The gas exchange surface in the lungs where oxygen enters the blood is the?

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Gas exchange in the lungs happens across the alveolar walls, where oxygen from the air inside the alveoli moves into the blood and carbon dioxide moves from the blood into the alveolar air to be exhaled. Alveoli are tiny sacs with walls only one cell thick and are wrapped in a dense network of capillaries, creating a very thin respiratory barrier and a huge surface area for diffusion. This setup allows oxygen to diffuse from the alveolar air (where its partial pressure is higher) into the blood, while carbon dioxide diffuses in the opposite direction. The bronchioles and trachea are conducting airways that transport air to the lungs but do not participate in gas exchange, and the diaphragm is a muscle that helps drive breathing rather than serving as a gas-exchange surface.