Blood carries oxygen through a network of vessels that would stretch a very long way.

The longer version

Oxygen transport relies on a branching network that reaches every corner of the body. Large arteries leave the heart and divide again and again into narrower vessels, ending in capillaries so thin that blood cells pass through almost single file.

Their enormous combined length is a consequence of that branching: to supply billions of cells, the network has to pass close to nearly all of them. Capillary walls are thin enough that oxygen and carbon dioxide diffuse straight through. The return side of the circuit collects blood into progressively larger veins, which carry it back to the heart and lungs to be refreshed.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.