and Activation

The changes that take place in blood platelets are necessary for clotting to take place. Normally, platelets are round cells that travel through the circulatory system, looking for damaged areas. When they encounter an injury, they alter their shape to become spiny and sticky. They also send out chemical messages to signal more platelets to come to that site and activate clotting factors to continue the clotting process. One of these factors is prothrombin, which when activated signals thrombin to make fibrin. Platelets also provide a support structure for the rest of the clotting activity to take place.


When an injury has become plugged with a clot, other mechanisms fall into place to ensure that clotting does not go out of control and fill the entire blood vessel. This is partially done via a feedback loop that stops the action of thrombin when the platelets have filled the hole and stop producing activated prothrombin. Another method of control is achieved via thrombin inhibitors, substances that increase in number in the blood as clotting is achieved. These substances also halt the action of thrombin.

Clotting Factors

Blood factors, or clotting factors, are substances in the blood that normally circulate in an inactive form. When activated by chemicals produced by platelets, these clotting factors carry out the necessary actions to complete the production of a blood clot. These are generally referred to by Roman numerals, such as Factor II, Factor VIII and Factor X. Each of these factors is activated in turn, and each sets off the activity of the next one needed in the clotting cascade.


Blood-clotting disorders can occur as a result of blood clotting too easily or blood not clotting when it should, according to Ivy Rose Holistic. Blood that does not clot properly may be the result of a genetic defect such as hemophilia, in which the clotting factors do not form properly, causing the clotting cascade to become interrupted midway through. Blood that clots too easily can result from overactive clotting factors, an excess of platelets or diseases that cause injuries to the arteries and lead to internal clots.

About this Author

Bridget Coila has been writing professionally since 1998 and specializes in health, nutrition, pregnancy and parenting topics. Some of her articles have appeared in “Oxygen,” “American Fitness” and on various websites. Coila has a Bachelor of Science in cell and molecular biology from the University of Cincinnati and over 10 years of medical research experience.