The heart is a muscular organ. Its function is to pump blood. The cardiac muscle of the heart contracts to pump the blood from the atria into the ventricles and from the ventricles into the arteries. The cardiac muscle of the heart requires energy from respiration to continue to contract, and is supplied with the glucose and oxygen required by coronary arteries.

The right side pumps blood through the pulmonary circuit (to the lungs), blood then returns to the left side which pumps blood through the systemic circuit (body).

Two separate pumps

In general, blood flows into one side of the heart from a vein, goes into an atrium, then a ventricle, and out through an artery.

The blood on the right side of the heart is deoxygenated. It has been around the body and supplied the cells with oxygen carried by the red blood cells. It is now depleted of oxygen and enters the heart through the main vein, the vena cava, into the right atrium.

The right atrium contracts and pushes the blood into the right ventricle which in turn contracts, pushing the blood out of the pulmonary artery to the lungs to become oxygenated.

Blood now returns to the left side of the heart through the pulmonary vein. It enters the left atrium which contracts to push the blood into the left ventricle. The left ventricle has a thick muscular wall to provide sufficient pressure to pump the oxygenated blood to the furthest areas of the body.

Blood leaves the heart through the main artery, the aorta.

The heart contains valves to prevent the blood flowing backwards:

• the right side has a tricuspid valve (a valve with three flaps)
• the left side has a bicuspid valve (a valve with two flaps)

Both sides have semilunar valves – at the entrances to the pulmonary artery and aorta.

Heart beat

The slideshow demonstrates how the heart functions.