The circulatory system
Overview — double circulation
Humans have a double circulatory system: blood passes through the heart TWICE for every complete circuit.
- Pulmonary circulation: heart → lungs → heart (blood picks up O₂, loses CO₂).
- Systemic circulation: heart → body → heart (blood delivers O₂ and nutrients, picks up CO₂).
This system is efficient because oxygenated and deoxygenated blood never mix, and blood is re-pressurised at the heart before delivery to the body.
The heart
The heart is a muscular pump made largely of cardiac muscle (myocardium), which never fatigues.
Right side Left side
Deoxygenated blood Oxygenated blood
from body from lungs
↓ ↓
[Right atrium] [Left atrium] ↓ ↓ [Right ventricle] [Left ventricle] ↓ ↓ Lungs Body (aorta)
Key valves prevent backflow:
- Atrioventricular (AV) valves (tricuspid on right; bicuspid/mitral on left): between atria and ventricles.
- Semilunar valves: between ventricles and arteries (pulmonary and aortic).
Left ventricle wall is thicker than the right: it must pump blood around the entire body (much greater distance and resistance), requiring more force.
Blood vessels
| Vessel | Wall | Lumen | Blood pressure | Direction |
|---|---|---|---|---|
| Artery | Thick, muscular, elastic | Narrow | High | Away from heart |
| Vein | Thin, less muscular | Wide | Low | Towards heart |
| Capillary | One cell thick | Very narrow | Lowest | Through tissues |
Veins have valves to prevent backflow (low pressure cannot keep blood moving on its own).
Capillaries are the site of exchange between blood and tissues (O₂, glucose, CO₂, waste).
Blood — four components
| Component | Structure | Function |
|---|---|---|
| Red blood cells (RBCs) | Biconcave, no nucleus, haemoglobin | Carry O₂ (and some CO₂) |
| White blood cells | Large nucleus, some granular | Immune defence (phagocytosis/antibodies) |
| Platelets | Cell fragments, no nucleus | Blood clotting at wound sites |
| Plasma | Yellow fluid | Transport (glucose, CO₂, urea, hormones, heat) |
Coronary heart disease (CHD)
The heart muscle receives its own blood supply via the coronary arteries. If these become blocked, the heart muscle is starved of oxygen → heart attack (myocardial infarction).
Causes of CHD:
- Fatty plaques (atherosclerosis) narrow coronary arteries.
- Blood clot (thrombosis) in a narrowed artery blocks it completely.
Risk factors: smoking, high blood pressure, high blood cholesterol, poor diet (saturated fat), lack of exercise, family history.
Treatments:
- Lifestyle changes (diet, exercise, stop smoking).
- Statins (drugs that lower blood cholesterol).
- Coronary angioplasty + stent (balloon opens artery; wire mesh keeps it open).
- Coronary bypass surgery (healthy blood vessel grafted to bypass the blockage).
AI-generated · claude-opus-4-7 · v3-ccea-combined-science