To describe the requirements of blood circulation in the body it is possible to express this in terms of cardiac output or the amount of blood pumped around the body in one minute. Cardiac output is made up of two factors, stroke volume (the amount of blood in each beat) and heart rate (the amount of heart beats per minute). The following equation illustrates the relationship of the two variables:

Cardiac Output (ml) = Stroke Volume (ml) X Heart Rate (bpm)

The heart is often said to be a double pump as each beat causes blood to be pumped to two different locations (the lungs or around the body). Within its structure there are a number of valves that stop backflow of blood between chambers and major blood vessels, with each beat. This system ensures that the blood flow is only in one direction.

Controlling the rate that the heart beats is the sino atrial node (SA Node), situated in the wall of the right atrium, it sends out the electrical impulse causing the atria to contract. This impulse then spreads to the atrioventricular node (AV Node) in the atrial septum which distributes the signal along septum's 'Bundles of His' and branching Purkinje fibres to the whole of the ventricles, causing them to contract.

The phases of the heart beat are:

Systole - the heart contracts.

Diastole - the heart relaxes and fills, ready for systole.

The circulation of blood aroud the body is split into two distinct systems. The Pulmonary Circuit takes blood from the heart to the lungs to be oxygenated and then returns it back to the heart to be pumped around the body in the Systemic Circuit to all the oxygen requiring tissues.

In both the Pulmonary and Systemic systems gas exchange takes place in the capillary beds.