Final answer:
The cross-current ventilation system results in a lower oxygen partial pressure in the exhaled medium compared to the blood, which is more efficient than tidal or concurrent systems at extracting oxygen.
Step-by-step explanation:
The ventilation system that results in an oxygen partial pressure of the exhaled medium that is lower than that of the excurrent blood is cross-current ventilation. This system can be observed in bird lungs where the flow of blood is at an angle to the flow of air, allowing for a higher efficiency in oxygen extraction than tidal or concurrent systems. In tidal ventilation, which is found in human lungs, air flows in and out of the alveoli in the same direction, and while it is efficient, the oxygen partial pressure in exhaled air is not lower than in the blood. In concurrent systems, both air and blood flow in the same direction; once equilibration is reached, no further oxygen can be extracted, hence the efficiency is lower than in cross-current systems.