# The Circulatory System
Humans have a double circulatory system: blood passes through the heart twice in one complete circuit. This is essential for delivering oxygen and nutrients to cells and removing waste.
1. Double Circulation
- Right side: heart → lungs → heart (pulmonary circuit)
- Left side: heart → body → heart (systemic circuit)
Advantage: maintains high pressure to the body for efficient delivery.
2. The Heart
| Chamber | Function |
|---|---|
| Right atrium | Receives deoxygenated blood from body (via vena cava) |
| Right ventricle | Pumps blood to lungs (via pulmonary artery) |
| Left atrium | Receives oxygenated blood from lungs (via pulmonary vein) |
| Left ventricle | Pumps blood to body (via aorta); thickest wall |
- Valves: prevent backflow of blood
- Pacemaker (SAN): controls heart rate; in the right atrium
- Left ventricle wall is thicker because it pumps blood to the whole body
3. Blood Vessels
| Vessel | Structure | Function |
|---|---|---|
| Arteries | Thick muscular walls, small lumen, elastic | Carry blood AWAY from heart at high pressure |
| Veins | Thin walls, large lumen, valves | Return blood TO heart at low pressure |
| Capillaries | One cell thick, very narrow | Exchange of substances with tissues |
4. Blood Components
| Component | Function |
|---|---|
| Red blood cells | Carry oxygen (contain haemoglobin); no nucleus; biconcave shape |
| White blood cells | Fight infection (phagocytosis, antibody production) |
| Platelets | Blood clotting at wound sites |
| Plasma | Liquid; transports dissolved substances (glucose, CO₂, urea, hormones) |
5. Cardiovascular Diseases
Coronary Heart Disease
- Coronary arteries blocked by fatty deposits (atherosclerosis)
- Reduces blood flow to heart muscle
- Treatments: stents, statins, bypass surgery
Risk Factors
- Smoking, high-fat diet, lack of exercise, obesity, genetics, high blood pressure
6. Practice Questions
- Describe the route of blood through the double circulatory system.
- Why is the left ventricle wall thicker than the right?
- Compare arteries and veins (structure and function).
- What is the function of haemoglobin?
- Explain how a stent treats coronary heart disease.
Want to check your answers and get step-by-step solutions?
Summary
- Double circulation: pulmonary (lungs) + systemic (body)
- Heart: 4 chambers; left ventricle has thickest wall
- Arteries: thick walls, away from heart; Veins: valves, to heart; Capillaries: exchange
- Blood: red cells (O₂), white cells (defence), platelets (clotting), plasma (transport)
- CHD: blocked coronary arteries; treated with stents/statins