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Ophthalmology => High Blood Pressure => Blood Pressure
Blood Pressure
Blood Pressure, pressure of circulating blood against the walls of the arteries. Blood pressure is an important diagnostic index, especially of circulatory function. Because the heart can pump into the large arteries a greater volume of blood than can be absorbed by the tiny arterioles and capillaries, the resulting back pressure is exerted against the arteries. Any condition that dilates or contracts the blood vessels or affects their elasticity, or any disease of the heart that interferes with its pumping power, affects the blood pressure. In a healthy animal, the blood pressure normal for its species is maintained within a certain average range with great constancy. Controlled by both cerebrospinal and sympathetic nerve centers, the complex nervous system mechanisms 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.
In 1996 scientists discovered that hemoglobin, the iron-protein compound that gives blood its red color, also plays a role in regulating local variation in blood pressure. Hemoglobin was found to carry nitric oxide, a gas that relaxes the blood vessel walls, thus increasing blood flow. Hemoglobin controls the expansion and contraction of blood vessels, and thus blood pressure, by regulating the amount of nitric oxide to which the vessels are exposed.
Blood pressure is measured at two points: the high point at which the heart contracts to empty its blood into the circulation, called systole; and the low point at which the heart relaxes to fill with blood returned by the circulation, called diastole. Pressure is measured in millimeters (mm) 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 arm and inflated by squeezing a rubber bulb connected to it by a tube. Meanwhile, the health professional 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. As the cuff is slowly deflated, a spurting sound can be heard when the heart contraction forces blood through the compressed artery. The cuff is then allowed gradually to deflate further until the blood is flowing smoothly again and no further spurting sound is heard. A reading at this point shows the diastolic pressure that occurs during relaxation of the heart. During a single cardiac cycle, or heartbeat, the blood pressure reaches maximum during systole and minimum during diastole. Usually, both measurements are given as a ratio expression of maximum over minimum-for example, 140/80. When a single figure is given, it is usually the higher, or systolic, pressure.
In healthy persons, blood pressure increases 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 force of heart contractions. Blood pressure is influenced by a wide range of factors and varies between individuals and in the same individual at different times. It is generally higher in men than in women and children and is lowest during sleep.
Abnormally high blood pressure, or hypertension, is considered a contributory cause of arteriosclerosis. Poisons generated within the body cause extreme hypertension in various disorders. Abnormally low blood pressure, or hypotension, is observed in infectious and wasting diseases, hemorrhage, and collapse. Many healthy persons, however, have habitual systolic pressures of from about 95 to 115 that are not associated with symptoms or disease. A systolic pressure of much lower than 80 is usually associated with shock.
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