Factors Influencing Flow and Pressure

The relationship between blood flow and blood pressure is one of the chief topics of this book. An engineer who is familiar with constant stroke volume pumps might imagine that flow from the heart is a simple function of heart rate. An engineer who designs pressure controllers may think of flow to a given capillary bed as related directly to the hydraulic resistance in that bed. In fact, the heart can sometimes be modeled as a constant stroke volume, variable-speed pump, but the system containing the heart and circulatory system is a complex network with a control system that includes pressure inputs, flow inputs, as well as neural and chemical feedback.

The pulse pressure in the aorta can be thought of chiefly as a function of heart rate, peripheral resistance, and stroke volume.

Figure 2.13 shows the effect of heart rate on pressure. If peripheral resistance and stroke volume remain constant, an increase in heart rate causes the heart to pump more blood into the aorta over a fixed time-period, and systolic blood pressure increases. At the same time, the increased rate of pumping leaves less time between heartbeats for the pressure in arteries to decrease, simultaneously the diastolic pressure also increases. As the heart rate increases, with fixed peripheral resistance and fixed stroke volume, cardiac output, or flow into the systemic circulation, increases.

On the other hand, if heart rate and peripheral resistance are constant, but stroke volume changes, pressure and flow are also affected. Figure 2.14 shows the effect of stroke volume on pressure. An increase in stroke volume increases pressure because of the extra volume of blood that is pumped into the aorta during each heart beat. However, a healthy human would not normally experience a case of constant heart rate and constant peripheral resistance with increasing stroke volume. As shown in the figure, under resting conditions heart rate drops and

P

n pd2

Rate = 2n

ps3 pd3

Rate = 2n/3

fs1 fd1 fs2 fd2 fs3 fd3

Time

Figure 2.13 Aortic pressures as a function of heart rate. PS is the systolic pressure and PD is the diastolic pressure. Picture 2, the middle picture, shows the fastest heart rate and the picture on the right, picture 3, shows the slowest heart rate.

Ps1 Pdi

\ Normal

\

stroke volume

\

i s

Increased

stroke volume

ps3

pd3

i s

Decreased

stroke volume

i s

fs1 fd1 fs2 fd2 fs3 fd3

Time

Figure 2.14 Shows the effect of stroke volume on pressure. PS represents pressure at systole and PD represents pressure at diastole.

peripheral resistance increases. The result is that a large stroke volume, as might be seen in well trained athletes, increases systolic pressure or perhaps maintains it at a steady level, while the decreased heart rate allows more time between beats and the diastolic pressure decreases. A very small stroke volume, as might be seen in a patient with severe heart damage, would normally result in an increased heart rate as the cardiovascular system attempts to maintain cardiac output although a very small amount of blood is ejected from the left ventricle during each heart beat.

The third parameter that controls blood pressure and flow is peripheral resistance. This peripheral resistance comes chiefly from the resistance found in capillary beds and the more capillaries that are open, the lower the resistance will be. If heart rate and stroke volume remain constant as the need for oxygenated blood increases in peripheral tissues, more capillaries open to allow more blood to flow into the tissue and this corresponds to a decrease in peripheral resistance. Figure 2.15 shows

Psi Pdi

Normal v. peripheral \resistance

i s

ps2 pd2

Increased peripheral . resistance i s

Decreased peripheral resistance i s

Time

Figure 2.15 Shows aortic pressure as a function of peripheral resistance while heart rate and stroke volume are held constant.

that blood pressure increases with increasing peripheral resistance when heart rate and stroke volume are held constant.

Heart rate, stroke volume, and peripheral resistance are the primary factors that control the relationship between blood pressure and flow, or cardiac output, but there are other factors which contribute. For example, in the case of diseased heart valves, blood leakage backward through the valve into the heart can alter the relationship. This type of leakage through a valve is known as regurgitance. Regurgitance in heart valves has a similar effect to that of decrease in stroke volume. Blood that has been ejected into the aorta can flow backwards into the left ventricle through a faulty aortic valve resulting in a decreased effective stroke volume.

Arterial compliance, a characteristic that is related to vessel stiffness, can also affect the pressure flow relationship. Very stiff arteries increase the overall hydrodynamic resistance of the system. This increase in resistance causes a decrease in blood flow to the peripheral tissue. The complex control system in the cardiovascular system will result in compensation that increases heart rate and therefore increases blood pressure to compensate for the increased resistance.

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