This unidirectional flow of blood through the heart shows that mammals have a double circulatory system. This means blood travels through the heart twice in one circulation of the body.

The circulatory system takes oxygen around the body. Find out more in this Bitesize Primary 2nd level Science guide.

Your body's circulatory system is a wondrous thing. It's made up of blood vessels that carry blood to and from your heart and also pumps blood to your lungs so you can breathe. It helps grow and ...