The functions of the carotid and aortic bodies

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Abstract The carotid and aortic bodies are structures whose specialized function it is to respond to changes in the chemical composition of the arterial blood by setting up afferent impulses which enter the central nervous system with the glossopharyngeal and vagus nerves, respectively. The nerve impulses in question are stimulant to the medullary centers (respiratory, vasomotor, and, in the case of the carotid body of the dog, cardioinhibitory). The impulses from the stretch receptors of the carotid sinuses and aortic arch (carried by the same nerves) are inhibitory to these centers (except the cardioinhibitory, which is stimulated by them). The chemically sensitive receptors (chemoreceptors) can be stimulated by anoxemia, asphyxia, increased carbon dioxide tension, or increased hydrogenion concentration, or by a variety of drugs and poisons which have no common chemical or physiologic attribute although the list includes inhibitors of oxidations (cyanide, sulfide) and substances with nicotinic properties (nicotine, lobeline, coniine, and choline and its derivatives). Reflexes from these structures are responsible for much, if not all, of the stimulant effects of anoxia on respiration and circulation. They probably are not concerned in the normal control of respiration, the sensitivity of the chemoreceptors to carbon dioxide being much less than that of the centers unless the latter is depressed; in that event these reflexes become an important factor in maintaining respiration. An explanation that is in accord with existing information on the subject is that the chemoreceptors represent a survival in relatively undifferentiated form of a reflex mechanism originally developed for a water-breathing ancestral form. The ability of these structures to set up a strong reflex stimulation to respiration when exposed to an environment that would depress or paralyze nerve cells (severe anoxia, very high carbon dioxide tension or acidity, deep narcosis) is probably related to their primitive status and responsible for much, if not all, of their value to the organism.