Gas exchange depends on ventilation, perfusion, diffusion surface area, and partial pressure gradients. Oxygen moves from alveoli into blood; CO₂ moves from blood into alveoli for exhalation. Hypoxemia (low PaO₂) differs from hypoxia (low tissue O₂) — anemia and CO poisoning can cause hypoxia with misleading oximetry context. Connect Gas Exchange to bedside cues you will reassess first: vitals trends, work of breathing, perfusion, mentation, and pain or ischemic equivalents when relevant. NCLEX items often reward recognizing when subtle instability outweighs reassurance, then selecting nursing actions that protect airway, circulation, and neurologic status before routine tasks.