A healthy 33-year-old woman relocates to an area approximately 8000 ft above sea level. On her first day, she begins to hyperventilate, but in 24 hr she shows signs of recovery. What is the probable cause of her condition?

a. Hypoxemia-mediated hyperventilation
b. Hypercapnia
c. Hyperoxia-mediated hyperventilation
d. Increased H+

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 H+. The resulting increase in ventilation is less than expected, though, because hyperventilation lowers the PaCO2 and raises arterial pH.