Final answer:
Hypoxemia is defined as having a PaO₂ of less than 80 mm Hg, which is below the normal partial pressure of oxygen in oxygenated blood.
Step-by-step explanation:
Hypoxemia occurs when the partial pressure of oxygen in the arterial blood (PaO₂) is less than the normal range. The normal partial pressure of oxygenated blood in pulmonary veins is about 100 mm Hg. When there is a decrease below this value, oxygen transfer across the respiratory membrane becomes less efficient, and this can lead to fewer oxygen molecules being bound by hemoglobin. Therefore, hypoxemia is defined as having a PaO₂ of less than 80 mm Hg.