Final answer:
In a tidal gas exchange system, the oxygen partial pressure of the excurrent blood is lower than that of the excurrent air or water. Option A is correct.
Step-by-step explanation:
In a tidal gas exchange system, the oxygen partial pressure of the excurrent blood is lower than that of the excurrent air or water.
This is because during tidal gas exchange, when an animal inhales, oxygen-rich air or water enters the respiratory system, and when it exhales, oxygen-poor air or water leaves the system. As a result, the oxygen partial pressure in the excurrent blood, which has already exchanged oxygen with the respiratory surface, is lower than that of the excurrent air or water.
For example, in fish, oxygen-rich water enters the gills during inhalation, and oxygen-poor water exits during exhalation. As a result, the oxygen partial pressure in the excurrent blood is lower than that of the excurrent water.
In a tidal gas exchange system, the oxygen partial pressure of the excurrent blood is lower than that of the excurrent air or water. The reason for this is that oxygen is constantly being removed by the blood in the lungs, which creates a pressure gradient that facilitates the diffusion of oxygen from the air or water into the blood. Thus, the correct answer is (a) lower than.