The heart, as we have learned, is a pump that sends the blood circulating through our body. The pumping action takes place when the left ventricle of the heart contracts. This forces the blood out into the arteries, which expand to receive the oncoming blood.

But the arteries have a muscular lining which resists this pressure, and thus the blood is squeezed out of them into the smaller vessels of the body. Blood pressure is the amount of pressure on the blood as a result of the heart’s pumping and the resistance of the arterial walls.

There are two kinds of pressure: maximum and minimum. The maximum pressure occurs when the left ventricle contracts; it is called the systolic pressure. The minimum pressure occurs just before the heartbeat which follows; it is called the diastolic pressure.

When your doctor measures your blood pressure, he uses an instrument which measures it in terms of a column of mercury, which rises and falls under the pressure. He reads it in millimeters rather than in inches. The average systolic pressure in a young man is about 120 millimeters (about 5 inches) of mercury. The diastolic pressure is about 80 millimeters of mercury. These figures are usually stated as 120/80, or 120 over 80.

When the blood pressure is in this range, it provides the body with a circulating supply of Wood without unduly straining the walls of the blood vessels. But there are many variations from this range which may be quite normal.

With age, the blood pressure gradually rises until, at 60 years, it is about 140/87. There are many factors that affect the blood pressure. Overweight people often have a higher blood pressure than people of normal weight. Tension, exercise, and even posture may affect the blood pressure.