Atherosclerosis, blood clotting and cardiovascular diseaseEdexcel A-Level Biology A: Revision notes

The inner lining of every artery is a single layer of cells called the endothelium. It is smooth, so blood flows without friction and does not clot. Endothelial dysfunction means this lining is damaged. Common causes are high blood pressure and chemicals from cigarette smoke, and a diet high in saturated fat that raises blood cholesterol also contributes.

Damaged endothelium allows lipids from the blood to enter the artery wall and triggers an inflammatory response.

After the endothelium is damaged, an inflammatory response occurs. White blood cells (monocytes, which become macrophages) move from the blood into the artery wall. They take up cholesterol and other lipids and become foam cells, building up a fatty streak.

Smooth muscle cells then migrate into the area and divide, and collagen (fibrous tissue) is deposited. The mixture of lipids, dead cells and fibrous tissue is called a plaque or atheroma. As it grows, it bulges into the lumen and may later rupture.

The plaque narrows the lumen and the artery becomes less elastic (it hardens). Resistance to blood flow rises, so blood pressure goes up. Raised pressure damages the endothelium further, so atherosclerosis becomes worse in a positive feedback cycle.

High blood pressure is both a cause and a consequence of atherosclerosis. Over time, narrow arteries reduce the blood supply to tissues, which causes chest pain (angina) if a coronary artery is affected.

Clotting stops blood loss from damaged vessels, but a clot inside an artery is dangerous.

1. Damaged tissue and platelets release thromboplastin.
2. Thromboplastin triggers the conversion of soluble plasma protein prothrombin into thrombin, an enzyme.
3. Thrombin catalyses the conversion of soluble fibrinogen into insoluble fibrin fibres.
4. The fibrin forms a mesh that traps red blood cells and platelets, producing a clot.

$\text{prothrombin} \xrightarrow{\text{thromboplastin}} \text{thrombin} \qquad \text{fibrinogen} \xrightarrow{\text{thrombin}} \text{fibrin}$

If a plaque ruptures, the endothelium is damaged and tissue is exposed, so clotting is triggered inside the artery. The clot (thrombus) can block an artery already narrowed by atheroma. A piece may break off as an embolus and lodge in a smaller vessel.

• A blocked coronary artery cuts off the oxygen and glucose supply to cardiac muscle, which dies: a myocardial infarction (heart attack).
• A blocked artery in the brain causes a stroke.

Atherosclerosis and clotting together are therefore the main cause of cardiovascular disease (CVD).