Measuring blood pressure

Systolic and Diastolic Pressure

The pumping of the heart allows blood to flow to all parts of the body.  For this to occur efficiently, pressure is generated due the contraction of the atria and the ventricles.  Both the atria contract together and empty blood into the ventricles.  The ventricles, in turn, contract simultaneously and force blood through the arteries into different parts of the body.  Both the atria and ventricles relax briefly before the cycle repeats itself.

Thus, when the ventricles contract, the blood pressure is relatively high and is called the systolic pressure.  When the ventricles relax, the blood pressure is relatively low and is called the diastolic pressure.

Measuring Blood Pressure

A stethoscope and sphygmomanometer are routinely used to measure diastolic and systolic pressure.  The different sounds of blood flow through the arm’s main artery correspond to the contraction and relaxation of the heart.

Systolic pressure is measured by applying pressure to the artery through the inflation of acuff.  The pressure in the cuff is slowly released, and as it descends it reaches a point where the blood can spurt with each pulse through the constricted artery, creating a tapping sound.  This spurting occurs only when the blood pressure is highest, which is during ventricular contraction.

Diastolic pressure is measured after continuing to let the air out the cuff.  The first moment when the sounds are no longer audible indicates that blood is flowing continuously through the artery.  This is the lower blood pressure reading - the diastolic pressure - and it occurs during ventricular relaxation.