The effectiveness of external respiration can be affected by which factor?

Altitude and air pressure significantly influence the effectiveness of external respiration. At higher altitudes, the atmospheric pressure decreases, resulting in a lower partial pressure of oxygen. This reduction means that, even though the percentage of oxygen in the air remains constant, there is less oxygen available for diffusion into the bloodstream during respiration. The lower availability of oxygen can lead to symptoms such as altitude sickness, as the body struggles to obtain adequate oxygen.

In contrast, factors like the color of the air, humidity levels, and wind speed do not play a direct role in the respiratory process itself. While humidity can affect comfort or irritate the respiratory system in certain conditions, it does not fundamentally change the efficiency of gas exchange. Similarly, wind speed does not impact the oxygen levels present in the air we breathe. Thus, altitude and air pressure are the most critical factors affecting external respiration's effectiveness.