Blood pressure may be defined as the pressure exerted by the blood against the vessel walls which contain it. This pressure is greatest in the arteries during ventricular contraction (systole), and is called systolic pressure. During ventricular relaxation (diastole), blood pressure falls, reaching a minimum pressure just prior to the next systole. The minimum pressure is referred to as diastolic pressure.

Average arterial pressure during systole is about 120 mm Hg in an adult, while the diastolic pressure is about 80 mm Hg. This is normally expressed as blood pressure of 120/80, the upper number indicating systolic pressure and the lower number indicating the diastolic pressure. From these figures it is obvious that there is a fluctuation in blood pressure during each heart beat. The difference between these two pressures, 40 mm Hg, is termed pulse pressure.

Blood pressure is measured by an apparatus called the sphygmomanometer, which measures blood pressure in an artery. When the blood pressure is taken, the cuff is wrapped around the arm just above the elbow over the brachial artery, and air is pumped into the bag. A stethoscope is placed over the artery so that the pulse can be read. Air is pumped untill flow stops in that artery and the sounds cease. Now the bag is slowly deflated until blood starts to flow and the pulse (sound) can just be heard. The manometer reading at this time indicates systolic pressure. Deflation of the bag is continued and a reading just before the last pulse sound is taken. This indicates diastolic pressure i.e. lowest arterial pressure caused by the ventricular diastole

Extreme deviations from the normal blood pressure (120/80) are indications of a heart malfunction, unusual blood volume in the system, arterial inelasticity (arteriosclerosis), kidney disease, etc. A decrease in the blood pressure below the normal level is referred to as hypotension or low blood pressure. This may happen due to low blood volume or sometimes due to a defect of the heart. A persistant increase in blood pressure above the normal level (say 160/95), is referred to as hypertension or high blood pressure. People with high blood pressure are susceptible to stroke, heart diseases, kidney failure, headaches, etc.