Basement membrane alveolar-capillary

In order to better understand the pathophysiology associated with these various forms of pulmonary edema, a review of the morphology associated with the capillary-alveolar—intestinal interlace is useful (Fig. 6). From this review it can be seen that fluid in pulmonary capillaries is separated from the alveolar interstitial tissue by the capillary endothelial cells and the capillary basement membrane (commonly called the endothelial barrier). The alveolar surface is separated from the interstitial space by the alveolar-airway barrier, which consists of the alveolar basement membrane, alveolar epithelium, and a layer of pulmonary surfactant within the alveolus. As described previously, the alveolar interstitial tissue is made up of connective tissue (elastin and collagen), fibronectin, and mucopolysaccharides. The interstitial space also contains the pulmonary lympatic system, which functions to drain proteins, large particulate matter, and excess fluid away from the tissue space and to return them to the blood. 

Most direct toxins entering the alveoli primarily affect Type I cells and their associated capillary endothelial cells. After acute injury, the epithelium and/or underlying capillary endothelial cells may swell and disrupt, distort, or lose their connections with others, leaving large areas of basement membrane uncovered. This allows fluid to move into the alveolar lumen from capillaries, with subsequent pulmonary edema. 

Barriers to pulmonary absorption of proteins and peptides include respiratory mucus, mucociliary clearance, pulmonary enzymes/proteases, alveolar lining layer, alveolar epithelium, basement membrane, macrophages and other cells [3, 18]. The molecular weight cutoff of tight junctions for alveolar type I cells is 0.6 nm, while endothelial junctions allow the passage of larger molecules (4-6 nm). In order to reach the bloodstream in the endothelial vasculature, proteins and peptides must cross this alveolar epithelium, the capillary endothelium, and the intervening extracellular matrix. 

Figure 1 Diagrammatic representation of the ultrastructure of the respiratory membrane. Arrows indicate the passage of drugs (horizontal heavy lines) through the respiratory membrane after alveolar or capillary exposure, or of metabolites (horizontal broken lines) generated in the epithelial or endothelial layers. Key (1) monomolecular surfactant layers, (2) thin fluid film, (3) interstitial space, (4) endothelial capillary basement membrane, (5) drug transport from the alveoli, (6) absorption of drug into endothelial cells from the circulation, (7) transport of drug from the circulation to alveolar epithelium, (8) transport of drug from the circulation to the alveoli. (From Ref. 102. Reproduced by permission, CRC Press, Inc.)...

The predominant involvement of the respiratory tract in oxygen toxicity is probably due to the fact that the lungs are the first and only organ to come in contact with the full Pio,. Dyspnea, capillary congestion, alveolar exudation, hemorrhage, atelectasis, swelling of alveolar walls with encroachment on alveolar spaces, fragmentation of basement membranes between alveolar and endothelial cell layers, accumulation of exudate between the basement... 

The type I alveolar cell constitutes over 90% of the alveolar surface. This cell is highly differentiated and very flat. It provides the thin surface through which gas exchange with the pulmonary capillaries occurs. It is separated from the endothelial cells that line the capillary lumen by a common basement membrane. 

FIGURE 1. A transmission electron micrograph of lung tissue illustrating attenuated type 1 pneu-mocytes (arrowheads), type II pneumocyte (arrow), and a pulmonary alveolar macrophage (PAM), a = alveolus c = capillary asterisk = platelet double arrowheads = capillary endothelium. The blood-air barrier consists of the type I pneumocyte, capillary endothelium, and the basement membrane separating these two cells. 

FIGURE 3. A transmission electron micrograph of a section of an alveolar septum of lung tissue of a rat pup exposed to nicotine via the placenta and mother s milk. Swelling of the endothelial cell (asterisks) of the blood-air barrier occurs. Note the focal degeneration of the endothelial cell cytoplasm (star). The blood-air barrier is composed of the endothelial cell (asterisks), type I cell (double arrowheads), and the basement membrane between these cells. Swollen mitochondria can be seen in the interstitial cells (arrowheads). Collagen fibers also occur (arrows), cap = capillary. 

Donald KJ, Edwards RL, McEvoy JD. Alveolar capillary basement membrane lesions in Goodpasture s syndrome and idiopathic pulmonary hemosiderosis. Am J Med 1975 59(5) 642 9. 

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