Peripheral chemoreceptors are located in the carotid and aortic bodies and detect changes in arterial blood Po2, Pco2, and pH. Carotid bodies are small (~ 2 mm wide) sensory organs located bilaterally near the bifurcations of the common carotid arteries near the base of the skull. Afferent nerves travel to the CNS from the carotid bodies in the glos-
sopharyngeal nerves. Aortic bodies are located along the ascending aorta and are innervated by vagal afferents.
As with the medullary chemoreceptors, increasing Paco2 stimulates peripheral receptors. H+ formed from H2CO3 within the peripheral chemoreceptors (glomus cells) is the stimulus and not molecular CO2. About 40% of the effect of Paco2 on ventilation is brought about by peripheral chemoreceptors, while central chemoreceptors bring about the rest. Unlike the central sensor, peripheral chemoreceptors are sensitive to rising arterial blood H+ and falling Po2. They alone cause the stimulation of breathing by hypoxia; hypoxia in the brain has little effect on breathing unless severe, at which point breathing is depressed.
Carotid chemoreceptors play a more prominent role than aortic chemoreceptors; because of this and their greater accessibility, they have been studied in greater detail. The discharge rate of carotid chemoreceptors (and the resulting minute ventilation) is approximately linearly related to Paco2. The linear behavior of the receptor is reflected in the linear ventilatory response to carbon dioxide illustrated in Figure 22.6. When expressed using pH, the response curve is no longer linear but shows a progressively increasing effect as pH falls below normal. This occurs because pH is a logarithmic function of [H + ], so the absolute change in [H + ] per unit change in pH is greater when brought about at a lower pH.
The response of peripheral chemoreceptors to oxygen depends on arterial Pao2, and not oxygen content. Therefore, anemia or carbon monoxide poisoning, two conditions that exhibit reduced oxygen content but have normal Pao2, have little effect on the response curve. The shape of the response curve is not linear; instead, hypoxia is of increasing effectiveness as Po2 falls below about 90 mm Hg. The behavior of the receptors is reflected in the ventilatory response to hypoxia illustrated in Figure 22.7. The shape of the curve relating ventilatory response to Po2 resembles that of the oxyhemoglobin equilibrium curve when plotted upside down (see Chapter 21). As a result, the ventilatory response is inversely related in an approximately linear fashion to arterial blood oxygen saturation.
The nonlinearities of the ventilatory responses to Po2 and pH, and the relatively low sensitivity across the normal ranges of these variables, cause ventilatory changes to be apparent only when Po2 and pH deviate significantly from the normal range, especially toward hypoxemia or acidemia. By contrast, ventilation is sensitive to Pco2 within the normal range, and carbon dioxide is normally the dominant chemical regulator of breathing through the use of both central and peripheral chemoreceptors (compare Figs. 22.6 and 22.7).
There is a strong interaction among stimuli, which causes the slope of the carbon dioxide response curve to increase if determined under hypoxic conditions (see Fig. 22.6), causing the response to hypoxia to be directly related to the prevailing Pco2 and pH (see Fig. 22.7). As discussed in the next section, these interactions, and interaction with the effects of the central carbon dioxide sensor, profoundly influence the integrated chemoresponses to a primary change in arterial blood composition.
Carotid and aortic bodies also can be strongly stimulated by certain chemicals, particularly cyanide ion and other poisons of the metabolic respiratory chain. Changes in blood pressure have only a small effect on chemorecep-tor activity, but responses can be stimulated if arterial pressure falls below about 60 mm Hg. This effect is more prominent in aortic bodies than in carotid bodies. Afferent activity of peripheral chemoreceptors is under some degree of efferent control capable of influencing responses by mechanisms that are not clear. Afferent activity from the chemoreceptors is also centrally modified in its effects by interactions with other reflexes, such as the lung stretch reflex and the systemic arterial baroreflex (see Chapter 18). Although the breathing interactions are not well understood in humans, they serve as examples of the complex interactions of cardiorespiratory regulation. Interactions among chemoreflexes, however, are easily demonstrated.
Was this article helpful?