Pulmonary alveolus

A pulmonary alveolus is a microscopic air sac at the end of the lung’s airways. Its thin, moist walls bring inhaled air close to capillary blood for oxygen and carbon dioxide exchange.

Alveolar–capillary membrane: The thin tissue barrier separating alveolar air from blood in pulmonary capillaries. Its small diffusion distance lets oxygen and carbon dioxide cross between air and blood.
Respiratory bronchiole: A small airway where alveoli begin appearing in its walls. It marks the transition from air conduction to regions that also exchange gases.
Pulmonary edema: Excess fluid in the lung tissue and air spaces that impairs breathing. Fluid in or around alveoli increases the distance gases must cross.
Conducting zone: The airways that move, warm, humidify, and filter air without exchanging respiratory gases. Unlike alveoli, these passages primarily transport air rather than exchange gases.
Pulmonary surfactant: A mixture of lipids and proteins that lowers surface tension at the air–liquid interface in the lungs. It helps keep alveoli open, especially during exhalation.
Alveolar duct: A passage lined with openings into alveoli, extending from respiratory bronchioles. Alveolar ducts lead into clusters of the air sacs.
Emphysema: A lung disease involving destruction of alveolar walls and persistent enlargement of air spaces. Loss of alveolar walls reduces the surface available for gas exchange.
Terminal bronchiole: The final airway in the conducting zone, before respiratory bronchioles begin. It conducts air but has no alveoli in its walls.
Type I pneumocyte: A thin epithelial cell that covers most of the alveolar surface. These cells form much of the short path gases cross into capillaries.
Alveolar sac: A cluster of alveoli opening into a shared space at the end of an alveolar duct. It shows how individual alveoli are grouped at airway endings.

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