The heart pumps blood to various organs with a force, which exerts pressure on the walls of the arteries. Hence, blood pressure is defined as the pressure exerted by the blood flow upon the walls of the blood vessel and is measured in millimetres of mercury (mmHg). 120/80 mmHg is considered as the normal reading for blood pressure, where 120 is the pressure when the heart beats to pump out blood (systolic pressure) and 80 is the pressure when the heart rests between beats (diastolic pressure). A person is said to have hypertension when the blood pressure readings are 140/90 mmHg or higher. Read about 10 myths about hypertension.