Altitude in feet at which blood volume decreases when collected?

• A. 1000
• B. 3000
• C. 5000
• D. 7000

Answer: (C)

At higher elevations, the body loses plasma volume due to diuresis and fluid shifts caused by hypoxia and increased water loss from the lungs. This makes the blood more concentrated and reduces the volume available in a drawn sample. This change becomes noticeable around 5,000 feet, with greater effects at higher elevations. While red cell mass may rise later with acclimatization, the immediate drop in blood volume from plasma loss explains why blood collected at around 5,000 feet shows a decreased volume.