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Why do people living at high altitudes tend to have more red blood cells?

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Blood plasma — blood Blood sediment in test tubes, showing plasma (clear fluid), red blood cells (red), and the release of hemoglobin into the surrounding plasma (pink).

People living at high altitudes tend to have more red blood cells as a physiological response to the lower atmospheric pressure and reduced partial pressure of oxygen in their environment.123

Because the air at high altitudes contains less available oxygen, the body must compensate to ensure that tissues receive an adequate supply. When the oxygen tension in the blood falls, the kidneys increase the production of a hormone called erythropoietin. This hormone travels through the plasma to the bone marrow, where it stimulates and accelerates the production of red blood cells.2314

This increase in red blood cells—a condition known as polycythemia—enhances the blood's capacity to transport oxygen to the body's tissues, helping individuals acclimatize to the oxygen-scarce environment. If these individuals move to a lower altitude, the production of erythropoietin is suppressed, and the red blood cell count eventually returns to normal levels.2314