Peripheral Chemoreceptors

Overview

The Peripheral Chemoreceptors refer to anatomical collections of chemoreceptors outside of the brain, the most important of which are those located in the carotid bodies of the carotid arteries as well as in the aortic bodies of the aortic arch. The peripheral chemoreceptors sense and respond to of a variety of blood molecules and are an important sensory component of a negative feedback loop which controls respiratory activity in an attempt to maintain relatively constant levels of arterial oxygen, arterial carbon dioxide, and blood pH as discussed further in integrated respiratory control.

Mechanism

The peripheral chemoreceptors are directly sensitive to the partial pressures of arterial oxygen and carbon dioxide as well as Blood pH; however, the mechanisms by which the concentrations of these molecules is connected to chemoreceptor activity is not well-understood. Although sensitive to all three stimuli, the physiological role of the peripheral chemoreceptors is most important for regulation of arterial oxygen and blood pH. In fact, the peripheral chemoreceptors appear to be the only locus within the body which can sense and respond to the partial pressure of arterial oxygen; consequently, these receptors are absolutely critical for responses to hypoxemia.

The peripheral chemoreceptors send increasingly rapid afferent impulses as the partial pressure of arterial oxygen declines; however, this relationship is extremely non-linear. In fact, the low basal rate of impulses only begins to rise noticeably after arterial partial pressures of roughly 100 mm Hg are reached. Indeed, the steepest rise in impulse frequency only occurs after arterial oxygen tension declines below 80 mm Hg, notably corresponding with the partial pressures at which steep portion of the Oxygen-hemoglobin Dissociation curve occurs (See: Oxygen Transport). It is perhaps reasonable to submit an evolutionary explanation for this observation that the partial pressures at which the oxygen sensory system of the body begins to signal alarm corresponds to the partial pressures at which the oxygen transport system of the body begins to lose significant carriage capacity.

Although the peripheral chemoreceptors can sense changes in the partial pressures of arterial carbon dioxide, their response does not appear to be nearly as influential as that of the central chemoreceptors which are the principal sensors of arterial CO₂.

Of the peripheral chemoreceptors only those in the carotid bodies can sense and respond to changes in blood pH. Importantly, these chemoreceptors are directly activated by any change in the pH of blood, whether the pathogenesis be that of respiratory acidosis, respiratory alkalosis, metabolic acidosis, or metabolic alkalosis.

Effects

The peripheral chemoreceptors send afferent fibers to the brainstem respiratory centers, especially the Inspiratory Center, and aid in modulation of respiratory drive. As described above, the peripheral chemoreceptors are extremely sensitive to abnormally low levels of arterial oxygen, which result in increased afferent impulses to the inspiratory brain stem respiratory centers, yielding an in increased respiratory drive. This improves alveolar ventilation and can return the arterial partial pressure of oxygen back to normal. It should be pointed out that although supra-normal partial pressures of arterial oxygen do reduce the basal rate of afferent impulses from the peripheral chemoreceptors, these do not significantly reduce respiratory drive. As mentioned, while partial pressures of arterial carbon dioxide can modulate the afferent activity of the peripheral chemoreceptors, these do not appear to exert a significant affect on the respiratory drive on their own. Finally, changes in blood pH do modulate the afferent output from the carotid peripheral chemoreceptors and these appear to the primary stimuli which coordinate the respiratory response to acid-base disturbances as described in Respiratory Acid-Base Control and Integrated Respiratory Control.

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