Topic 7 of 7Cambridge O Levels

Transport in Humans

Blood, heart, blood vessels and the circulatory system

The circulatory system is a double circulatory system: blood passes through the heart twice per circuit.

Pulmonary circuit: Heart → lungs → heart (deoxygenated → oxygenated)

Systemic circuit: Heart → body → heart (delivers oxygen, removes CO₂)

The Heart: Four chambers — **right atrium** (receives deoxygenated blood from body via vena cava) → **right ventricle** (pumps to lungs via pulmonary artery) → **left atrium** (receives oxygenated blood from lungs via pulmonary vein) → **left ventricle** (pumps to body via aorta). Left ventricle has thicker wall (pumps further). **Valves** prevent backflow.

Blood vessels:

Arteries: Thick walls, carry blood AWAY from heart, high pressure, no valves

Veins: Thinner walls, carry blood TO heart, lower pressure, HAVE valves

Capillaries: One cell thick, allow diffusion of substances

Blood components: **Red blood cells** (contain haemoglobin, carry O₂), **White blood cells** (fight infection), **Platelets** (clotting), **Plasma** (liquid, carries dissolved substances).

Key Points to Remember

1Double circulatory system: pulmonary + systemic
2Arteries: thick walls, away from heart; Veins: thinner, have valves
3Capillaries: one cell thick for diffusion
4Red blood cells carry oxygen via haemoglobin

Pakistan Example

Heart Disease in Pakistan — Understanding the Pump

Pakistan has one of the highest rates of heart disease in South Asia. Understanding that blocked coronary arteries reduce blood supply to heart muscle helps explain why fatty foods (ghee, fried snacks) increase risk. Aga Khan University Hospital performs thousands of heart surgeries annually.

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Disease & Immunity