This episode introduces the concept of hypobaria—the reduction in barometric pressure at altitude—and explains how it lowers partial pressure of oxygen (PO₂), creating hypoxia. We review the alveolar gas equation to show how lower barometric pressure reduces alveolar oxygen and how hyperventilation is the body’s first acute response. We also explore how hyperventilation triggers respiratory alkalosis, shifting the oxygen-hemoglobin dissociation curve to the left, improving oxygen loading in the lungs. Lastly, we touch on initial cardiovascular changes, including increased heart rate and submaximal cardiac output.