## Blood Pressure

Standard Units of Pressure. Blood pressure almost always is measured in millimeters of mercury (mmHg) because the mercury manometer (shown in Figure 14-7) has been used since antiquity as the standard

Recording arterial pressure with a mercury manometer, a method that has been used in the manner shown for recording pressure throughout the history of physiology.

reference for measuring pressure. Actually, blood pressure means the force exerted by the blood against any unit area of the vessel wall. When one says that the pressure in a vessel is 50 mm Hg, one means that the force exerted is sufficient to push a column of mercury against gravity up to a level 50 mm high. If the pressure is 100 mm Hg, it will push the column of mercury up to 100 millimeters.

Occasionally, pressure is measured in centimeters of water (cm HJO). A pressure of 10 cm H2O means a pressure sufficient to raise a column of water against gravity to a height of 10 centimeters. One millimeter of mercury pressure equals 1.36 cm water pressure because the specific gravity of mercury is 13.6 times that of water, and 1 centimeter is 10 times as great as 1 millimeter.

High-Fidelity Methods for Measuring Blood Pressure. The mercury in the mercury manometer has so much inertia that it cannot rise and fall rapidly. For this reason, the mercury manometer, although excellent for recording steady pressures, cannot respond to pressure changes that occur more rapidly than about one cycle every 2 to 3 seconds. Whenever it is desired to record rapidly changing pressures, some other type of pressure recorder is needed. Figure 14-8 demonstrates the basic principles of three electronic pressure transducers commonly used for converting blood pressure and/or rapid changes in pressure into electrical signals and then recording the electrical signals on a high-speed electrical recorder. Each of these transducers uses a very thin, highly stretched metal membrane that forms one wall of the fluid chamber. The fluid chamber in turn is connected through a needle or catheter to the blood vessel in which the pressure is to be measured. When the pressure is high, the membrane bulges slightly, and when it is low, it returns toward its resting position.