Final answer:
The tidal ventilation system results in a lower oxygen partial pressure in exhaled air compared to excurrent blood, due to the mixing of inhaled air with residual air within the lungs.
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 in the options provided is tidal ventilation. Tidal ventilation is a breathing pattern seen in humans and many other animals where air enters and exits the lungs in the same pathway. This causes exhaled air, which is mixed with residual air in the lungs, to have a lower partial pressure of oxygen compared to the blood leaving the lungs because oxygen from this mixed air has already been transferred to the blood.
So, in tidal systems, the excurrent air is not as rich in oxygen as the excurrent blood. This differs from countercurrent and cross-current systems, which are more efficient, and often found in birds and fish, where the blood and air flow in such a way as to maximize oxygen extraction.