Factors Controlling Arterial Blood Pressure
The mean arterial blood pressure is equal to the product of the cardiac output (about 5 L/min at rest) and the total peripheral resistance (TPR). Because the total drop of mean pressure across the systemic circulation is about 100 mmHg, TPR is calculated to be 100 mmHg/5000 mL/min, or 0.02 mmHg/mL/min. The unit mmHg/mL/min is referred to as a peripheral resistance unit (PRU), so that TPR is normally about 0.02 PRU.
Systolic pressure is mainly influenced by the stroke volume, the left ventricular ejection velocity and aortic/arterial stiffness, and rises when any of these increase. Conversely, diastolic pressure rises with an increase in TPR. Arterial pressure falls progressively during diastole (see Figure 16), so that a shortening of the diastolic interval associated with a rise in the heart rate also increases diastolic pressure.
Blood Pressure and Flow in the Arteries
The blood flow in the aorta and the larger arteries is pulsatile, as a result of the rhythmic emptying of the left ventricle.
As blood is ejected from the left ventricle during systole, it hits the column of blood already present in the ascending aorta, creating a pressure wave in the aortic blood which is rapidly (at between 4 and 10 m/s) conducted towards the arterioles. As this pulse pressure wave passes each point along the aorta and the major arteries, it sets up a transient pressure gradient that briefly propels the blood at that point forward, causing a pulsatile flow wave. The blood in the arteries therefore moves forward in short bursts, separated by longer periods of stasis, so that its average velocity in the aorta is about 0.2 m/s.
The pressure wave also causes the elastic arterial wall to bulge out, thereby storing some of the energy of the wave. The arterial wall then rebounds, releasing part of this energy to drive the blood forward during diastole (diastolic run-off). This pumping mechanism of the elastic arteries is termed the Windkessel function (Figure 19).
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