Blood pressure is the pressure produced when your heart pumps blood into the arteries of the body. Your blood needs to be under pressure to move it forward so that the arteries can carry it to the organs, tissues and cells of the body (arteries carry blood away from the heart, veins generally carry blood towards the heart). As the heart beats, the pressure rises to a maximum level, called the systolic blood pressure, and as it relaxes, it falls to a minimum level called the diastolic blood pressure. When your doctor tells you that you have a blood pressure of 120 over 80 (written 120/80) it means your systolic blood pressure is 120 and your diastolic blood pressure is 80 millimetres of mercury; that is, your blood pressure is rising to a maximum of 120 millimetres of mercury as the heart beats, and falling to a minimum of 80 millimeters of mercury as the heart relaxes.

Why does the pressure not fall to zero when the heart relaxes? This is because of the elasticity of the arterial walls. Imagine you are blowing up a rubber balloon. You force air into the balloon. When you stop blowing and take your mouth away the air is forced back as the balloon deflates, because of the elastic nature of the rubber. If you close the opening with your fingers, the air remains under pressure inside. The great arteries of the body behave somewhat like the balloon. They are stretched when the heart contracts, and by 'springing back' after the heartbeat, they cause the blood to remain under pressure. This residual pressure is what we call the diastolic pressure.

What is the significance of systolic and diastolic blood pressure?

When you are exercising, or under mental stress, it is your systolic blood pressure which changes first. Changes in diastolic blood pressure occur more gradually and more evenly. Doctors have therefore assumed that a series of diastolic blood pressure readings will give a better indication of your average blood pressure, than a series of systolic readings. They have put more emphasis on diastolic blood pressure when evaluating your risk of stroke or heart disease.

Recently however, some studies have suggested systolic blood pressure is important too, and it is now generally agreed that 'systolic hypertension' (a series of readings in which systolic blood pressure is found to be raised most of the time, diastolic being normal) should be treated.

Keeping the blood pressure at appropriate levels is an exquisite example of the body's ability to maintain its internal constancy (homeostasis). In the great vessels of the body, the aorta and the carotid arteries, are tiny pressure receptors, called baroreceptors, which monitor, second by second, the pressure within the artery walls. These send messages to a part of the brain, the brain stem. There the messages are processed and result in nerve impulses being sent to the heart, telling it either to speed up or slow down, or increase or decrease its force of contraction. Other messages are also sent to the smooth muscles of arterioles telling them to contract or relax, while still others are received from and sent back to the kidneys which have a part to play in the control of blood pressure through the secretion of a hormone called renin. This is a simplified picture only. Cardiac peptides and hormones within the blood vessel wall are among numerous other checks and balances in a complex neural and hormonal interplay. The common final pathway is our blood pressure being at the right level for the moment, every minute of the day.