Which structure forms the barrier where gas exchange occurs between alveoli and capillaries?

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Gas exchange happens by diffusion across a very thin barrier between the air in the alveolus and the blood in surrounding capillaries. This barrier is the alveolar-capillary membrane, formed by the alveolar epithelium (primarily Type I pneumocytes), the capillary endothelium, and their shared basement membranes. Its extreme thinness and large surface area allow oxygen to move into blood and carbon dioxide to move out efficiently as they diffuse down their partial pressure gradients. The pleural lining surrounds the lungs but isn’t the site of gas exchange. Alveolar Type II cells secrete surfactant to keep the alveoli from collapsing; they don’t form the gas-exchange barrier. Bronchial epithelium lines the airways and is not involved in gas diffusion. So, the barrier where gas exchange occurs is the alveolar-capillary membrane.