20

20 40 60

Arterial Pco2 (mm Hg)

FIGURE 7 Relation between cerebral blood flow and the partial pressure of CO2 (normal Pco2 = 40mmHg). Note that elevating Pco2 elicits a profound increase in blood flow.

mechanism for maintaining a constant pH in neuronal tissue, which is an important objective in view of the profound inhibitory effects of acidosis on neuronal activity.

Skeletal Muscle Circulation

The circulations of skeletal muscle and cardiac muscle have much in common; however, the two vascular beds differ in several respects. The more notable differences include a fiber type (tonic versus phasic)-based heterogeneity of O2 demand and blood perfusion in skeletal muscle, a greater role for extrinsic factors in regulating skeletal muscle blood flow, and a larger potential for increasing O2 extraction and the number of perfused capillaries in skeletal muscle. Blood flow in resting skeletal muscle consisting of mixed fiber types (tonic and phasic) is about 3 mL/min per 100 g, with a corresponding low basal O2 consumption (0.3 mL/min per 100 g). Flow in resting phasic (glycolytic, rapid twitch) muscle is about 20% the value of resting tonic (oxidative, postural) muscle, but the former can increase more than 20-fold during exercise. The differences in resting flow to the two muscle types can be attributed to the greater need for O2 by tonic muscles, a premise supported by the correspondingly larger capillary density surrounding tonic fibers.

Skeletal muscle arterioles are richly innervated by sympathetic vasoconstrictor fibers, whose tonic discharge contributes to resting arteriolar tone, as evidenced by an increased muscle blood flow after sympathetic denervation. Further stimulation of sympathetic nerves to skeletal muscle results in an increased arteriolar resistance and a reduction in blood flow. In this manner, resting skeletal muscle can contribute to the regulation of total peripheral resistance and arterial pressure because of the shear mass of skeletal muscle in the body. During periods of exercise, however, vascular resistance falls in skeletal muscle because of a dominance of metabolic vasodilation over vasoconstrictor signals derived from the autonomic nervous system. The active hyperemia observed in exercising skeletal muscle is due almost entirely to local metabolic vasodilation, rather than the relatively modest increase in arterial pressure.

Unlike the heart, resting skeletal muscle extracts only 25-30% of the O2 from blood. The low O2 extraction and low blood flow in resting skeletal muscle appears more than sufficient to meet the low O2 demand of this tissue. However, during severe exercise, blood flow increases up to 20-fold, the number of perfused capillaries increases 3- to 4-fold, and O2 extraction can reach 90%. All of these responses are geared toward meeting the 50- to 75-fold increase in O2 demand of skeletal muscle known to occur in healthy, untrained humans during strenuous exercise.

When skeletal muscle contracts, it compresses the intramuscular arteries and arterioles sufficiently to reduce blood flow, as shown in Fig. 8. However, between contractions, blood flow is high because of the dilated arterioles, and the transient hyperemic condition is enough to meet the elevated O2 demand. If a strong contraction is sustained, then the fibers become hypoxic and lactate accumulates, causing ischemic pain, which will eventually remind the individual to relax the muscle momentarily so that its O2 debt can be repaid. The massaging effect of rhythmic muscle contractions on the deep veins of the calf muscle appears to assist limb perfusion because this muscle pumping action lowers venous pressure and consequently provides a larger pressure gradient for blood perfusion.

Clinical Note

Physicians must be aware of the unique permeability properties of the blood-brain barrier in order to treat diseases of the nervous system effectively. For example, the antibiotics chlortetracycline and penicillin enter the brain to a very limited degree. Erythromycin, on the other hand, enters quite readily.

Rhythmic Exercise

FIGURE 8 Blood flow through the vascular bed of a contracting skeletal muscle. The muscle begins to contract rhythmically approximately once every 13 sec. Note that this causes an increase in mean blood flow because of active hyperemia. Also note that each contraction of the skeletal muscle transiently inhibits its blood flow by mechanically compressing the blood vessels inside the muscle.

Rhythmic Exercise

FIGURE 8 Blood flow through the vascular bed of a contracting skeletal muscle. The muscle begins to contract rhythmically approximately once every 13 sec. Note that this causes an increase in mean blood flow because of active hyperemia. Also note that each contraction of the skeletal muscle transiently inhibits its blood flow by mechanically compressing the blood vessels inside the muscle.

Splanchnic Circulation

The splanchnic circulation includes the vasculature of the gastrointestinal tract, liver, spleen, and pancreas. The principal functions of this vascular bed are to transport absorbed nutrients to the liver and systemic circulation and to mobilize blood to the systemic circulation during periods of whole body stress. Some of the features that distinguish the splanchnic circulation from other regional circulations include an active hyperemia elicited by food ingestion, an exquisite sensitivity of splanchnic arterioles to extrinsic control mechanisms, and reciprocal blood flow control between the liver and other splanchnic organs.

