With each contraction, the left ventricle ejects a volume of blood into the aorta that then perfuses the arterial tree. As the ensuing pressure wave moves rapidly through the arterial system it generates the arterial pulse. Although the pressure wave travels quickly, many times faster than the blood itself, a palpable delay between ventricular contraction and peripheral pulses makes the pulses in the arms and legs unsuitable for timing events in the cardiac cycle.
Blood pressure in the arterial system varies during the cardiac cycle, peaking in systole and falling to its lowest trough in diastole (Fig. 1). These are the levels that are measured with the blood pressure cuff, or sphygmomanometer. The difference between systolic and diastolic pressures is known as the pulse pressure.
Factors Affecting Blood Pressure
● Left ventricular stroke volume
● Distensibility of the aorta and the large arteries
● Peripheral vascular resistance, particularly at the arteriolar level
● Volume of blood in the arterial system
FIGURE 1. Blood pressure and pulse pressure in the cardiac cycle.
Changes in any of these four factors alter systolic pressure, diastolic pressure, or both. Blood pressure levels fluctuate strikingly throughout any 24-hour period, varying with physical activity, emotional state, pain, noise, environmental temperature, use of coffee, tobacco, and other drugs, and even time of day.
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