Final answer:
An increase in altitude leads to a a) decrease in atmospheric pressure. This results in a lower oxygen saturation level at high altitudes compared to sea level.
Step-by-step explanation:
An increase in altitude leads to a decrease in atmospheric pressure. The proportion of oxygen relative to gases in the atmosphere remains at 21 percent, but its partial pressure decreases. As a result, it becomes more difficult for the body to achieve the same level of oxygen saturation at high altitudes compared to sea level. Hemoglobin saturation, for example, is lower at high altitudes, reaching about 67 percent at 19,000 feet above sea level, compared to about 98 percent at sea level.