Background information respiration | Anatomy | Microscopic structures | Alveoli

Histologic constituents of alveoli

The tissue layer separating the gas in the alveoli from the capillary blood is extremely thin in order to facilitate diffusion of oxygen and carbon dioxide. It also provides a mechanical barrier to prevent bleeding into the alveole.

The overall surface of the alveolar walls comprises an area of 50-100 m² (about as large as a tennis court and 50times larger than the body surface). Such an enormous area only can be generated when the lung divides into millions (ca. 300) of small alveoli with an average diameter of 0.3mm.

Each alveole is a cup-shaped, polyhedral thin-walled sac, which lacks one wall and opens into a respiratory bronchiole, an alveolar duct, or an alveolar sac. The surfaces facing the air are lined by an epithelium made of two types of cells, type I and type II alveolar cells (pneumocytes). Most of the surface (about 95%) is lined by type I alveolar cells, which are extremely squamous. Their nuclei are usually far apart from very thinly extending cytoplasmic processes.

Type II alveolar cells are found interspersed among the type I cells, singly or in clusters, often in the angles of the alveolar walls. They are fairly large cuboidal cells that have a large nucleus and vacuolated cytoplasm. Type II cells secrete surfactant, a phospholipid that spreads over the alveolar surfaces to reduce surface tension and prevent the alveoli from collapsing upon expiration. Surfactant also has a bactericidal effect.

Within the septal wall are found capillaries, elastic and collagen fibres, fibroblasts and macrophages called dust cells. Exchange of gases occurs in the portions of the septa, which only contain the type I cell, the fused basement membranes of the type I cell and the endothelial cell, and the endothelial cell of a capillary.

Literature:

Klinke R, Silbernagel S. Lehrbuch der Physiologie. 2001. Thieme: Stuttgart

Ross MH, Gordon KI, Pawlina W. Histology: A text and atlas. 2002. Lippincott Williams and Wilkins: Philadelphia

Microscopic structures of the alveoli