At what conditions does oxygen transport become diffusion limited?

• A. Low altitudes
• B. During sleep
• C. During extreme heat
• D. High altitudes

Answer: (D)

Oxygen transport becomes diffusion limited primarily at high altitudes due to the decreased partial pressure of oxygen in the atmosphere. As altitude increases, the barometric pressure drops, which leads to a reduction in the availability of oxygen. This lower partial pressure means that the driving force for oxygen to diffuse from the alveoli in the lungs into the bloodstream is significantly reduced, making it more difficult for oxygen to move efficiently across the alveolar-capillary membrane.

At high altitudes, the body's ability to maintain sufficient oxygen levels is challenged, often leading to conditions such as hypoxia. The diffusion of oxygen becomes the limiting factor, impacting not just the transportation of oxygen to tissues but also how effectively cells can utilize oxygen for metabolic processes.

In contrast, conditions like low altitudes, sleep, or extreme heat do not typically create the same limitations on diffusion due to either normal atmospheric pressure, unchanged metabolic demands, or the body's ability to adapt to heat stress, respectively. Thus, it is at high altitudes that oxygen transport becomes diffusion limited, resulting from the reduced partial pressure and subsequent challenges in oxygen uptake.