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How Does the Blood Clot?

Blood Vessel Walls, Hemophilia, Thrombosis, Liver & Platelets

Aug 25, 2009 Noreen Kassem

The body's blood clotting system is a complex cascading mechanism that is linked to and affected by other body systems and organs including the liver.

When skin or tissue ruptures due to a cut, scrape or other injury, the body forms a blood clot within a few minutes to fill the wound and stop the bleeding - if the rupture is not too large. This process is caused by polymerization (or cutting and extension) of plasma fibrinogen molecules that are found in the blood but formed in the liver, into long fibrin threads. The fibrin threads then form a net at the injury site that entraps large numbers of red blood cells, white blood cells and platelets (solid fragments of cells called megakaryocytes) and plasma to form a soft gelatinous mass - a blood clot. The fibrin threads gradually contract and strengthen, expelling most of the plasma from the clot leaving a protective barrier in the opening of the vessel.

How is a Blood Clot Formed?

Prothrombin is a plasma protein also formed by the liver that can be split into two smaller molecules, one of which is thrombin. Thrombin is an enzyme that causes the polymerization of the plasma fibrinogen molecules into the fibrin threads that lead to blood clotting. In normal circulation very little prothrombin is converted into thrombin and blood clotting does not occur. Two conditions that lead to clotting are:

Damage to the blood vessel: when an injury occurs, damage to the walls of blood vessels in the skin or tissue releases a substance called tissue thromboplastin, almost like an SOS signal. This substance is composed mainly of phospholipids from the damaged tissues. The thromboplastin then triggers a complicated series of enzymatic reactions among multiple blood plasma proteins called blood coagulation factors. These reactions eventually form prothrombin activator (also called prothrombinase), which causes the prothrombin to change into the enzyme thrombin. Thrombin initiates coagulation by converting the soluble fibrinogen into insoluble fibrin threads that will bind the clot.

Damage to blood itself causes direct activation of special protein blood coagulation factors. Damage to platelets of the blood releases platelet thromboplastin, whch has similar effects to those of tissue thromboplastin released by torn blood vessels. This is why blood outside the body will become thick and coagulate.

Blood Clotting Disorders

Thrombosis

Like all complex body systems, blood clotting can also cause disorders. If blood clots too easily or quickly thrombosis may occur. This is when blood clots form on unbroken blood vessels or on atherosclerotic (hardened) plaques in the arteries and can cause heart attacks or strokes. Blood clots forming in the veins of the legs may cause a painful condition called phlebitis which can break away and travel to the lungs to cause a dangerous condition called pulmonary embolus.

Hemophilia

Hemophilia occurs exclusively in males, usually because of a deficiency in Factor VIII, or antihemophilic factor, which is one of the factors required to form fibrin from fibrinogen in the blood clotting mechanism.

Liver Disease and Vitamin K Deficiency

Most of the blood clotting proteins are formed in the liver. Therefore hepatitis, cirrhosis and other diseases of the liver can reduce normal blood clotting, causing a person to bleed excessively. Another cause of decreased coagulation factor production by the liver is vitamin K deficiency. Vitamin K is necessary for formation of prothrombin, Factor VIII, Factor IX, and Factor X.

Platelets Can Also Form Blood Clots

The platelets are tiny, solid cellular fragments, made in the bone marrow, that travel in the bloodstream. When injury or holes occur they are the first line of defense to stop the bleeding. Like blood vessel walls, platelets have the capabilities to induce clotting by secreting thromboplatin. However they can also plug up holes and tears via chemical reactions which make the surface of the platelet sticky. These activated platelets begin adhering to the wall of the blood vessel at the site of bleeding, and within a few minutes they form a white clot. Platelets can then plug up small holes in the blood vessels and organs without using the complex clotting system.

Thrombocytopenia is a disorder in which the number of platelets in the blood is greatly reduced. In individuals with thrombocytopenia the primary clotting with platelets is lost and as a result the person develops many minute bleeding spots called petechial hemorrhages, throughout all the organs and beneath the skin where they are easily seen.

The copyright of the article How Does the Blood Clot? in General Medicine is owned by Noreen Kassem. Permission to republish How Does the Blood Clot? in print or online must be granted by the author in writing.