Final answer:
A person's red blood cell count increases after several weeks at high altitudes due to production of more erythropoietin by the kidneys, which stimulates erythrocyte production to enhance the body's oxygen transport capacity, allowing for acclimatization to lower oxygen levels.
Step-by-step explanation:
An individual's red blood cell count will increase after several weeks at moderate to high altitudes as a physiological response to lower oxygen levels. When a person ascends to high altitudes, the lower atmospheric pressure results in a lower partial pressure of oxygen, which in turn leads to decreased oxygen saturation of hemoglobin in the blood. The kidneys detect this decrease in oxygen and respond by producing more of the hormone erythropoietin (EPO). This hormone stimulates the production of erythrocytes (red blood cells), thereby increasing the individual's capacity to transport oxygen and gradually restoring oxygen levels in the body to maintain homeostasis.
With an increased number of erythrocytes carrying hemoglobin, there is an improved capability for oxygen transport despite the lower saturation of each hemoglobin molecule. This physiological change—known as acclimatization—allows individuals to better partake in physical exertion without developing acute mountain sickness (AMS). Over time, residents of high-altitude areas naturally maintain a higher hematocrit level compared to those living at sea level due to this continued exposure to lower partial pressure of oxygen.