The circulatory system: transport around the body — Biology, 11–13 years
The heart, blood and blood vessels form a delivery and collection network. It carries oxygen and nutrients to cells, then carries carbon dioxide and other wastes away.
A moving delivery network
Blood is not just a red liquid; it is a moving transport system. The heart pushes it through arteries, tiny capillaries and veins. Red blood cells carry much of the oxygen, plasma carries dissolved substances, and white blood cells help defend the body.
Why does the body need circulation?
A large body cannot let every cell collect oxygen and food directly from the outside. It needs a network that reaches almost everywhere and also collects waste. The circulatory system solves this distance problem; its parts were understood gradually through dissections, observations of the pulse and experiments on blood flow.
Following one oxygen molecule
Trace one oxygen molecule: it crosses from an alveolus into a red blood cell. The blood travels through a vein to the heart, then the heart pumps it through an artery. In a muscle capillary, oxygen leaves the blood and enters a cell. Carbon dioxide then begins the return journey.
Arteries are not defined by oxygen
Many people think arteries always carry oxygen-rich blood and veins always carry oxygen-poor blood. This is a reasonable shortcut because it is true for most of the body. The real rule is direction: arteries carry blood away from the heart, while veins carry it towards the heart. The lung vessels are the important exception to the shortcut.
Pulse, injuries and health
You can feel the circulatory system at your wrist because each heartbeat sends a pressure wave through an artery. Knowing the network helps explain why a cut artery can be dangerous and why pressure is applied to a wound. Doctors also use pulse, blood pressure and blood tests to check transport around the body.
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