Chemoreceptors pO2/pH

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  1. Peripheral chemoreceptors
    1. small, vascular bodies in the aortic arch and medial to the carotid sinuses that are sensitive to changes in paO2, paCO2 and pH of the blood and are primarily concerned with regulation of respiration but influence the vasomotor regions to a minor degree
    2. reduction in paO2 stimulates the chemoreceptors which stimulate the vasoconstrictor regions resulting in increased tone of the resistance and capacitance vessles; chemoreceptors are also stimulated by increased arterial paCO2 and reduced pH but the reflex effect is small compared to the direct effect of hypercapnia and hydrogen ions on the vasomotor regions in the medulla; when the hypoxia and hypercapnia occur at the same time the stimulation of the chemoreceptors is greater than when they are stimulated individually
      1. hypoxia increases ventilation primarily by exciting sensors in the carotid body innervated by CN 9 and to a lesser extent the aortic body innervated by CN 10
      2. partial pressure of O2 rather than O2 content is the main stimulus
    3. chemoreceptors with sympathetic afferent fibers are also in the heart and are activated by ischemia and transmit the precordial pain (angina pectoris) associated with an inadequate blood supply to the myocardium
      1. NOTE: baroreceptors are stretch receptors located in the carotid sinuses and aortic arch; impulses travel in the carotid sinus up the sinus nerve (nerve of Hering) to the glossopharyngeal nerve and then to the nucleus of the tractus solitarius in the medulla; fibers from the aortic arch travel in the vagus nerve; the tractus solitarius is the central projection of the chemoreceptors and baroreceptors; stimulation of the tractus solitarius inhibits sympathetic nerve impulses to the peripheral blood vessels while lesions of the tractus solitarius produce vasoconstriction
  2. Central chemoreceptors of the ventrolateral medulla
    1. increases of paCO2 stimulate the vasoconstrictor regions increasing peripheral resistance; central chemoreceptors account for 80% of the CO2 induced increases in ventilation
    2. hypoxia does not have a great effect on central chemoreceptors
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