Blood pressure is the pressure (force per unit area) exerted by the blood on the walls of the blood vessels. Unless indicated otherwise, blood pressure refers to the pressure in the large arteries, such as the brachial artery (in the arm). The pressure of the blood in other blood vessels differs from the arterial pressure. Measurement of pressures in the venous system and the pulmonary vessels plays an important role in intensive care medicine but requires invasive techniques.
Arterial blood pressure is usually measured in millimetres of mercury (mm Hg) using a sphygmomanometer. This is an inflatable cuff placed around the upper arm, at roughly the same vertical height as the heart in a sitting person, attached to a manometer. The cuff is inflated until the artery is completely occluded. Listening with a stethoscope to the brachial artery at the elbow, the examiner slowly releases the pressure in the cuff. When bloodflow barely begins again in the artery, a "whooshing" or pounding sound is heard. The pressure is noted at which this sound began. This is the systolic blood pressure. The cuff pressure is further released until the sound can no longer be heard. This is the diastolic blood pressure. The peak pressure in the arteries during the cardiac cycle is the systolic pressure, and the lowest pressure (at the resting phase of the cardiac cycle) is the diastolic pressure.
Normal ranges for blood pressure in adult humans are:
- Systolic between 90 and 135 mmHg
- Diastolic between 50 and 90 mmHg
If the blood pressure exceeds these normal values, one speaks of arterial hypertension. Blood pressure that is too low is known as hypotension.
Any level of blood pressure puts mechanical stress on the arterial walls. The higher the pressure, the more stress that is present. At branch points in blood vessels, this stress becomes a shearing force, capable of injuring the tender lining of the blood vessels, the endothelium. Elevated shearing force on the endothelium, along with other factors (nutritional, inflammatory, toxic) may contribute to the development of atherosclerosis.
When blood pressure is very low, the perfusion of the brain may be critically decreased (i.e. the blood supply is not sufficient), causing lightheadedness, weakness and fainting. Sometimes the blood pressure drops significantly when a patient stands up. This is known as orthostatic hypotension. Other causes of low blood pressure include:
- Toxins including toxic doses of blood pressure medicine
- Hormone abnormalities, such as Addison's disease