The heart itself is simply a muscular pump that pushes the blood through the different vessels in order that various substances it transports circulate to where they need to go. In order to understand how the heart works it is necessary to recognise 12 key structures: 4 chambers, 4 blood vessels and 4 valves. The heart has four chambers: two upper chambers called atria, and two lower chambers called ventricles. The average adult has approximately five litres of blood distributed throughout the cardiovascular system in a number of different vessels. There are three main types of blood vessel: arteries, veins and capillaries. Structurally, arteries and veins are very similar. Although both arteries and veins are constructed from the same three layers of tissue there are number of distinct anatomical differences between them. The final type of blood vessels is capillaries. Capillaries microscopic tubes consist of a single layer of endothelial cells through which plasma and other small molecules can pass.