How the Heart Beats
Obviously, the blood supply to the heart muscle has to be special. To beat efficiently, the heart has first to push blood from the upper chambers, called the atria, to the lower and larger chambers, the ventricles. The atria beat first, shifting the blood that has returned from the body or lungs into the ventricles. Once filled and stretched by the incoming blood, the ventricles then beat, pushing the blood out into the lungs and the rest of the body.
Once the contraction is over, the heart relaxes. Valves placed in the outflow vessels - the pulmonary artery (to the lungs) and the aorta (to the rest of the body) - prevent the blood just expelled from the heart from flowing back again into it. Instead, even when the heart is in its state of relaxation, the blood is driven onwards by the tension in the closed valves and in the muscles within the walls of the main blood vessels. Meanwhile, more blood is sucked into the atria as the heart relaxes, and the cycle starts again.
This system means that there are two distinct phases of the heart beat. The first is the contraction that forces the blood out through the open valves: this is called systole. The pressure exerted on the blood in this action is called the systolic pressure.
The second is the phase of relaxation of the heart. This is called diastole. The pressure of blood within the body's blood vessels while the heart is relaxing is the diastolic pressure. Both the systolic and the diastolic pressures are noted when the blood pressure is taken. The first is a measure of the force of the heart beat, the second of the resistance of the blood vessels to the flow of blood within them. Together, they give the doctor a good idea of the state of the circulation.
Blood pressure is measured in units of millimeters of mercury (mmHg), which corresponds to the height of the column of mercury in the sphygmomanometer - the instrument doctors usually use for taking the blood pressure. A figure of 120-140 mmHg is normal for the systolic pressure, and 70-80 for the diastolic pressure. So a normal pressure would be recorded as 120/80 mmHg. A blood pressure of 150/95 is moderately raised. One of 170/105 or above needs investigation.
High blood pressure can be due to excessive force in the heart beat (raising the systolic pressure), or to narrowing of the small blood vessels in the skin and muscles of the limbs (which raises the diastolic pressure). Usually, there is a combination of both.
Blood pressure is needed, of course, to maintain the blood supply to all the organs and structures of the body. The action of systole distributes the blood onwards, and can be felt in the pulse, say, at the wrist or throat.
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