PaO2 has little direct effect on the medullary vasomotor region. The primary effect of hypoxia is mediated by reflexes via the carotid and aortic chemoreceptors. A moderate reduction in PaO2 stimulates the vasomotor region, but a severe reduction depresses vasomotor activity in the same manner by which other areas of the brain are depressed by very low O2 tension.
Balance Between Extrinsic and Intrinsic Factors in Regulation of Peripheral Blood Flow
Dual control of peripheral vessels by intrinsic and extrinsic mechanisms evokes a number of important vascular adjustments. Such regulatory mechanisms enable the body to direct blood flow to areas where it is most needed and away from areas that have fewer requirements. In some tissues, the effects of the extrinsic and intrinsic mechanisms are fixed; in other tissues, the ratio is changeable and depends on the state of activity of that tissue.
Cerebralischemia,whichmayoccurbecauseofexcessivepressureexertedbyanexpandingintracranialtumor,resultsinamarkedincreaseinperipheralvasoconstriction.ThestimulationisprobablycausedbylocalaccumulationofCO2andareductioninO2andpossiblybyexcitationofintracranialbaroreceptors.Withprolonged,severeischemia,centraldepressioneventuallysupervenes,andbloodpressurefalls.
CHAPTER 18 Regulation of the Heart and Vasculature
In the brain and heart, which are vital structures with limited tolerance for a reduced blood supply, intrinsic flow-regulating mechanisms are dominant. For instance, massive discharge of the vasoconstrictor region via the sympathetic nerves, which might occur in severe, acute hemorrhage, has negligible effects on the cerebral and cardiac resistance vessels, whereas the cutaneous, renal, and splanchnic blood vessels become greatly constricted.
In the skin, extrinsic vascular control is dominant. The cutaneous vessels not only participate strongly in a general vasoconstrictor discharge but also respond selectively via hypothalamic pathways to subserve the functions of heat loss and heat conservation required for regulation of body temperature. However, intrinsic control can be elicited by local temperature changes that modify or override the central influence on resistance and capacitance vessels (see also
In skeletal muscle, the extrinsic and intrinsic mechanisms interact. In resting skeletal muscle, neural control (vasoconstrictor tone) is dominant, as can be demonstrated by the large increase in blood flow that occurs immediately after section of the sympathetic nerves to the tissue. After the onset of exercise, the intrinsic flow-regulating mechanism assumes control, and vasodilation occurs in the active muscles because of the local increase in metabolites. Vasoconstriction occurs in the inactive tissues as a manifestation of the general sympathetic discharge. However, constrictor impulses that reach the resistance vessels of the active muscles are overridden by the local metabolic effect. Operation of this dual control mechanism thus increases blood flow where it is required and shunts it away from relatively inactive areas (see also
Chapter 17).
Similar effects may be achieved in response to an increase in PCO2. Normally, the hyperventilation associated with exercise keeps PCO2 at normal levels. However, if PCO2 is increased, generalized vasoconstriction would occur because CO2 stimulates the medullary vasoconstrictor region. In active muscles, where [CO2] would be highest, the smooth muscle of the arterioles would relax in response to the local PCO2. Factors that affect and are affected by the vasomotor region are summarized in