Figure 1434

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Pressures in the vascular system.

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

TABLE 14-7 Functions of Endothelial Cells

1. Serve as a physical lining of heart and blood vessels to which blood cells do not normally adhere.

2. Serve as a permeability barrier for the exchange of nutrients, metabolic end products, and fluid between plasma and interstitial fluid; regulate transport of macromolecules and other substances.

3. Secrete paracrine agents that act on adjacent vascular smooth-muscle cells; these include vasodilators—prostacyclin and nitric oxide (endothelium-derived relaxing factor, EDRF)—and vasoconstrictors—notably endothelin-1.

4. Mediate angiogenesis (new capillary growth).

5. Play a central role in vascular remodeling by detecting signals and releasing paracrine agents that act on adjacent cells in the blood vessel wall.

6. Contribute to the formation and maintenance of extracellular matrix (Chapter 1).

7. Produce growth factors in response to damage.

8. Secrete substances that regulate platelet clumping, clotting, and anticlotting.

9. Synthesize active hormones from inactive precursors (Chapter 16).

10. Extract or degrade hormones and other mediators (Chapter 15).

11. Secrete cytokines during immune responses (Chapter 20).

12. Influence vascular smooth-muscle proliferation in the disease atherosclerosis.

These principles can be applied to an analysis of arterial blood pressure. The contraction of the ventricles ejects blood into the pulmonary and systemic arteries during systole. If a precisely equal quantity of blood were to flow simultaneously out of the arteries, the total volume of blood in the arteries would remain constant and arterial pressure would not change. Such is not the case, however. As shown in Figure 14-35, a volume of blood equal to only about one-third the stroke volume leaves the arteries during systole. The rest of the stroke volume remains in the arteries during systole, distending them and raising the arterial pressure. When ventricular contraction ends, the stretched arterial walls recoil passively, like a stretched rubber band being released, and blood continues to be driven into the arterioles during diastole. As blood leaves the arteries, the arterial volume and therefore the arterial pressure slowly fall, but the next ventricular contraction occurs while there is still adequate blood in the arteries to stretch them partially. Therefore, the arterial pressure does not fall to zero.

The aortic pressure pattern shown in Figure 14-36a is typical of the pressure changes that occur in all the large systemic arteries. The maximum arterial pressure reached during peak ventricular ejection is called systolic pressure (SP). The minimum arterial pressure occurs just before ventricular ejection begins and is called diastolic pressure (DP). Arterial pressure is generally recorded as systolic/diastolic—that is, 125/75 mmHg in our example (see Figure 14-36b for average values at different ages in the population of the United States).

The difference between systolic pressure and diastolic pressure (125 — 75 = 50 mmHg in the example) is called the pulse pressure. It can be felt as a pulsation or throb in the arteries of the wrist or neck with each heartbeat. During diastole, nothing is felt over the

Movement Blood Cardiac Cycle

FIGURE 14-35

Movement of blood into and out of the arteries during the cardiac cycle. The lengths of the arrows denote relative quantities flowing into and out of the arteries and remaining in the arteries.

FIGURE 14-35

Movement of blood into and out of the arteries during the cardiac cycle. The lengths of the arrows denote relative quantities flowing into and out of the arteries and remaining in the arteries.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

Systolic pressure

-

- Aortic valve

closure

Mean pressure

-Diastolic

pressure

Time

Cardiac Cycle Guyton

Age (years)

FIGURE 14-36

(a) Typical arterial pressure fluctuations during the cardiac cycle. (b) Changes in arterial pressure with age in the U.S. population.

Adapted from Guyton.

Age (years)

FIGURE 14-36

(a) Typical arterial pressure fluctuations during the cardiac cycle. (b) Changes in arterial pressure with age in the U.S. population.

Adapted from Guyton.

artery, but the rapid rise in pressure at the next systole pushes out the artery wall, and it is this expansion of the vessel that produces the detectable throb.

The most important factors determining the magnitude of the pulse pressure—that is, how much greater systolic pressure is than diastolic—are (1) stroke volume, (2) speed of ejection of the stroke volume, and (3) arterial compliance. Specifically, the pulse pressure produced by a ventricular ejection is greater if the volume of blood ejected is increased, if the speed at which it is ejected is increased, or if the arteries are less compliant. This last phenomenon occurs in atherosclerosis, the "hardening" of the arteries that progresses with age and accounts for the increasing pulse pressure seen so often in older people.

It is evident from Figure 14-36a that arterial pressure is continuously changing throughout the cardiac cycle. The average pressure (mean arterial pressure,

MAP) in the cycle is not merely the value halfway between systolic pressure and diastolic pressure because diastole usually lasts longer than systole. The true mean arterial pressure can be obtained by complex methods, but for most purposes it is approximately equal to the diastolic pressure plus one-third of the pulse pressure (SP — DP), largely because diastole lasts about twice as long as systole:

Thus, in our example: MAP = 75 + 1/3 (50) = 92 mmHg.

The MAP is the most important of the pressures described because it is the pressure driving blood into the tissues averaged over the entire cardiac cycle. We can say mean "arterial" pressure without specifying to which artery we are referring because the aorta and other large arteries have such large diameters that they offer only negligible resistance to flow, and the mean pressures are therefore similar everywhere in the large arteries.

One additional important point should be made: We have stated that arterial compliance is an important determinant of pulse pressure, but for complex reasons, compliance does not influence the mean arterial pressure. Thus, for example, a person with a low arterial compliance (due to atherosclerosis) but an otherwise normal cardiovascular system will have a large pulse pressure but a normal mean arterial pressure. The determinants of mean arterial pressure are described in Section E.

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  • venla
    What are the three most important factors determining the magnitude of the pulse pressure?
    8 years ago

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