The cardiovascular system is the body’s delivery network, and it makes most sense studied from the inside out. Start at the pump: a fist-sized muscle with four chambers and four one-way valves that never lets blood run backwards. Then follow the blood out into the pipework — the arteries that carry it away under pressure, the capillaries where the real exchange happens, and the veins that bring it home. Finally look at the cargo itself, the red cells, white cells, platelets and plasma that make blood far more than a red liquid. Three levels, one continuous journey from heartbeat to bloodstream.
Start here: the four chambers, the four valves and the muscular pump at the centre of circulation.
Level up to arteries, veins and capillaries and the two circuits that carry blood around the body.
Finish with what blood is made of — red cells, white cells, platelets and plasma — and the blood-group system.