Describe the structure and function of the alveoli. What is the process by which gases move across the cell wall of an alveolus; is the body capable of active transport of oxygen across the cell wall of an alveolus?
Each air sac has multiple bulges in its walls that dramatically increase its surface area. These bulges are called alveoli, the cell walls of which are one cell thick, surrounded by a dense capillary bed, and lined with a film of water. All gas exchange takes place through the alveoli's cell walls: Oxygen moves from the air to the blood and carbon dioxide moves from the blood to the air. Oxygen, which is in a higher concentration in the air than in the blood, and carbon dioxide, which is in a higher concentration in the blood than in the air, move across the alveoli's cell walls by simple diffusion. In fact, the body is incapable of active transport of oxygen across the alveolar wall.
