The heart is the centre of the circulatory system acting as a pump circulating the blood through arteries and veins. Blood collects in the left auricle. As the left auricle fills up an impulse is triggered that results in the first chamber contracting and pushing the blood into the left ventricle.The ventricle is the main pumping chamber of the heart. When this is full it contracts and pumps the oxygenated blood into the main artery (dorsal aorta) and around the whole body.

In the tissues, blood flows into capillaries, allowing transfer of nutrients and waste products. Waste products are then carried away in veins to relevant organs that dispose of them. The veins lead into the anterior and posterior vena cava and so to the right ventricle.

The right ventricle contracts and sends the blood into the pulmonary artery where it is taken to the lungs to dispose of carbon dioxide and pick up oxygen. The blood leaves the lungs via the pulmonary vein and goes back to the left ventricle where the cycle start again.The two sides of the heart work in synchrony. The two auricles and ventricles contract in pairs.A heart murmur can be heard when the valves in the ventricle leak. This allows an eddy of blood to flow out and can be heard with a stethoscope.

The blood is about 65% liquid (plasma) with nutrients such as sugar disolved in it. The remaining 35% is cells.The Red blood cells carry oxygen around the body. They are very small, about 9 million to a cubic millilitre of blood.The white cells are less common: 6-12 million per millilitre of blood. The white cells are responsible for fighting infections: Neutrophils killing bacteria, Lymphocytes attacking viruses, and Eosinophils being involved with parasites and allergic reactions. To buy 'Functional Anatomy' or one of over 40 titles in the Threshold Picture Guides series, priced at only £4.95 including delivery