alveolos

Alveolos are the tiny air-filled units at the ends of the airways in the lungs where most gas exchange occurs. They form clustered structures known as alveolar ducts and alveolar sacs and are the primary sites where oxygen enters the blood and carbon dioxide is released.

An alveolus is lined by a very thin epithelium composed mainly of type I pneumocytes, with a smaller population of type II pneumocytes that produce surfactant. The walls are separated from surrounding capillaries by a thin respiratory membrane made up of the alveolar epithelium, the interstitial space, and the capillary endothelium. Surfactant, secreted by type II cells, reduces surface tension and helps keep the alveoli from collapsing at the end of expiration. Pores of Kohn can interconnect neighboring alveoli, allowing collateral ventilation in some circumstances. Alveolar spaces also host alveolar macrophages, which help remove inhaled particles and pathogens.

Gas exchange occurs primarily by diffusion across the respiratory membrane, driven by differences in partial pressures of O2 and CO2 between the air in the alveoli and the blood in the pulmonary capillaries. The extensive capillary network and large surface area of the alveoli facilitate efficient transfer of gases.

Clinical relevance: diseases that fill or damage alveoli, such as pneumonia, pulmonary edema, or acute respiratory distress syndrome (ARDS), impair gas exchange and can cause hypoxemia. Chronic conditions like emphysema reduce alveolar surface area, lowering overall gas transfer capacity.

Developmentally, alveoli form and mature after birth, continuing to develop into early childhood in humans, contributing to the lung’s full gas-exchange capability.