What would be a primary physiological effect of flying at high altitudes without supplemental oxygen?

Flying at high altitudes without supplemental oxygen primarily leads to decreased oxygen levels in the bloodstream. At higher altitudes, the atmospheric pressure is lower, which results in a reduced amount of oxygen available to the body. As altitude increases, the partial pressure of oxygen decreases, leading to hypoxia, a condition in which the body or a region of the body is deprived of adequate oxygen supply.

This lack of oxygen can cause various physiological responses and symptoms, including impaired cognitive function, increased heart rate as the body attempts to compensate for the reduced oxygen availability, and symptoms like dizziness and fatigue. Therefore, understanding the relationship between altitude and oxygen levels is crucial for pilots and aviation personnel to ensure safety and mitigate the risks associated with hypoxia during flight.