The reflex response to changes in blood gases and pH begins with chemoreceptors located peripherally in the carotid bodies and aortic bodies and centrally in the medulla (see Chapter 22). The peripheral chemoreceptors of the carotid bodies and aortic bodies are specialized structures located in approximately the same areas as the carotid sinus and aortic baroreceptors. They send nerve impulses to the NTS and are sensitive to elevated Pco2, as well as decreased pH and Po2. Peripheral chemoreceptors exhibit an increased firing rate when (1) the Po2 or pH of the arterial blood is low, (2) the Pco2 of arterial blood is increased, (3) the flow through the bodies is very low or stopped, or (4) a chemical is given that blocks oxidative metabolism in the chemoreceptor cells. The central medullary chemoreceptors increase their firing rate primarily in response to elevated arterial Pco2, which causes a decrease in brain pH.
The increased firing of both peripheral and central chemoreceptors (via the NTS and RVL) leads to profound peripheral vasoconstriction. Arterial pressure is significantly elevated. If respiratory movements are voluntarily stopped, the vasoconstriction is more intense and a striking bradycardia and decreased cardiac output occur. This response pattern is typical of the diving response (discussed later). As in the case of the baroreceptor reflex, the coronary and cerebral circulations are not subject to the sympathetic vasoconstrictor effects and instead exhibit vasodila-tion, as a result of the combination of the direct effect of the abnormal blood gases and local metabolic effects.
In addition to its importance when arterial blood gases are abnormal, the chemoreceptor reflex is important in the cardiovascular response to severe hypotension. As blood pressure falls, blood flow through the carotid and aortic bodies decreases and chemoreceptor firing increases— probably because of changes in local Pco2, pH, and Po2.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.