alveolaircapillaire

The alveolaircapillaire, or alveolo-capillary barrier, is the thin interface between the air-filled alveoli and the surrounding pulmonary capillaries. It serves as the functional unit for gas exchange and helps regulate the fluid balance of the lung.

Structure and components: The barrier comprises the alveolar epithelium (predominantly type I pneumocytes forming the gas-exchange surface, with type II pneumocytes producing surfactant and capable of repair), the thin capillary endothelium, and a shared basement membrane that couples the two layers. Interposed interstitial space and fibers provide support. Alveolar macrophages reside within the airspaces, while surfactant lowers surface tension to prevent collapse during breathing.

Function: The barrier is extremely thin and expansive, maximizing diffusion of oxygen from alveolar air into blood and carbon dioxide in the opposite direction. Gas transfer follows partial pressure gradients and is described by diffusion laws; the process depends on membrane thickness, surface area, and the driving pressures of O2 and CO2. The alveolar surface is lined with surfactant to stabilize alveoli, and active ion transport helps maintain low fluid levels in the airspaces, protecting the barrier from edema.

Clinical relevance: Thickening or damage to the alveolaircapillaire impairs gas exchange. Conditions such as pulmonary edema, interstitial fibrosis, and acute respiratory distress syndrome (ARDS) increase diffusion distance and reduce diffusion capacity (DLCO). Evaluations of DLCO and imaging help assess barrier integrity and function.

Notes: The alveolaircapillaire is also referred to as the blood–air barrier and is central to respiratory physiology, pathology, and clinical assessment of lung function.