After ingestion of a meal, all organs of the digestive system become metabolically active. The increased acid secretion in the stomach, nutrient absorption and increased motility in the small bowel, and formation of digestive juices in the pancreas all impose a need for more blood flow to the respective tissues. The postprandial hyperemia in the gastrointestinal tract appears to be mediated by multiple factors, many of which are unique to specific regions of this organ system. For example, in the proximal small intestine, ingested long-chain fatty acids are the most potent stimulants for the active hyperemia, whereas bile acids dominate as mediators of the hyperemia in the distal ileum. Metabolic factors, gastrointestinal hormones, and neuropep-tides released from cholinergic nerve terminals have all been implicated in the increased blood flow that is elicited by a meal. Figure 9 illustrates the time course of changes in resistance of the splanchnic vessels during and after ingestion of a meal. There is a slowly developing decrease in vascular resistance over the first hour after eating. As the meal is digested and absorbed over a period of several hours, vascular resistance and blood flow return to their resting levels.

Splanchnic blood vessels are frequently called on to sacrifice their blood flow for the remainder of the circulation. The ability of the splanchnic circulation to contribute in this way toward whole-body homeostasis is made possible by the dense innervation of splanchnic arterioles by sympathetic nerves and by the highly responsive nature of these vessels to circulating vasoconstrictors, such as vasopressin and angiotensin II. Sympathetic stimulation elicits an intense reduction in intestinal blood flow that is blunted only somewhat by intrinsic metabolic factors. The combined vasoconstrictor effects of norepinephrine, vasopressin, and angiotensin II likely account for the intense reduction in splanchnic blood flow observed after severe hemorrhage.

The liver receives blood from two sources, the portal vein and the hepatic artery. The portal vein, with its low O2 content (much of the O2 is extracted in the gastrointestinal tract), normally provides 75% of liver blood flow, whereas the hepatic artery, with its fully O2 saturated blood, delivers the remainder. Blood flows in the hepatic artery and portal vein vary reciprocally. When liver blood flow increases through the portal vein, arterioles derived from the hepatic artery constrict, thereby preventing sudden increases in flow and pressure in downstream liver sinusoids. Because the sinusoids are extremely permeable to water and plasma

Clinical Note

The pale, cold skin characteristic of hypovolemic shock reflects a rise in cutaneous vascular resistance that appears to help support arterial blood pressure. During World War I, it was noticed that men rescued quickly and warmed in blankets (producing cutaneous vasodilation)

were less likely to survive than men who could not be reached for some time and who retained their natural cutaneous vasoconstriction. Hence, patients in shock should not be warmed to the point at which their body temperature rises.

FIGURE 9 Active hyperemia in the gastrointestinal tract. Vascular resistance in the splanchnic circulation before, during, and after ingesting a meal is plotted. During eating, resistance increases transiently but is followed quickly by a slowly developing and long-lasting decrease in resistance that increases blood flow to the gut. As the meal is digested and absorbed over the next several hours, vascular resistance (and blood flow) return to the levels seen during ingestion of a meal.

FIGURE 9 Active hyperemia in the gastrointestinal tract. Vascular resistance in the splanchnic circulation before, during, and after ingesting a meal is plotted. During eating, resistance increases transiently but is followed quickly by a slowly developing and long-lasting decrease in resistance that increases blood flow to the gut. As the meal is digested and absorbed over the next several hours, vascular resistance (and blood flow) return to the levels seen during ingestion of a meal.

proteins, the hepatic arterial constrictor response to increases in portal flow serves to prevent excessive filtration across this microvascular bed.

Cutaneous Circulation

The metabolic requirements of the skin are modest and blood flow through this tissue is controlled primarily by sympathetic fibers, whose activity is linked to body temperature. The amount of heat lost from the body is regulated by the amount of blood flowing through the skin. The skin's ability to serve as the body's radiator is due largely to the existence of arteriovenous anastomoses (AVAs) beneath the skin surface. These AVAs are low-resistance shunt pathways that are richly innervated by sympathetic vasoconstrictor fibers. When body core temperature is low, thermo-sensitive regions of the hypothalamus induce an increased sympathetic activity to the fibers innervating the AVAs. The resulting reduction in skin blood flow leads to a diminished dissipation of heat from blood to the environment and body temperature rises. Conversely, AVAs are dilated when core temperature is high because of a reduced sympathetic drive. In this instance, the cutaneous vasodilation favors the delivery of more blood to the skin, where heat readily crosses the vessel walls to reduce body temperature to a normal level. These temperature-induced changes in cutaneous vascular resistance account for the observation that blood flow can vary between 1 and 200 mL/min per 100 g in human skin.

Cutaneous vessels are exposed to an additional vasodilatory stimulus when increased body temperature is accompanied by sweating. The heat-induced parasympathetic stimulation of sweat glands results in the liberation of bradykinin, a potent vasodilator that acts on AVAs situated in proximity to the sweat glands. Figure 10 illustrates the effect of changes in ambient temperature on skin blood flow in the hand of an individual at rest and during exercise. In the resting state, an increased ambient temperature results in increased skin blood flow. During exercise, however, comparable increases in ambient temperature elicit more profound increments in hand blood flow. The difference between the two conditions is that the rise in body core temperature is greater in the exercising state and thus the stimulus for vasodilation is greater.

Fetal Circulation

Several of the anatomic and functional characteristics of the fetal circulation distinguish it from the adult circulation. Because the lungs of the fetus are collapsed and nonfunctional, some of the unique features of the fetal circulation are directed toward ensuring the absence of lung perfusion. In the fetus, the placenta serves as the functional equivalent of the lung, providing the oxygen and nutrients needed for survival, growth, and development of the fetus. Oxygenated fetal blood

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