Christopher Crawford, age 11, of Portland, Ore., for his question:
WHAT IS NORMAL BLOOD PRESSURE?
Low blood pressure is called hypotension, a condition that rarely indicates serious disease. In most cases, doctors give no treatment at all to those with low blood pressure.
Blood pressure usually rises with age because the arteries become less elastic when a person gets older, and this condition will often slow down the flow of the blood.
Blood pressure is the pressure that blood exerts against the walls of the arteries. The amount depends on the strength and rate of the heart's contraction, the volume of blood in the body and also the elasticity of the arteries.
Blood pressure is taken by an instrument called a sphygmomanometer. The equipment includes a glass tube with mercury, a cuff or wide rubber band that can be filled with air, and a hollow rubber ball which is used to pump air into the cuff.
A doctor takes a blood pressure reading by wrapping the cuff around a patient's arm and placing a stethoscope over the arteries just below the cuff. He pumps air into the cuff which presses down on the arteries and stops the flow of blood. The doctor then slowly lets the air 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.
The doctor determines the systolic pressure by reading the scale on the mercury tube. He then lets more air out of the cuff until the sound becomes muffled. The pressure at this point is called the diastolic pressure. It represents the blood pressure while the heart is relaxing.
Blood pressure is measured with two numbers, such as 120/80. The first number refers to systolic pressure and the second number refers to diastolic pressure. Normal systolic blood pressure for adults is about 120 millimeters. Pressures over 150 millimeters are considered high. Doctors also consider diastolic pressures over 90 millimeters to be high.
Doctors call high blood pressure essential hypertension when its cause is unknown.
Regular medical checkups will include testing of the blood pressure. If it is too high, diet and medication will often be prescribed to correct the problem.
Scientists synthesized, or made chemically, the s substance in the blood believed to cause high blood pressure. This discovery was made in 1957, and since that time researchers have been using the substance to study causes of hypertension. The chemical substance is called angiotensin II.