alveolarcapillary

The alveolar-capillary, or alveolar-capillary membrane, is the thin barrier through which gas exchange occurs between the air-filled alveoli of the lungs and the blood in the surrounding pulmonary capillaries. It comprises the alveolar epithelium, the capillary endothelium, and the intervening basement membranes, sometimes including a small alveolar interstitial space. This unit forms the primary site where oxygen diffuses into the blood and carbon dioxide diffuses out for exhalation.

Structure and components: The alveolar lining consists largely of type I pneumocytes, which provide most of the gas-exchange surface, and type II pneumocytes, which secrete surfactant and contribute to repair. The capillary side features a continuous endothelial layer. The basal laminae of the epithelial and endothelial cells are typically fused, producing an exceptionally thin diffusion barrier. Surfactant within the alveolar space reduces surface tension, stabilizing alveolar walls and maintaining efficient gas exchange.

Function and physiology: The thin barrier and large surface area enable rapid diffusion of gases driven by partial pressure gradients. In healthy lungs, the diffusion distance is on the order of a fraction of a micrometer, and the total alveolar surface area is large enough to support efficient transfer of oxygen into, and carbon dioxide out of, the bloodstream. The diffusion capacity for carbon monoxide (DLCO) is a clinical measure of overall alveolar-capillary gas transfer efficiency.

Clinical relevance: Pathologies can thicken or disrupt the alveolar-capillary barrier or reduce surface area, impairing gas exchange. Conditions such as edema, inflammation, fibrosis, or pneumonia can increase diffusion distance, while emphysema reduces surface area. Assessment of DLCO and imaging help evaluate alveolar-capillary function in various lung diseases.