The first documented measurement of blood pressure was accomplished by Stephen Hales (1677-1761), an English clergyman and physiologist. Hales inserted a cannula into the artery of a horse and measured the heights to which blood would rise in the vertical tube. The height of this blood column bounced between the systolic pressure at its highest and the diastolic pressure at its lowest, as the heart went through its cycle of systole and diastole. Modern clinical blood pressure measurements, fortunately, are less direct. The indirect, or auscultatory, method is based on the correlation of blood pressure and arterial sounds.
In the auscultatory method, an inflatable rubber bladder within a cloth cuff is wrapped around the upper arm, and a stethoscope is applied over the brachial artery (fig. 14.28). The artery is normally silent before inflation of the cuff because blood normally travels in a smooth, laminar flow through the arteries. The term laminar means "layered"— blood in the central axial stream moves the fastest, and blood
Cardiac Output, Blood Flow, and Blood Pressure
Cardiac Output, Blood Flow, and Blood Pressure
■ Figure 14.28 A pressure cuff and sphygmomanometer are used to measure blood pressure. The examiner is listening for the Korotkoff sounds.
flowing closer to the artery wall moves more slowly. There is little transverse movement between these layers that would produce mixing.
The laminar flow that normally occurs in arteries is smooth and silent. When the artery is pinched, however, blood flow through the constriction becomes turbulent. This causes the artery to produce sounds, much like the sounds produced by water flowing through a kink in a garden hose. The tendency of the cuff pressure to constrict the artery is opposed by the blood pressure. Thus, in order to constrict the artery, the cuff pressure must be greater than the diastolic blood pressure. If the cuff pressure is also greater than the systolic blood pressure, the artery will be pinched off and silent. Turbulent flow and sounds produced by the artery as a result of this flow, therefore, occur only when the cuff pressure is greater than the diastolic blood pressure and lower than the systolic pressure.
Let's say that a person has a systolic pressure of 120 mmHg and a diastolic pressure of 80 mmHg (the average normal values). When the cuff pressure is between 80 and 120 mmHg, the artery will be closed during diastole and open during systole. As the artery begins to open with every systole, turbulent flow of blood through the constriction will create vibrations that are known as the sounds of Korotkoff, as shown in figure 14.29. These are usually "tapping" sounds because the artery becomes constricted, blood flow stops, and silence is restored with every diastole. It should be understood that the sounds of Korotkoff are not "lub-dub" sounds produced by closing of the heart valves (those sounds can be heard only on the chest, not on the brachial artery).
Initially, the cuff is usually inflated to produce a pressure greater than the systolic pressure, so that the artery is pinched off and silent. The pressure in the cuff is read from an attached meter called a sphygmomanometer. A valve is then turned to allow the release of air from the cuff, causing a gradual decrease in cuff pressure. When the cuff pressure is equal to the systolic pressure, the first Korotkoff sound is heard as blood passes in a turbulent flow through the constricted opening of the artery.
Korotkoff sounds will continue to be heard at every systole as long as the cuff pressure remains greater than the dia-stolic pressure. When the cuff pressure becomes equal to or less than the diastolic pressure, the sounds disappear because the artery remains open and laminar flow resumes. (fig. 14.30). The last Korotkoff sound thus occurs when the cuff pressure is equal to the diastolic pressure.
Different phases in the measurement of blood pressure are identified on the basis of the quality of the Korotkoff sounds (fig. 14.31). In some people, the Korotkoff sounds do not disappear even when the cuff pressure is reduced to zero (zero pressure means that it is equal to atmospheric pressure). In these cases—and often routinely—the onset of muffling of the sounds (phase 4 in fig. 14.31) is used as an indication of diastolic pressure rather than the onset of silence (phase 5).
The average arterial blood pressure in the systemic circulation is 120/80 mmHg, whereas the average pulmonary arterial blood pressure is only 22/8 mmHg. Because of the Frank-Starling relationship, the cardiac output from the right ventricle into the pulmonary circulation is matched to that of the left ventricle into the systemic circulation. Since the cardiac outputs are the same, the lower pulmonary blood pressure must be caused by a lower peripheral resistance in the pulmonary circulation. Because the right ventricle pumps blood against a lower resistance, it has a lighter workload and its walls are thinner than those of the left ventricle.
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