Changes in cardiac performance as a result of stimulation of central and peripheral chemoreceptors have been described previously in this chapter. These effects usually predominate. However, O2 and CO2 do have direct effects on the myocardium.
Hypoxia has a biphasic effect on myocardial performance. Mild hypoxia stimulates performance, but more severe hypoxia depresses performance because oxidative metabolism is limited.
An increase in partial pressure of carbon dioxide (PCO2)— which results in a decrease in pH—has a direct depressant effect on the heart. This effect is mediated by changes in intracellular pH. A reduction in intracellular pH, induced by an increase in PCO2, diminishes the amount of Ca++ released from the sarcoplasmic reticulum in response to excitation. The diminished pH also decreases the sensitivity of the myofilaments to Ca++ . Increases in intracellular pH have the opposite effect; that is, they enhance sensitivity to Ca++ .
Regulation of the Peripheral Circulation
The peripheral circulation is essentially under dual control: centrally through the nervous system and locally by conditions in tissues surrounding the blood vessels. Nervous and humoral regulation of vascular smooth muscle is described in
Fig. 14.7
Table 14.1 ), in which transmitters, hormones and their receptors are discussed. Aspects of local control are discussed in , in which the relative importance of these two control mechanisms is shown to vary in different tissues.
The arterioles are involved in regulating the rate of blood flow throughout the body. These vessels offer the greatest resistance to the flow of blood pumped to the tissues by the heart, and thus these vessels are important in the maintenance of arterial blood pressure. The arteriole walls are composed in large part of smooth muscle fibers that allow the diameter of the vessel lumen to vary (see
Fig.
15.2 ). When this smooth muscle contracts strongly, the endothelial lining folds inward and completely obliterates the vessel lumen. When the smooth muscle is completely relaxed, the vessel lumen is maximally dilated. Some resistance vessels are closed at any given time. In addition, the smooth muscle in these vessels is partially contracted (which accounts for the tone of these vessels). If all the resistance vessels in the body dilated simultaneously, arterial blood pressure would fall precipitously.
Vascular smooth muscle controls total peripheral resistance, arterial and venous tone, and the distribution of blood flow throughout the body. The properties of vascular smooth muscle are discussed in . In the following sections, intrinsic and extrinsic control of vascular smooth muscle tone, and thus perfusion of peripheral tissues, is reviewed.
Intrinsic or Local Control of Peripheral Blood Flow
In certain tissues, blood flow is adjusted to the existing metabolic activity of the tissue. Furthermore, when tissue metabolism is steady, changes in perfusion pressure (arterial blood pressure) evoke changes in vascular resistance that tend to maintain a constant blood flow. This myogenic mechanism, which is illustrated graphically in
Fig. 18.22 , is commonly referred to as autoregulation of blood flow. When pressure is abruptly increased or decreased from a control pressure of 100 mm Hg, flow increases or decreases, respectively. However, even with pressure maintained at its new level, blood flow returns toward the control level within 30 to 60 seconds.