Simone was hypoxemic (i.e., she had a decreased Pao₂). However, she was not hypercapnic (i.e., she did not have CO₂ retention or an increased Paco₂). In fact, at 37 mm Hg, her Paco₂ was slightly lower than normal. Simone clearly has a problem with O₂ exchange, but she does not seem to have a problem with CO₂ exchange. How can hypoxemia occur in the absence of hypercapnia?