The Heart and Circulatory System
The circulatory system
Humans have a double circulatory system: the heart pumps blood on two loops — one to the lungs to pick up oxygen, and one to the rest of the body to deliver it. "Double" means blood passes through the heart twice for each full circuit, which keeps blood pressure high and delivery efficient.
The heart
The heart is a muscular organ with four chambers:
| Chamber | Receives / sends |
|---|---|
| Right atrium | Receives deoxygenated blood from the body (vena cava) |
| Right ventricle | Pumps deoxygenated blood to the lungs (pulmonary artery) |
| Left atrium | Receives oxygenated blood from the lungs (pulmonary vein) |
| Left ventricle | Pumps oxygenated blood to the whole body (aorta) |
- The left ventricle has a thicker muscular wall because it must pump blood at high pressure all around the body.
- Valves stop blood flowing backwards, keeping it moving in one direction.
- The heart's natural resting rate is controlled by a group of cells called the pacemaker in the right atrium. Artificial pacemakers correct irregular heartbeats.
- The heart muscle itself is supplied with oxygen by the coronary arteries.
The blood vessels
| Vessel | Job | Features |
|---|---|---|
| Arteries | Carry blood away from the heart (usually oxygenated) | Thick, elastic, muscular walls; small lumen; high pressure |
| Veins | Carry blood back to the heart (usually deoxygenated) | Thinner walls; large lumen; valves to prevent backflow; low pressure |
| Capillaries | Exchange of substances with tissues | One-cell-thick walls; huge surface area; very narrow |
The blood
Blood is a tissue made of four parts:
- Red blood cells – carry oxygen using haemoglobin; no nucleus; biconcave shape for large surface area.
- White blood cells – part of the immune system (engulf pathogens, make antibodies).
- Platelets – small fragments that help blood clot at wounds.
- Plasma – yellow liquid that carries cells, dissolved glucose, CO₂, hormones and waste.
Coronary heart disease
Fatty deposits build up in the coronary arteries, narrowing them and reducing blood/oxygen to the heart muscle. Treatments include stents (hold arteries open) and statins (reduce blood cholesterol).
Worked example
Why is the left ventricle wall thicker than the right?
- The left ventricle pumps blood to the whole body at high pressure, so it needs a thicker, more muscular wall to generate that force. The right ventricle only pumps to the nearby lungs. ✓
Common mistakes
- Swapping the sides: the right side handles deoxygenated blood (to lungs); the left handles oxygenated (to body).
- Saying arteries always carry oxygenated blood — the pulmonary artery carries deoxygenated blood to the lungs.
- Forgetting capillary walls are one cell thick to speed up exchange.
Exam tips
- Learn the path of blood: body → right atrium → right ventricle → lungs → left atrium → left ventricle → body.
- Match each vessel to its features (arteries = thick/high pressure; veins = valves; capillaries = one cell thick).
- Explain the thick left ventricle in terms of pumping to the whole body.
Key facts to remember
- Double circulation: blood passes through the heart twice (lungs loop + body loop).
- Four chambers; left ventricle wall is thickest; valves prevent backflow; pacemaker sets the rhythm.
- Arteries (away, high pressure), veins (back, valves), capillaries (one-cell-thick exchange); blood = red cells, white cells, platelets, plasma.