The Heart and Circulatory System
A double circulatory system
1. Heart → lungs → heart (picks up oxygen).
2. Heart → rest of the body → heart (delivers oxygen).
The heart (key structures)
- Atria (top) receive blood; ventricles (bottom) pump it out.
- Right side pumps deoxygenated blood to the lungs; left side pumps oxygenated blood to the body (thicker wall — further to pump).
- Valves stop backflow. The natural pacemaker is in the right atrium.
Blood vessels
| Vessel | Carries | Wall | Feature |
|---|---|---|---|
| Artery | blood away from heart | thick, muscular | high pressure |
| Vein | blood to the heart | thinner | valves prevent backflow |
| Capillary | exchange | one cell thick | swaps O₂/CO₂/nutrients |
Blood components
- Red blood cells – carry oxygen (haemoglobin; no nucleus).
- White blood cells – fight pathogens.
- Platelets – help clotting.
- Plasma – liquid carrying cells, nutrients, CO₂, urea, hormones.
Exam tip
Arteries carry blood Away; veins have valves. The left ventricle wall is thickest because it pumps blood the furthest (around the whole body).