Blood clotting and cardiovascular disease

Molecules, Transport and Health · Cardiovascular disease and its causes · note 2 of 5

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In short

Blood clotting is a series of enzyme-controlled steps. Damaged tissue and platelets release thromboplastin, which catalyses the conversion of prothrombin to thrombin. Thrombin converts soluble fibrinogen to insoluble fibrin fibres that form a mesh trapping blood cells. A clot on a ruptured plaque can block an artery and cause a heart attack or stroke.

When a blood vessel is damaged, platelets stick to the damaged surface and release chemicals. Platelets and the damaged tissue release thromboplastin (a clotting factor). This starts a cascade of reactions.

Thromboplastin (with calcium ions) catalyses the conversion of the inactive plasma protein prothrombin to the enzyme thrombin.
Thrombin catalyses the conversion of soluble fibrinogen to insoluble fibrin.
Fibrin forms long fibres that make a mesh. The mesh traps red blood cells and platelets, forming a clot that seals the damage and stops blood loss.

Flow diagram: a damaged vessel wall and platelets release thromboplastin; thromboplastin with Ca²⁺ converts prothrombin to the enzyme thrombin; thrombin converts soluble fibrinogen to insoluble fibrin; fibrin fibres form a mesh that traps red blood cells and platelets in a clot sealing the cut. — The clotting cascade: prothrombin → thrombin, then fibrinogen → fibrin, which forms the clot.

Role in cardiovascular disease

A plaque can rupture and expose the artery wall. Clotting is triggered and a thrombus forms on the plaque, blocking a narrowed artery.
If the blocked artery is a coronary artery, the cardiac muscle supplied is starved of oxygen and glucose, cannot respire aerobically and dies. This is a myocardial infarction (heart attack).
If the blocked artery supplies the brain, the result is a stroke.
A clot can also break away and travel in the blood (an embolus) until it lodges in a narrower vessel.

Common mistake:

Fibrinogen is soluble and fibrin is insoluble, and thrombin is an enzyme made from inactive prothrombin. Do not swap these pairs, and keep the order: thromboplastin, prothrombin to thrombin, fibrinogen to fibrin.

Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026

Frequently asked questions

What causes atherosclerosis?

Atherosclerosis starts with damage to the artery’s endothelium, for example from high blood pressure or smoking. LDL cholesterol enters the wall, white blood cells engulf it, and a plaque of cholesterol, cells and fibres forms. The artery narrows and stiffens, raising blood pressure and causing more damage.

How does a blood clot cause a heart attack?

If a plaque in a coronary artery ruptures, clotting starts and a thrombus forms in the narrowed artery. Blood flow to part of the cardiac muscle is blocked, so those cells lack oxygen, cannot respire aerobically and die. This is a myocardial infarction.

What are the risk factors for cardiovascular disease?

The risk factors are genetics, a diet high in saturated fat and salt, age, being male, high blood pressure, smoking and inactivity. They raise the probability of CVD by damaging the artery lining, raising LDL cholesterol or blood pressure, or making clots form more easily, and together they raise the risk further.

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