Blood–air barrier

The thin tissue interface separating alveolar air from pulmonary capillary blood. Oxygen and carbon dioxide cross it by diffusion.

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Alveolar epithelium: The epithelial lining of pulmonary alveoli, formed mainly by thin type I cells and surfactant-secreting type II cells. Type I cells form the air-facing cellular layer of the barrier.
Pulmonary alveolus: A microscopic air sac in the lung where gas exchange with surrounding capillaries occurs. The barrier forms the tissue boundary of each gas-exchanging air sac.
Pulmonary edema: Excess fluid accumulation in lung tissue or air spaces that impairs respiratory function. Interstitial fluid increases the distance gases must cross before reaching alveolar air.
Blood-brain barrier: A selective interface between blood and brain tissue, formed primarily by specialized cerebral endothelial cells. Unlike the lung interface, it restricts many substances rather than enabling rapid gas exchange.
Capillary endothelium: The thin endothelial cell lining of blood capillaries. Its closely apposed cells form the blood-facing layer of the barrier.
Pulmonary capillary: A small blood vessel in the lungs that carries blood alongside alveoli for gas exchange. Capillary blood lies immediately beyond the barrier’s vascular layer.
Acute respiratory distress syndrome: A severe inflammatory lung injury causing diffuse alveolar damage, pulmonary edema, and impaired oxygenation. Its injured, leaky interface illustrates failure of the barrier’s thin, selective structure.
Placental barrier: The tissue layers separating maternal blood from fetal blood in the placenta. It exchanges gases and nutrients between circulations without direct blood mixing.
Basement membrane: A thin extracellular-matrix sheet that supports and anchors cells in tissues. Alveolar and capillary basement membranes often fuse into a narrow diffusion path.
Alveolar-capillary membrane: The tissue interface through which respiratory gases pass between alveoli and pulmonary capillaries. This closely related name often denotes the same gas-exchange interface.