The human circulatory system
Why a transport system?
A single cell exchanges materials directly with its environment by diffusion. Larger organisms have a small surface-area-to-volume ratio and longer diffusion distances, so they need a mass-transport system to move oxygen, glucose, CO₂ and urea between cells and exchange surfaces.
Double circulation
Humans have a double circulation: blood passes through the heart twice for one full body circuit.
- Pulmonary circuit — right side of heart → lungs → left side. Picks up O₂, drops CO₂.
- Systemic circuit — left side → body tissues → right side. Delivers O₂/glucose, picks up CO₂.
This separation keeps oxygenated and deoxygenated blood from mixing, and lets the systemic side run at a higher pressure.
The heart
Four chambers: right and left atria (top, receive); right and left ventricles (bottom, pump). Valves (atrioventricular and semilunar) prevent backflow. The left ventricle has the thickest muscle wall — it pumps blood to the whole body. Coronary arteries supply heart muscle with oxygenated blood.
Blood vessels
| Vessel | Wall | Lumen | Direction | Pressure |
|---|---|---|---|---|
| Artery | Thick, elastic, muscular | Narrow | Away from heart | High |
| Vein | Thinner, valves | Wide | Towards heart | Low |
| Capillary | One cell thick | Tiny | Exchange | Falling |
Capillaries are the exchange vessels — short diffusion distance and large surface area.
Blood components
- Red blood cells — biconcave, no nucleus, packed with haemoglobin → carry O₂.
- White blood cells — defence (phagocytes engulf, lymphocytes make antibodies).
- Platelets — fragments that trigger clotting.
- Plasma — straw-coloured liquid carrying CO₂, urea, hormones, glucose, heat.
OCR exam tip
Link structure to function in three steps: feature → property → use. e.g. "biconcave shape → larger surface area → faster O₂ diffusion".
AI-generated · claude-opus-4-7 · v3-ocr-combined-science-leaves