Respiratory Tissue: Structure, Histophysiology, Cytodynamics Part II. New Approaches and Interpretations

Abstract

This chapter focuses on the alveolar cell. Examination of respiratory tissues from a variety of animal species revealed that there appeared to exist little appreciable difference in alveolar wall structure between most mammalian species. The framework of the alveolar wall is predominantly composed of reticulin. Most of the reticulin occurs in the form of basement membranes. Elastic tissue is present chiefly in the form of individual fibers, which apparently occur at a rather random distribution in the alveolar wall. A higher concentration of the elastic tissue is found at the level of the alveolar duct, particularly surrounding the opening to alveoli. Several distinct cell types are distinguished in the respiratory tissue are: (1) pulmonary surface epithelial cells, ( 2 ) alveolar cells, occurring as two morphological forms, (3) endothelial cells of the pulmonary capillaries, and ( 4 ) migratory blood cells. The dense network of blood capillaries are physiologically the most important components of the alveolar tissue. These are externally protected by the continuous, simple squamous pulmonary surface epithelium, preventing the direct exposure of capillary loops to the alveolar air. A direct connection has to exist between the external environment and the innermost alveolar spaces of the lung, contamination of the latter by inhaled extraneous material cannot be avoided. The alveolar tissue is protected in this respect by millions of alveolar cells, busily engaged in the phagocytosis and ultimate elimination of this material.