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Blood pressure is the pressure that blood exerts against the walls of the arteries. The amount of pressure depends upon the strength and rate of the heart's contraction, the volume of blood in the circulatory system, and the elasticity of the arteries. Blood pressure is measured with an instrument called a sphygmomanometer. It consists of (1) a cuff or wide rubber band that can be filled with air; (2) a hollow rubber bulb, which pumps air into the cuff; and (3) a gauge, or a glass tube containing a column of mercury. To take blood pressure, the cuff is wrapped around the patient's arm. A stethoscope is placed over the arteries of the arm just below the cuff. The pulsations of blood in the arteries can then be heard. Air is pumped into the cuff, causing it to press down on the arteries. This stops the flow of blood, and the sounds stop. Then air is slowly let out of the cuff. When the pressure of the cuff becomes less than the blood pressure, the blood flow returns. The pressure at which the flow of blood resumes is called the systolic pressure. It represents the blood pressure when the heart is contracting. This pressure is determined by reading the gauge or the scale on the mercury tube. As more air is let out of the cuff, the sounds become muffled. The pressure at this point is called the diastolic pressure. It represents the blood pressure while the heart is relaxing. Measurements of blood pressure consist of two numbers, such as 120/80. The first number refers to the systolic pressure, and the second number to the diastolic pressure. Normal systolic blood pressure for adults is about 120 millimetres. Some doctors consider pressures over 140 millimetres to be high. They also consider diastolic pressures of over 90 millimetres to be high. Other doctors believe a reading greater than 150/95 indicates high blood pressure. Strong emotion, such as anger or fear, may lead to a temporary rise in blood pressure. Severe shock may cause a sharp fall in blood pressure. Blood pressure usually rises with age
because the arteries become less elastic and slow down the flow of blood.
High blood pressure may cause heart failure, a stroke, or kidney failure.
Doctors call high blood pressure essential hypertension when its cause
is unknown. In 1957, scientists synthesized (made chemically) a substance
in the blood believed to cause high blood pressure. Researchers use
this substance, called angiotensin II, to study causes of hypertension. |
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