Blood Clots

Blood clots are healthy and lifesaving when they stop bleeding. However, blood clots can also form abnormally, causing a heart attack, stroke, or other serious medical problem.

How Does Blood Clot?

Blood has a seemingly impossible job: it must flow continuously and smoothly for an entire lifetime, but quickly form a blood clot when bleeding occurs. Blood achieves this through complex interactions between substances in blood and the blood vessel walls.

The major parts of blood clot formation are:

1. The platelet plug forms. Platelets are tiny components in blood that initiate blood clots. Platelets become stimulated when they encounter a damaged blood vessel, and flock to the site. The platelets clump together and form a plug, which reduces bleeding. Platelets also release substances that start the chemical reaction of blood clot formation.

2. Chemical reactions grow the blood clot. Blood contains dissolved proteins, also called clotting factors, which promote blood clots. (Most of the proteins have Roman numerals for names, including factors V, VII, VIII, IX, X, and XI). These signal to and amplify each other’s activity in massive numbers at the site of bleeding. This results in a rapid chemical chain reaction whose end product is fibrin, the main protein forming blood clots. A fibrin-formed blood clot is much tougher and more durable than the platelet plug.

3. Anti-clotting processes halt the blood clot’s growth. Once formed, the blood clot must be prevented from growing and spreading through the body, where it could cause damage. Numerous anti-clotting proteins (antithrombin, protein C, protein S, and others) exist in a natural balance with clotting factors. The anti-clotting enzymes neutralize excess clotting factors, preventing them from extending the blood clot farther than it should go.

4. The body slowly breaks down the blood clot. As the damaged tissue heals, the body slowly degrades the blood clot and reabsorbs it. An enzyme called plasmin is responsible for dissolving the tough fibrin strands in a blood clot. Various other substances act together to activate plasmin and help it break down the blood clot.

What Causes Blood Clots?

The process of blood clotting is triggered whenever flowing blood is exposed to certain substances. There are many different such substances, which are called thrombogenic because they promote formation of thrombus (another name for a clot). Many thrombogenic substances are located in the skin or in blood vessel walls. Normally safely separated from flowing blood, their contact with blood usually means the blood vessel wall is ruptured and bleeding. Examples of these thrombogenic substances are tissue factor, collagen, and von Willebrand factor.

Blood clots are essential to stop bleeding after injury, but harmful blood clots can also form, causing serious damage. Most heart attacks and strokes result from the sudden formation of a blood clot on a cholesterol plaque inside an artery in the heart or brain. When the plaque ruptures suddenly, thrombogenic substances inside the plaque are exposed to blood, triggering the blood clotting process.

Blood clots may also form when blood fails to flow properly. Atrial fibrillation is an abnormal heart rhythm in which blood pools in the heart, potentially forming blood clots. If a blood clot dislodges and travels to the brain, it can cause a stroke. Prolonged immobilization can reduce blood flow in the legs, increasing the risk for blood clots in leg veins (deep venous thrombosis, or DVT).