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FIGURE 17 As blood flows from the root of the aorta to the vena cava, the pressure within the vascular system decreases. The decrease is due to the resistance to blood flow. As this figure illustrates, the greatest decrease in pressure (due to large resistance) occurs in the arterioles. There is little resistance to flow in the large arteries or in the veins.

FIGURE 17 As blood flows from the root of the aorta to the vena cava, the pressure within the vascular system decreases. The decrease is due to the resistance to blood flow. As this figure illustrates, the greatest decrease in pressure (due to large resistance) occurs in the arterioles. There is little resistance to flow in the large arteries or in the veins.

(TPR) would increase. By Ohm's law:

Mean arterial pressure = CO x TPR (19)

If, on the other hand, stimulation were confined to a single organ, the effect on TPR and thus mean arterial blood pressure might be small. Blood flow to the stimulated area would be reduced, whereas that to all other tissues would remain the same. Modulation of vascular resistance is a primary means of controlling pressures and flow in the cardiovascular system.

Pressure Differs in Each Vessel Type

Time

FIGURE 16 Arterial blood under constant pressure is introduced into an artery supplying an organ. The sympathetic nerves to that organ are stimulated at various rates. Note that flow falls because the resistance to flow increases with stimulation rate. These nerves cause constriction of vascular smooth muscle in the arterioles which decreases their caliber.

Time

FIGURE 16 Arterial blood under constant pressure is introduced into an artery supplying an organ. The sympathetic nerves to that organ are stimulated at various rates. Note that flow falls because the resistance to flow increases with stimulation rate. These nerves cause constriction of vascular smooth muscle in the arterioles which decreases their caliber.

Because total energy for blood flow through the blood vessels comes from the heart, total energy (expressed as end pressure) is highest in the aorta. As blood flows through the vessels, energy is lost because of friction, and total energy will be lowest in the vena cava. However, as one might suspect from the preceding discussion of resistances in the system, the drop in energy is not the same across each vessel type. Figure 17 illustrates the mean pressure recorded at all levels throughout the system. Because the vessel types are essentially in series with each other in each organ, the resistance of each vessel type will be proportional to the drop in pressure across it. As mentioned previously, little pressure is lost as blood moves through the larger

10. Hemodynamics

Clinical Note

Measuring Blood Pressure

Blood pressure is measured in the human noninvasively with a device called a sphygmomanometer. Basically, an inflatable cuff is wrapped around the upper arm and inflated to about 170 mm Hg. A stethoscope is placed over the brachial artery just below the cuff and the air is slowly let out of the cuff. To begin with, the brachial artery is collapsed by the pressure. As the pressure in the cuff falls below the systolic blood pressure, the vessel pops open briefly during each systole. As the flow starts and stops it is turbulent and audible. As cuff pressure falls below diastolic pressure, the vessel remains patent throughout the cardiac cycle and the sounds disappear. The cuff pressure where the sounds are first heard is taken to be the patient's systolic pressure and the pressure at which they disappear as the diastolic pressure.

arteries and veins. Those vessels serve as efficient conduits, bringing blood to and away from all parts of the body. The greatest pressure drop occurs across the arterioles. These vessels also have abundant smooth muscle in their walls that can change their resistance to flow moment to moment. Because they constitute a major portion of the resistance to flow of an organ, arterioles are the ideal site for local regulation of blood flow.

Arterial Pressure is Pulsatile

The major energy source for blood flow in the cardiovascular system is the heart. During systole the heart ejects blood into the aorta, raising the pressure. During the rest of the cycle (diastole), the aortic pressure is falling as the heart refills with blood from the veins. The shaded areas depicted in Fig. 17 are meant to illustrate the pulsations in pressure. Note that the pulsations are damped out in the microcirculation. These pulsations are better appreciated in Fig. 18 where pressure in an artery is plotted against time. The difference between the peak pressure (systolic) and the minimum pressure (diastolic) over the cycle is referred to as the pulse pressure. The mean pressure refers to the average pressure with respect to time as indicated by the dotted line labeled 100 in Fig. 18. If the aortic pressure varied as a perfect sign wave then the mean aortic pressure would be simply halfway between the systolic and diastolic pressure. Unfortunately, the aortic pressure has a very peculiar shape. In practice, the mean aortic pressure can be closely approximated by adding 1/3 of the pulse pressure to the diastolic pressure.

The basis for the pulse pressure is as follows. The equation for compliance states that the pressure in the distensible arterial tree at any instant depends on the volume of fluid in those vessels divided by their capacitance. Because arterial compliance changes little from moment to moment, the factor that determines arterial pressure is the volume of blood in the arterial tree. This volume depends on the balance between inflow from the heart and outflow through the resistance vessels. Inflow occurs primarily during the rapid ejection period of the heart (see Chapter 13) while outflow depends on the peripheral resistance and the aortic pressure and occurs throughout the cardiac cycle. Because the rapid ejection period of systole is so brief, little outflow occurs during that period, and it is as if the entire stroke volume suddenly appears in the aorta. Thus, pulse pressure (PP) will be closely approximated by:

Root of the Aorta

20cm from Aortic Valve

VV/WlA/V

40cm from Aortic Valve i

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