The pressure of the arterial blood is regulated by the blood volume, total peripheral resistance, and the cardiac rate. Regulatory mechanisms adjust these factors in a negative feedback manner to compensate for deviations. Arterial pressure rises and falls as the heart goes through systole and diastole.
Resistance to flow in the arterial system is greatest in the arterioles because these vessels have the smallest diameters. Although the total blood flow through a system of arterioles must be equal to the flow in the larger vessel that gave rise to those arterioles, the narrow diameter of each arteriole reduces the flow in each according to Poiseuille's law. Blood flow and pressure are thus reduced in the capillaries, which are located downstream of the high resistance imposed by the arterioles. (The slow velocity of blood flow through capillaries enhances diffusion across the capillary wall.) The blood pressure upstream of the arterioles—in the medium and large arteries—is correspondingly increased (fig. 14.23).
The blood pressure and flow within the capillaries are further reduced by the fact that their total cross-sectional area is much greater, due to their large number, than the cross-sectional areas of the arteries and arterioles (fig. 14.24). Thus, although each capillary is much narrower than each arteriole, the capillary beds served by arterioles do not provide as great a resistance to blood flow as do the arterioles.
Variations in the diameter of arterioles as a result of vasoconstriction and vasodilation thus affect blood flow through capillaries and, simultaneously, the arterial blood pressure "upstream" from the capillaries. In this way, an increase in total peripheral resistance due to vasoconstriction of arterioles can raise arterial blood pressure. Blood pressure can also be raised
Clinical Investigation Clues
Remember that Charlie's skin was cold to the touch.
■ What does this indicate about his cutaneous blood flow?
■ What produced this effect?
■ What benefit does Charlie derive from this mechanism?
Test Yourself before You Continue
1. Define the term autoregulation and describe how this process is accomplished in the cerebral circulation.
2. Explain how hyperventilation can cause dizziness.
3. Explain how cutaneous blood flow is adjusted to maintain a constant deep-body temperature.
■ Figure 14.23 The effect of vasoconstriction on blood pressure. A constriction increases blood pressure upstream (analogous to the arterial pressure) and decreases pressure downstream (analogous to capillary and venous pressure).
430 Chapter Fourteen
■ Figure 14.24 The relationship between blood pressure and the cross-sectional area of vessels. As blood passes from the aorta to the smaller arteries, arterioles, and capillaries, the cross-sectional area increases as the pressure decreases.
by an increase in the cardiac output. This may be due to elevations in cardiac rate or in stroke volume, which in turn are affected by other factors. The three most important variables affecting blood pressure are the cardiac rate, stroke volume (determined primarily by the blood volume), and total peripheral resistance. An increase in any of these, if not compensated for by a decrease in another variable, will result in an increased blood pressure.
Arterial blood ^ pressure cardiac output x cardiac output
Cardiac rate total peripheral resistance
Blood pressure can thus be regulated by the kidneys, which control blood volume and thus stroke volume, and by the sympa-thoadrenal system. Increased activity of the sympathoadrenal
■ Figure 14.25 The effect of blood pressure on the baroreceptor response. This is a recording of the action potential frequency in sensory nerve fibers from baroreceptors in the carotid sinus and aortic arch. As the blood pressure increases, the baroreceptors become increasingly stretched. This results in a higher frequency of action potentials transmitted to the cardiac and vasomotor control centers in the medulla oblongata.
system can raise blood pressure by stimulating vasoconstriction of arterioles (thus raising total peripheral resistance) and by promoting an increased cardiac output. Sympathetic stimulation can also affect blood volume indirectly, by stimulating constriction of renal blood vessels and thus reducing urine output.
Blood pressure is measured in units of millimeters of mercury (mm Hg). In performing this measurement, the blood pushes on one surface of a U-shaped column of mercury while the atmosphere pushes on the other surface (see chapter 16, fig. 16.19). If the blood pressure were equal to the atmospheric pressure, the measurement would be zero mm Hg. For the same reason, a mean arterial pressure of 100 mm Hg indicates that the blood pressure is 100 mm Hg higher than the atmospheric pressure. Instruments used to measure blood pressure, called sphygmomanometers, thus contain mercury or are spring-loaded devices that are calibrated against mercurial instruments.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.