The pressure at which blood is pumped around the body by the heart is called blood pressure. The blood pressure of a person is always expressed in the form of two values called ‘systolic pressure’ and ‘diastolic pressure’. In order to understand this, we should first know the meaning of ‘systole’ and ‘diastole’.

The phase of the heart beat when the heart contracts and pumps the blood into arteries is called ‘systole’. And the phase of heart beat when the heart relaxes (or expands) and allows the chambers to fill with blood is called ‘diastole’.

The maximum pressure, at which the blood leaves the heart through the main artery (aorta) during contraction phase, is called the systolic pressure. This high pressure in the main artery maintains a steady flow of blood in all the arteries towards the capillaries.

The minimum pressure in the arteries during the relaxation phase of heart is called the diastolic pressure. The value of diastolic pressure is always lower than that of the systolic pressure. The blood pressure of a person is expressed in terms of millimetres of mercury (which is written as mm Hg). The normal blood pressure values are:


Systolic pressure: 120 mm Hg Diastolic pressure: 80 mm Hg

This is usually written as 120/80

The blood pressure values vary from person to person and from time to time. They also vary with age. For example, a young person may have blood pressure of 110/75 but at the age of 60 years it could be 145/90. High blood pressure is called hypertension. High blood pressure is caused by the constriction (narrowing) of very small arteries (called arterioles) which results in increased resistance to blood flow. Very high blood pressure can lead to rupture of an artery and internal bleeding.

How to Measure Blood Pressure:


Blood pressure is measured by using an instrument called sphygmomanometer. Two readings of blood pressure are taken: systolic pressure (when the heart is contracting and pumping out blood), and diastolic pressure (when the heart relaxes and fills with blood). The various steps in measuring the blood pressure of a person are as follows:

(i) A rubber cuff (which is a flat rubber tube) is wrapped around the person’s arm. The rubber cuff is inflated by pumping air into it to give a pressure of about 200 mm Hg to the brachial artery (which runs down the arm). This pressure can be seen on the scale of the instrument sphygmomanometer. If a stethoscope is now placed on the artery of the arm, no sound is heard through it.

(ii) With stethoscope still placed on artery, the cuff pressure is reduced gradually by deflating it. The cuff pressure when the heart beat is first heard as a soft tapping sound through the stethoscope gives us the systolic pressure.

(iii) The cuff pressure is reduced further by deflating it more and more. The cuff pressure when the tapping sound in stethoscope just disappears, gives us the diastolic pressure.


The above observations can be explained as follows: When a high pressure of about 200 mm Hg is applied to the arm by the cuff, then the brachial artery gets closed fully and hence no blood flows in it. Since no blood flows in the brachial artery at this stage, therefore, no tapping sound is heard in the stethoscope.

When the cuff pressure is reduced and becomes equal to the systolic pressure, then the brachial artery opens up slightly and there is an intermittent blood flow in it due to which a soft tapping sound just begins to be heard in the stethoscope. And finally, when the cuff pressure is reduced further and it becomes equal to diastolic pressure, then the brachial artery opens up fully, the blood flow in it is fully restored and hence the tapping sound just disappears.