More information about the Heart and Blood Vessels

The Heart and Blood Vessels

The heart consists of two pumps side by side, which beat together. The right side of the heart receives blood full of carbon dioxide from the body into its upper chamber - the atrium - from where it is passed to the right ventricle. From the ventricle it is pumped round the lungs. In the lungs the carbon dioxide is exchanged for oxygen. The blood then passes into the left atrium, from where it proceeds to the left ventricle and thence around the body and back to the right side of the heart. In the foetus, of course, the lungs do not function, and the blood is oxygenated by the placenta. The oxygenated blood returns to the foetus via the umbilical vein and enters the right side of the heart. In order that it can cross to the left side and on around the foetal circulation there are two communications - one between the two atria at the top of the heart, called the foramen ovale, and one between the pulmonary artery and the aorta, called the ductus arteriosus, which bypasses the circulation around the lungs. At birth, when the baby takes its first breath, the pressure on the left side of the heart rises and the foramen ovale closes. The ductus arteriosus usually closes shortly after birth.

The blood vessels leading from the heart are arteries, those bringing blood back are veins. The whole of the heart and the roots of the great vessels are contained in a lining - the pericardium - which contains a little fluid. This lubricates the surface of the heart as it beats.

At rest, the adult heart pumps about five litres of blood every minute, which is called the cardiac output, delivering 250 cc's of oxygen and picking up carbon-dioxide and other waste material, which it delivers to the lungs and the kidneys. The values in the child are correspondingly lower, and increase as the child grows. During exercise the figures can increase dramatically. The action of the two pumps of the heart, working in parallel side by side, causes the blood to circulate around the body - which is the basis of the circulation.

When the heart contracts, the right side to send blood to the lungs and the left to the body, it is called systole. When the heart relaxes, and the chambers fill, it is called diastole. When we measure blood pressure, we measure the pressure at which blood is squeezed out of the heart during contraction - the systolic blood pressure - and the pressure when it relaxes - the diastolic blood pressure.

So that blood only flows one way around the circulation, there are valves between the chambers of the heart, and between the heart and the vessels that are attached to it. When we listen to the heart, the sounds that we hear are the sounds of these valves closing.

The rate and strength of the heartbeat is under the influence of nerves and hormones. Usually the heart contracts as a result of an electrical change which starts at a place in the right atrium called the sinu-atrial node. This electrical change passes through fibres spread throughout the heart muscle and arranged in such a way that it contracts from the tip of the ventricle upwards. The rate speeds up under the influence of adrenalin, and slows down under the influence of a nerve - the vagus nerve - which is part of our autonomic nervous system.