The heart is a pump, and the blood goes round, and round, and round all the time. And why does it do that? Why can't it just have a rest? Well, the blood has to be kept moving around all the time ...

Examples are the contraction of the heart and peristalsis which pushes food through the digestive system. Voluntary contraction of the skeletal muscles is used to move the body and can be finely ...

Narrator: Muscles! Really? They don't look much use. When a muscle contracts (bunches up), it gets shorter and so pulls on the bone it is attached to. When a muscle relaxes, it goes back to its normal ...