RESPIRATORY SENSORS



The respiratory sensors consist of the central and peripheral chemoreceptors and the chest wall and intrapulmonary sensory receptors. The cen­tral chemoreceptors, located on the ventral sur­face of the medulla oblongata, rapidly respond to any increase in C02 or hydrogen ion concentra­tion by increasing ventilation . Under normal circumstances these receptors are very sensitive, keeping the Paco2 constant despite marked variability in C02 production. In contrast,hypoxia does not act as a central respiratory stim­ulant but instead depresses the central chemore­ceptors. Conversely, the peripheral chemorecep­tors, located at the bifurcation of the carotid arteries and along the aortic arch, are activated mainly by hypoxia and less so by C02 and hy­drogen ion. They are also sensitive to a fall in blood pressure, which may partly account for hy­perventilation seen in shock. Unlike the central response to Pco2, the peripheral chemoreceptors cause little increase in ventilation until there is significant hypoxemia (Po2 less than 60 mm Hg) . Mechanoreceptors in the chest wall respond to stretch of the intercostal muscles and reflexly modulate the rate and depth of breathing. Tidal volume and respiratory frequency may also be reflexly affected by stimuli arising in (1) airway irritant receptors, which respond to physical or chemical stimulation, (2) pulmonary stretch re­ceptors, which respond to marked increases in lung volume (Hering-Breuer reflex), or (3) J re­ceptors found in the juxtacapillary junctions, which respond to vascular engorgement and congestion.