ALVEOLI

Occurring only in the respiratory portion (which their presence distinguishes from the conducting portion), these small (about 200 um in diameter) sacs open into a respiratory bronchiole, an alveolar duct, an atrium, or an alveolar sac. They are separated by thin walls termed interalveolar (or alveolar) septa.

A. Interalveolar Septa: The structural features of these septa, which are specialized for gas exchange, are critical to respiratory function. The septa consist of 2 simple squamous epithelial layers with the interstitium sandwiched between them. The interstitium consists of continuous (nonfenestrated) capillaries embedded in an elastic connective tissue that includes elastic and collagen fibers, ground substance, fibroblasts, mast cells, macrophages, leukocytes, and contractile interstitial cells that contract in response to epinephrine and histamine. This elastic tissue is an important component of the ventilating mechanism. Gas exchange occurs between the air in the alveolar lumen and the blood in the interstitial capillaries.

1. Blood-air barrier. This term refers to the structures that oxygen and CO2, must cross to be exchanged. Varying from 0. 1-1.5 um in thickness, it includes the following layers: a. The film of pulmonary surfactant on the alveolar surface. b. The cytoplasm of the squamous cpithelial (type I alveolar) cells. c. The fused basal laminae sandwiched between the type I alveolar and capillary endothelial cells. d. The cytoplasm of the squamous endothelial cells lining the intcrstitial capillaries.

2. Alveolar pores. Each septum may be interrupted by one or more pores from 10 to 15 um in diameter. These connect adjacent alveoli and may help to equalize pressure and allow collateral air circulation, thus maximizing the use of available alveoli when some small airways are blocked.