How refreshing the air in sacs helps to maintain a steep diffusion gradient?

Refreshing the air in sacs helps to maintain a steep diffusion gradient by introducing new oxygen into the alveoli and removing carbon dioxide. This process ensures that there is a higher concentration of oxygen in the alveoli than in the pulmonary capillaries, creating a steep diffusion gradient for oxygen to move from the alveoli into the bloodstream. As oxygen diffuses from the alveoli into the capillaries, carbon dioxide diffuses from the capillaries into the alveoli. By continuously refreshing the air in the sacs, the concentration of carbon dioxide in the alveoli is kept low, creating a steep diffusion gradient for carbon dioxide to move from the blood into the alveolar air.