Each 2.54 square centimetre (a square inch) of human skin has six metres (20ft) of blood vessels. Veins hold about 65 per cent of the body's blood and have one-way valves tohelp blood move.

Grasshoppers have what's called an open circulatory system. Fluid isn't enclosed inside blood vessels but is pumped into a cavity inside their bodies where it circulates around their organs.