Why does it take approximately 24 hr for a full ventilatory response to develop to acute hypox-emia?
a. Initial cerebrospinal fluid alkalemia blunts the hypoxic ventilatory stimulus.
b. Peripheral chemoreceptors are slow to re-spond to decreased blood O2 levels.
c. Renal compensation for respiratory alkalo-sis increases chemoreceptor sensitivity.
d. A full ventilatory response is not possible until after the muscles become fatigued.
ANS: A
High altitude causes a healthy person's ventilation to increase because low barometric pressure decreases the inspired PO2, and thus the arterial PO2, which in turn raises the sensitivity of pe-ripheral chemoreceptors to hydrogen ions. The resulting increase in ventilation is less than ex-pected though, because hyperventilation lowers the PaCO2 and raises arterial pH. The increased pH depresses the medullary respiratory center, counteracting the excitatory effect of a low PaO2 on peripheral chemoreceptors.
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