Which structure is the primary site of gas exchange in the lungs?

Which structure is the primary site of gas exchange in the lungs?

• A. Alveoli
• B. Bronchi
• C. Pleura
• D. Trachea

Answer: (A)

Gas exchange happens where gases can diffuse between air and blood, and this occurs most efficiently across the thin, moist walls of the alveoli. Alveoli are tiny sacs with a vast surface area and are surrounded by a dense network of capillaries. The barrier between air and blood, the respiratory membrane, is made up of the alveolar epithelium, the capillary endothelium, and their shared basement membranes, all very thin to allow rapid diffusion. Surfactant from type II cells keeps the alveoli from collapsing and helps maintain the surface tension needed for efficient gas exchange. Because the alveolar walls are so thin and there’s a large surface area, oxygen moves from the alveolar air (where its partial pressure is higher) into the blood, and carbon dioxide moves from the blood into the alveolar air (where its partial pressure is higher). This diffusion follows partial pressure gradients and is aided by the moist environment inside the alveoli. The bronchi and trachea are airways that conduct air to the lungs but do not participate in gas exchange—their walls are too thick for efficient diffusion. The pleura are membranes that surround the lungs and help with expansion and contraction, not with gas transfer.

Gas exchange happens where gases can diffuse between air and blood, and this occurs most efficiently across the thin, moist walls of the alveoli. Alveoli are tiny sacs with a vast surface area and are surrounded by a dense network of capillaries. The barrier between air and blood, the respiratory membrane, is made up of the alveolar epithelium, the capillary endothelium, and their shared basement membranes, all very thin to allow rapid diffusion. Surfactant from type II cells keeps the alveoli from collapsing and helps maintain the surface tension needed for efficient gas exchange.

Because the alveolar walls are so thin and there’s a large surface area, oxygen moves from the alveolar air (where its partial pressure is higher) into the blood, and carbon dioxide moves from the blood into the alveolar air (where its partial pressure is higher). This diffusion follows partial pressure gradients and is aided by the moist environment inside the alveoli.

The bronchi and trachea are airways that conduct air to the lungs but do not participate in gas exchange—their walls are too thick for efficient diffusion. The pleura are membranes that surround the lungs and help with expansion and contraction, not with gas transfer.