blood circulation

Blood circulation is the never-ending journey of blood as it loops from the heart out to the tissues and back again. Picture a delivery truck that never parks: it drops off oxygen and nutrients, picks up waste, and returns to base to be reloaded — round and round, all day and night.

Driven by the heart's pumping pressure, blood leaves through arteries, slows down in the fine capillaries where it trades oxygen and nutrients for carbon dioxide and waste, then drains into veins and flows back. A separate short loop sends it through the lungs to release carbon dioxide and pick up fresh oxygen. The whole circuit takes roughly a minute at rest.

The idea that blood truly circulates — rather than being made and consumed — was established by William Harvey in 1628 and is one of the foundations of modern physiology. When circulation falters, tissues are starved of oxygen, which is the underlying threat in conditions from a heart attack to shock.