Coronary heart disease (CHD) is usually caused by atherosclerosis, which narrows the coronary arteries, sometimes followed by a blood clot that completely occludes an artery. This reduces oxygen delivery to cardiac muscle and can cause angina or a heart attack.
The mechanism
The coronary arteries supply the heart muscle, or myocardium, with oxygen and nutrients. CHD develops through the following sequence:
- The inner lining of a coronary artery becomes damaged or dysfunctional.
- Low-density lipoprotein cholesterol accumulates in the artery wall, stimulating inflammation.
- Lipids, inflammatory cells and fibrous tissue form an atherosclerotic plaque, also called an atheroma.
- The plaque narrows the artery’s lumen, reducing blood flow. During exercise, oxygen supply may become insufficient for aerobic cell respiration in cardiac muscle, causing angina.
- If the plaque ruptures, platelets aggregate and a thrombus forms. Complete occlusion stops blood flow, causing prolonged oxygen deprivation and death of cardiac muscle: a myocardial infarction.
Risk factors increase the probability of this process but are not all direct causes.
| Risk factor | How it increases risk |
|---|---|
| Smoking | Damages artery linings and promotes blood clotting |
| High LDL cholesterol | Increases lipid deposition in artery walls |
| High blood pressure | Increases mechanical damage to artery walls |
| Diabetes | Damages blood vessels and accelerates atherosclerosis |
| Physical inactivity or obesity | Can contribute to hypertension, diabetes and abnormal blood lipid levels |
| Age or genetic predisposition | Increases susceptibility but cannot be modified |
A common misconception is that dietary fat simply sticks inside an artery like grease in a pipe. Plaque formation is instead a complex inflammatory process occurring within the artery wall.
Exam technique
This is assessed at SL and HL in B3.2 Transport. For an explain question, give a linked sequence: plaque formation → narrowed lumen → reduced coronary blood flow → reduced oxygen supply → impaired aerobic respiration. When evaluating epidemiological data, distinguish correlation from causation and identify possible confounding variables.
