Michelle LeBlanc, age 14, of Laconia, N. H., for her question:
HOW IS BLOOD PRESSURE MEASURED?
Blood pressure is the pressure of circulating blood against the walls of the arteries. Pressure is measured in inches of mercury by an instrument called a sphygmomanometer, consisting of an inflatable rubber cuff connected to a pressure detecting device with a dial.
The cuff is wrapped around the upper left arm and inflated by squeezing a rubber bulb connected to it by a tube. Meanwhile, the doctor making the examination listens to a stethoscope applied to an artery in the lower arm. As the cuff expands, it gradually compresses the artery. The point at which the cuff stops the circulation and at which no pulsations can be heard is read as the systolic pressure. It is more usually read, as the cuff is slowly deflated, at the point when the circulation is restored.
A spurting sound can then be heard as the heart contractions forces blood through the artery. The cuff is then allowed gradually to deflate further until the blood is flowing smoothly again. A reading at this point shows the diastolic pressure that occurs during relaxation of the heart.
Again, the high point at which the heart contracts to empty its blood into the circulation system is called systole, and the low point at which the heart relaxes to fill with blood returned by the circulation is called diastole.
During a single cardiac cycle or heartbeat, the blood pressure varies from maximum during systole to minimum during diastole. Usually both measurements are given as a ratio expression of the highest over the lowest, for example, 140/80.
In healthy persons, blood pressure varies from about 80/45 in infants, to about 120/80 at age 30, to about 140/85 at age 40 and over. This increase occurs when the arteries lose the elasticity that in younger people absorbs the shock of heart contractions. Blood pressure varies between individuals an in the same individual at different times.
Blood pressure is generally higher in men than in women and children, is lowest during sleep and is influenced by a wide range of factors.
Many healthy persons have habitual systolic pressures of from about 95 to 115 not associated with symptoms or disease. Abnormally high blood pressure, or hypertension, is considered a contributory cause of arteriosclerosis.
Poisons generated with in the body cause extreme hypertension in various disorders.
Abnormally low pressure, or hypotension, is observed in infectious and wasting disease, hemorrhage and collapse. A systolic pressure of much lower than 80 is unusually associated with shock.
Controlled by both cerebrospinal and sympathetic nerve centers, the complex nervous mechanism that balance and coordinate the activity of the heart and arterial muscles permit great local variation in the rate of~blood flow without disturbing the general blood pressure.