How altitude influences blood volume?

At high altitudes, the air has lower oxygen pressure, so the body adapts to maintain oxygen delivery. Initially, plasma volume decreases slightly, which thickens the blood and concentrates red blood cells, improving oxygen transport temporarily. Within days to weeks, the kidneys release erythropoietin (EPO), a hormone that stimulates the bone marrow to produce more red blood cells.

This adaptation increases the hematocrit (the percentage of red cells in blood) and total blood volume. The process helps carry more oxygen per liter of blood, compensating for the thin air. Over time, people living permanently at high altitudes develop chronic polycythemia, meaning their blood has a naturally higher red cell count.

However, excessive red cell production can make blood too viscous, straining the heart and circulation. Hydration is critical at high altitudes to balance plasma loss.
In contrast, when descending to sea level, the body gradually reduces red cell production and restores normal plasma levels. In short, altitude triggers dynamic changes in blood composition and volume that enhance oxygen delivery in low-oxygen environments.