Question

when a man goes on high altitude it RBC count increase. but a man who live there is adapted to this environment. i.e he has high no of RBC count. when he comes at low altitude , is his RBC count decreases.
and If yes , then why[i know that he didn't't need enough RBC ] . but is more oxygen reaches in blood is harmful.

Answer 1

At high altitudes, the concentration of oxygen decreases so to meet the demands of oxygen, the people dwelling in hilly areas generally have higher number of RBC's. High number of RBC's are required to provide enough oxygen to every tissues in the body. People dwelling in low altitude have normal count of RBC's as the amount of oxygen available for them is enough. The count of RBC's depends on the availability of the oxygen.