alveolar sac

The alveolar sac is composed of multiple alveoli (typically 2 or more) arranged around a central space that opens into an alveolar duct, which in turn arises from a respiratory bronchiole. Alveolar sacs are the terminal structures of the conducting/respiratory pathway, forming the bulk of the functional gas-exchange surface of the lung.

Each alveolus within the sac is lined by type I pneumocytes (95% of surface area, thin squamous cells ideal for gas diffusion) and type II pneumocytes (produce surfactant, serve as progenitor cells for regeneration after injury). The alveolar-capillary interface (blood-air barrier) consists of the type I pneumocyte, fused basement membrane, and capillary endothelial cell, allowing rapid diffusion of O2 and CO2.

Pores of Kohn connect adjacent alveoli within and between alveolar sacs, allowing collateral ventilation and equalization of pressure, which is clinically relevant in conditions causing localized obstruction (they can allow air to bypass blocked airways but also facilitate spread of infection).

Clinical relevance: Diseases affecting alveolar sacs include emphysema (destruction of alveolar walls leading to enlarged, fewer alveolar sacs and decreased surface area for gas exchange), pulmonary fibrosis (thickening of interstitium impairing diffusion), and neonatal respiratory distress syndrome (surfactant deficiency causing alveolar collapse). Understanding the respiratory zone anatomy (respiratory bronchioles → alveolar ducts → alveolar sacs → alveoli) is essential for understanding pulmonary function tests, gas exchange physiology, and pathology of restrictive vs. obstructive lung diseases.