The Heart and Circulatory System
Overview
The human heart is a muscular pump that drives blood around a double circulatory system. "Double" means the blood passes through the heart twice for each complete circuit of the body — once to the lungs and once to the body organs.
Structure of the Heart
The heart has four chambers:
- Right atrium — receives deoxygenated blood from the body via the vena cava
- Right ventricle — pumps deoxygenated blood to the lungs via the pulmonary artery
- Left atrium — receives oxygenated blood from the lungs via the pulmonary vein
- Left ventricle — pumps oxygenated blood to the body via the aorta
Key valves prevent backflow of blood:
- Atrioventricular (AV) valves — between atria and ventricles (tricuspid on right, mitral/bicuspid on left)
- Semilunar valves — at the exits of the ventricles (in pulmonary artery and aorta)
Note: The left ventricle has a thicker muscular wall than the right ventricle because it must pump blood all the way around the body (systemic circulation) at higher pressure, whereas the right ventricle only pumps to the lungs (shorter distance, lower pressure).
The Double Circulation
Pulmonary circulation (right side): Right ventricle → Pulmonary artery → Lungs (gas exchange) → Pulmonary vein → Left atrium
Systemic circulation (left side): Left ventricle → Aorta → Body organs (delivers O₂, collects CO₂) → Vena cava → Right atrium
Advantage of double circulation: Blood is returned to the heart after passing through the lungs (where it loses pressure), then repressurised to be pumped strongly to the body. This maintains high pressure for efficient delivery of O₂ and glucose.
The Cardiac Cycle
- Diastole — heart relaxes; atria fill with blood from veins
- Atrial systole — atria contract; blood pushed into ventricles
- Ventricular systole — ventricles contract; blood pumped out to lungs and body; AV valves close (lub), semilunar valves open
Heart rate is controlled by the sinoatrial node (SAN) — the heart's natural pacemaker — in the wall of the right atrium. It sends electrical signals to trigger each heartbeat.
Coronary Heart Disease (CHD)
The heart muscle itself is supplied with blood by the coronary arteries (branches of the aorta). If coronary arteries become blocked:
Atherosclerosis: fatty deposits (atheroma/plaque) build up inside coronary artery walls → walls thicken → lumen narrows → blood flow restricted.
Consequence: Heart muscle cells receive less O₂ and glucose → myocardial infarction (heart attack).
Risk factors for CHD:
- High blood cholesterol (from diet high in saturated fat)
- High blood pressure (hypertension) — damages artery walls
- Smoking — carbon monoxide reduces O₂-carrying capacity; nicotine raises blood pressure
- Obesity, lack of exercise, diabetes, family history (genetics)
Treatments:
| Treatment | How it works |
|---|---|
| Lifestyle changes | Reduce fat intake, exercise, stop smoking |
| Statins | Lower blood cholesterol levels |
| Beta-blockers | Slow heart rate, reduce blood pressure |
| Angioplasty + stent | Inflatable balloon widens narrowed artery; stent holds it open |
| Bypass surgery | A vein from the leg is grafted to bypass the blockage |
| Heart transplant | Replace severely damaged heart (requires donor; risk of rejection) |
Wales Context — WJEC Note
Wales has higher rates of CHD than the England average. WJEC past papers often link CHD to lifestyle factors relevant to Welsh public health campaigns. Be prepared to evaluate data on risk factors and justify treatment choices.
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