Key Points

• The myogenic theory of blood flow regulation invokes a role for arteriolar wall tension, rather than blood flow, as the controlled variable in the vasculature.

• The metabolic theory of blood flow regulation links blood flow to the metabolic activity of the tissue, with vasodilator metabolites (e.g., adenosine) and low tissue Po2 serving as mediators of metabolic hyperemias.

• Activation of sympathetic nerves, via release of norepinephrine, increases resistance to blood flow in vascular beds that are richly innervated with a-adrenergic nerve fibers.

• Blood flow to the heart is governed almost entirely by intrinsic metabolic factors.

• The influence of autonomic nerve stimulation on coronary blood flow is dictated by the action of the released neurotransmitter on cardiac work, with sympathetic stimulation resulting in an increased flow and parasympathetic stimulation in a reduced flow.

• Cerebral arterioles respond with dilation to increases in metabolic activity in the brain.

• Cerebral arterioles are exquisitely sensitive to the vasodilatory action of an increased arterial blood PCO2.

• Exercise can elicit a profound increase (20-fold) in skeletal muscle blood flow that is mediated by vasodilator metabolites.

• The increased O2 demand that is associated with exercise is met by both an increased blood flow and an increased O2 extraction.

• Gastrointestinal blood flow increases after ingestion of a meal, with digested nutrients, metabolic factors, gastrointestinal hormones, and neuropeptides mediating the active hyperemia.

• Splanchnic blood flow is profoundly reduced during sympathetic activation and with increased blood levels of angiotensin II and vasopressin.

• The amount of heat lost from the body is regulated by the amount of blood flowing through the skin.

• Blood flow through the skin is primarily controlled by sympathetic fibers, whose activity is linked to body temperature.

Essential Medical Physiology, Third Edition

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