Biology · Both
Circulation & the Heart
Structure of the heart, blood vessels, components of blood and the double circulatory system.
The circulatory system transports oxygen, nutrients, carbon dioxide and waste. Humans have a double circulatory system – blood passes through the heart twice on each full circuit.
The heart
Four chambers:
• Right atrium – receives deoxygenated blood from the body
• Right ventricle – pumps deoxygenated blood to the lungs
• Left atrium – receives oxygenated blood from the lungs
• Left ventricle – pumps oxygenated blood to the body (thickest wall)
Valves prevent backflow.
Pacemaker (SAN) controls the heartbeat.
Blood vessels
• **Arteries** – carry blood away from the heart; thick muscular walls; high pressure
• **Veins** – carry blood towards the heart; thinner walls; valves; lower pressure
• **Capillaries** – very thin walls (one cell thick); exchange of substances with tissues
Blood components
• **Red blood cells** – carry oxygen (haemoglobin)
• **White blood cells** – defence against pathogens
• **Platelets** – help blood clot
• **Plasma** – liquid; carries dissolved substances (CO₂, glucose, urea, hormones)
Practice questions
Try these without looking at the answers first.
Q1.Which chamber pumps blood to the body?
Show answer
Left ventricle
Q2.Which vessels have valves?
Show answer
Veins
Q3.What do red blood cells carry?
Show answer
Oxygen