How can altitude and dehydration affect hematologic function during flight?

• A. Hydration is irrelevant during flight.
• B. Dehydration increases viscosity and DVT risk; altitude diuresis can contribute; ensure adequate hydration within safety limits.
• C. Dehydration decreases blood viscosity.
• D. Hydration should be avoided to prevent edema.

Answer: (B)

Altitude exposure reduces circulating plasma volume through altitude diuresis, so dehydration concentrates the blood. As plasma volume falls, the hematocrit rises and the blood becomes more viscous. Thicker blood flows more slowly, which can impair tissue perfusion and oxygen delivery, and it also raises the risk of venous thromboembolism during prolonged flights. Staying adequately hydrated helps preserve plasma volume and keeps blood viscosity closer to normal, reducing these risks, but it’s important to drink within safe limits to avoid edema or electrolyte disturbances.