The heart, lungs and blood vessels
Physical Education · Key Stage 3 · The School
Two pumps, not one
The heart has four chambers: two ATRIA on top, two VENTRICLES below. The RIGHT side pumps blood to the lungs to pick up oxygen. The LEFT side pumps that oxygen-rich blood round the rest of the body. Both sides beat at once, but they drive two separate loops.
The lungs
Air reaches tiny air sacs called ALVEOLI, surrounded by very thin-walled blood vessels. Oxygen moves from the air into the blood there. Carbon dioxide moves the other way, out of the blood and into the air, to be breathed out. Millions of alveoli give a huge surface for this swap to happen fast.
Three kinds of vessel
ARTERIES carry blood AWAY from the heart, under high pressure, with thick muscular walls. VEINS carry blood BACK to the heart, at low pressure, and have valves to stop blood flowing backward. CAPILLARIES are tiny and thin-walled, just one cell thick, which is where oxygen and nutrients actually pass into the tissues.
Start in the LEFT VENTRICLE. It is pumped out through an ARTERY to the muscles, where a CAPILLARY lets oxygen out and carbon dioxide in. It returns through a VEIN to the RIGHT ATRIUM, then the RIGHT VENTRICLE, which pumps it to the lungs. In the lungs it picks up fresh oxygen and drops off carbon dioxide. It returns to the LEFT ATRIUM, then the LEFT VENTRICLE — back where it started, ready to go again.