Friday, September 26, 2008

Blood Pressure

Blood pressure is the pressure exerted by circulating blood on the walls of the arteries, and it constitutes one of the main vital signs. Blood pressure results from two forces. One is created by the heart as it pumps blood into the arteries and through the circulatory system. The other is the force of the arteries as they resist the blood flow.

The term blood pressure generally refers to arterial pressure, which is the pressure in the larger arteries, the blood vessels that take blood away from the heart. Arterial pressure is most commonly measured via a sphygmomanometer, which historically used the height of a column of mercury to reflect the circulating pressure. Today blood pressure values are reported in millimeters of mercury (mmHg), though aneroid and electronic sphygmomanometers do not use mercury.

The systolic arterial pressure is defined as the peak pressure in the arteries, which occurs when the heart contracts to pump blood to the body. The diastolic arterial pressure is the lowest pressure when the heart relaxes between beats. The average pressure throughout the cardiac cycle is reported as mean arterial pressure. The pulse pressure reflects the difference between the maximum and minimum pressures measured.

Typical values for a healthy adult human are approximately 120 mmHg systolic and 80 mmHg diastolic, written as 120/80 mmHg, and spoken as one twenty over eighty, with large individual variations. These measures of arterial pressure are not static, but undergo natural variations from one heartbeat to another and throughout the day. They also change in response to stress, nutritional factors, drugs, or disease.

Measurement of blood pressure: Arterial pressures can be measured invasively and non-invasively. The former is usually restricted to a hospital setting, while the latter is more common, simpler and quicker. The non-invasive method uses a stethoscope and a sphygmomanometer. This comprises an inflatable (Riva-Rocci) cuff placed around the upper arm at roughly the same vertical height as the heart, attached to a mercury or aneroid manometer. The cuff is inflated manually by repeatedly squeezing a rubber bulb until the artery is completely occluded. Listening with the stethoscope to the brachial artery at the elbow, the examiner slowly releases the pressure in the cuff. When blood just starts to flow in the artery, the turbulent flow creates a pounding. The pressure at which this sound is first heard is the systolic blood pressure. The cuff pressure is further released; at the point when no sound can be heard is the diastolic arterial pressure.