The circulatory system transports materials around the body. It consists of the heart (the pump), blood vessels (the tubes) and blood (the transport medium).
Components of Blood
| Component | Function |
|---|---|
| Red blood cells | Carry oxygen using haemoglobin |
| White blood cells | Fight infection (defence/immunity) |
| Platelets | Help blood to clot |
| Plasma | Transports dissolved nutrients, hormones, carbon dioxide and urea |
Blood Vessels
- Arteries carry blood away from the heart. They have thick, elastic, muscular walls to withstand high pressure.
- Veins carry blood towards the heart. They have thinner walls and valves to prevent backflow of blood.
- Capillaries are tiny vessels (one cell thick) where exchange of materials occurs between blood and body cells.
Double Circulation
Humans have double circulation: blood passes through the heart twice in one complete circuit.
- Pulmonary circulation: heart → lungs → heart (blood picks up oxygen and releases carbon dioxide).
- Systemic circulation: heart → body → heart (oxygenated blood is delivered to the body).
Remember the rule: The right side of the heart pumps deoxygenated blood to the lungs; the left side pumps oxygenated blood to the body. The left ventricle has the thickest wall because it pumps blood to the whole body.
Summary
The circulatory system uses the heart to pump blood through arteries, capillaries and veins. Blood carries oxygen (in red blood cells), fights infection (white blood cells), clots wounds (platelets) and transports nutrients (plasma). Double circulation keeps oxygenated and deoxygenated blood separate.
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