The heart is a large muscle which consists of four hollow spaces. The top two cavities in the heart are called atria and the bottom two, chambers. The regular contraction of the atria and chambers causes the blood to be pumped around the body. The blood supplies muscles and organs with oxygen and nutrients. The heart consists of heart muscle cells. When these are stimulated electrically, the heart contracts. The electrical stimulus begins in the sinus node (which determines the heart rhythm) and then causes the atria and then the chambers to contract via a conduction system.