Neurohumoral Mechanisms Regulate Blood Pressure and BloodVoIume to Ensure Adequate Blood Flow for All Body Organs
The influences of the nervous system and hormones on the cardiovascular system are referred to collectively as the neuro- humoral mechanisms of cardiovascular control. The neurohumoral mechanisms are also called extrinsic control mechanisms because they act on organs from the outside.
As described in Chapter 24, the mechanisms of cardiovascular control that act locally, within individual tissues, are referred to as intrinsic control mechanisms. The local, or intrinsic, mechanisms predominate over extrinsic mechanisms in the control of blood flow to the “critical” organs, which include the heart (i.e., coronary circulation), brain, and working (exercising) skeletal muscle. In contrast, neurohumoral, or extrinsic, control mechanisms predominate over the intrinsic mechanisms in the control of blood flow to the “noncritical” organs, which include the kidneys, the splanchnic organs, and resting skeletal muscle. The noncritical organs are those that can withstand temporary reductions in blood flow (and metabolism) to make extra blood flow available for the critical organs, whose optimal function may be necessary for survival in a “fight or flight” situation.Neurohumoral mechanisms also control the heart rate and cardiac contractility. This allows cardiac output to be adjusted to provide adequate blood flow for all the systemic organs, or at least for the critical organs. An important distinction is that cardiac muscle is under neurohumoral control, whereas the coronary blood vessels are primarily under local control. When neurohumoral mechanisms increase the heart rate and cardiac contractility, the cardiac metabolic rate also increases. The increased metabolic rate works through local metabolic control mechanisms to dilate coronary arterioles, which increases coronary blood flow.
To appreciate the importance of neurohumoral control mechanisms, consider what would happen in their absence.
For example, what would occur during exercise if all the body organs simply relied on local control mechanisms to adjust their blood flow? At the onset of exercise, metabolic control mechanisms would cause vasodilation in the exercising skeletal muscles. Vascular resistance would decrease in the exercising muscles, and the blood flow through the muscles would increase. However, the decrease in resistance in skeletal muscles would also lower the total peripheral resistance (TPR). As a consequence, arterial blood pressure would decrease. This would decrease the perfusion pressure for all the systemic organs, and blood flow would therefore decrease below normal levels in the brain, kidneys, splanchnic organs, and so forth.The decreased blood flow in these organs would trigger autoregulatory responses, and these organs would vasodilate. However, the vasodilation would lower the TPR even further, which would reduce arterial pressure even more. This in turn would limit the increase in skeletal muscle blood flow. The end result would be some increase in blood flow in the exercising muscle and decreased blood flow elsewhere, but none of the organs (including skeletal muscle) would be receiving sufficient blood flow to meet their metabolic needs. Arterial pressure would be dangerously low, and the animal would exhibit profound exercise intolerance.
Neurohumoral control mechanisms allow an animal to avoid these complications. First, cardiac output is increased sufficiently to meet the increased need for blood flow in the exercising muscle (and in the coronary circulation) while keeping all the other organs supplied with a normal blood flow. If cardiac output cannot be increased sufficiently to meet all these needs, the control mechanisms take the additional step of temporarily reducing blood flow in the noncritical organs and making this extra flow available to the critical organs.
How do the neurohumoral control systems “know” when cardiac output is sufficiently high to meet the needs of all the organs and when to initiate vasoconstriction in the noncritical organs? An indirect “strategy” is used: cardiac output is increased enough to keep arterial pressure at a normal level.
As long as arterial pressure is maintained at normal level, local metabolic control mechanisms can successfully match blood flow to metabolic need in each individual organ. If cardiac output cannot be sufficiently increased to keep arterial pressure from falling, vasoconstriction of the noncritical organs is initiated. Thus, neurohumoral control mechanisms deprive noncritical organs of an ideal level of blood flow if more flow is needed by the critical organs than can be supplied by the heart.There are many important neurohumoral control mechanisms, but four are emphasized in the following presentation. Two of them are cardiovascular reflexes. The arterial baroreceptor reflex works to keep arterial pressure at its “set point” through the continual adjustment of cardiac output and vascular resistance (in the noncritical organs). The atrial volume receptor reflex works in conjunction with the arterial baroreceptor reflex to regulate arterial pressure by adjusting cardiac preload. The other two neurohumoral mechanisms described in this chapter exemplify psychogenic influences on the cardiovascular system: the defense-alarm reaction and vasovagal syncope (the “playing dead” reaction).