For internal respiration, which of the following must be true:
I. The total amount of CO₂ exchanged equals the amount of O₂ exchanged.
II. Carbon dioxide will move down its pressure gradient to enter the blood.
III. The partial pressure of O₂ in the tissue before gas exchange is ≥ 40 mm Hg.

When determining the directions that molecules will move in external and internal respiration:

What is the approximate partial pressure of oxygen in the blood as it enters and leaves the capillaries of the alveoli?

What is one difference between air in the alveoli compared to air in the atmosphere?

Which gradient most directly determines the direction that oxygen and carbon dioxide molecules will move between the air in the alveoli and the blood:

Choose which of the following statements is correct.