Final answer:
People living on high mountains have more red blood cells than those in lowlands due to the decrease in oxygen concentration at higher altitudes. The body compensates by producing more red blood cells, which helps carry oxygen.
Step-by-step explanation:
People living on high mountains have more red blood cells than those living in lowlands because the oxygen concentration decreases as altitude increases. At high altitudes, there is less oxygen available for the body, so the body compensates by producing more red blood cells, which contain hemoglobin that carries oxygen. This adaptation helps people living at high altitudes have a higher level of oxygen in their blood despite the lower saturation of each hemoglobin molecule.