from collapsing during the drop in internal air pressure that results from inhalation. The trachea branches at its lower end into two bronchi, one leading toward and entering each lung. Each bronchus branches repeatedly into smaller ducts called bronchioles. In turn, the bronchioles branch repeatedly into even smaller ducts called alveolar ducts, which each terminate in an air sac. 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.
