The Human Body

Blood Pressure

Blood Pressure, pressure of circulating blood against the walls of the arteries (blood vessels that carry blood from the heart to the rest of the body). Blood pressure is an important indicator of the health of the circulatory system. Any condition that dilates or contracts the arteries or affects their elasticity, or any disease of the heart that interferes with its pumping power, affects blood pressure.

In a healthy human being, blood pressure remains within a certain average range. 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.

Hemoglobin, the iron-protein compound that gives blood its red color, also plays a role in regulating local variation in blood pressure. Hemoglobin carries nitric oxide, a gas that relaxes the blood vessel walls. 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.

Two measurements are used to describe blood pressure. Systolic pressure measures blood pressure when the heart contracts to empty its blood into the circulatory system. Diastolic pressure measures blood pressure when the heart relaxes and fills with blood. Systolic and diastolic pressure are measured in millimeters of mercury (abbreviated mm Hg) using an instrument called a sphygmomanometer. This instrument consists 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, a health-care professional listens to a stethoscope applied to an artery in the lower arm. As the cuff inflates, it gradually compresses the artery. The point at which the cuff stops the circulation and at which no pulsations can be heard through the stethoscope 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. Normal blood pressure in an adult is less than 120/80 mm Hg. The first number describes systolic pressure, while the second number describes diastolic pressure.

Blood pressure is influenced by a wide range of factors and varies between individuals and in the same individual at different times. For instance, blood pressure naturally increases with age because the arteries lose the elasticity that, in younger people, absorbs the force of heart contractions. Other factors, such as emotions, exercise, or stress, may temporarily raise blood pressure.

Abnormally high blood pressure, known as hypertension, that remains untreated can lead to stroke, heart attack, and kidney or heart failure. Hypertension may have no known cause or it may result from heart or blood vessel disorders or from diseases affecting other parts of the body. Abnormally low blood pressure, known as hypotension, may be caused by shock, malnutrition, or some other disease or injury.