Why Do We have to Move Fluid to be Able to Breathe?

Abstract

The ability to breathe air represents a fundamental step in vertebrate evolution that was accompanied by several anatomical and physiological adaptations. The morphology of the air-blood barrier is highly conserved within air-breathing vertebrates. It is formed by three different plies, which are represented by the alveolar epithelium, the basal lamina, and the endothelial layer. Besides these conserved morphological elements, another common feature of vertebrate lungs is that they contain a certain amount of fluid that covers the alveolar epithelium. The volume and composition of the alveolar fluid is regulated by transepithelial ion transport mechanisms expressed in alveolar epithelial cells. These transport mechanisms have been reviewed extensively. Therefore, the present review focuses on the properties and functional significance of the alveolar fluid. How does the fluid enter the alveoli? What is the fate of the fluid in the alveoli? What is the function of the alveolar fluid in the lungs? The review highlights the importance of the alveolar fluid, its volume and its composition. Maintenance of the fluid volume and composition within certain limits is critical to facilitate gas exchange. We propose that the alveolar fluid is an essential element of the air-blood barrier. Therefore, it is appropriate to refer to this barrier as being formed by four plies, namely (1) the thin fluid layer covering the apical membrane of the epithelial cells, (2) the epithelial cell layer, (3) the basal membrane, and (4) the endothelial cells.

Glycemic Variability in Diabetes Increases the Severity of Influenza

“…To determine whether endothelial exposure to glucose variability could lead to more severe influenza, the effect of glucose variability on an established in vitro model of the epithelial-endothelial respiratory barrier was assessed (17,29). This in vitro model reflects the distal lung region where epithelial cells are covered in a fluid layer, called the alveolar lining fluid (30). Accordingly, the cells are cultured in medium instead of at an air-liquid interface, unlike cells in in vitro models of the upper respiratory tract (31).…”

Mouse Models of Acute Respiratory Distress Syndrome

“…The net movement of NaCl from the airspacelining fluid into interstitial tissue creates an osmotic gradient across the alveolar epithelium, which causes water to translocate to the interstitium. When normal processes that regulate ionic equilibrium are impaired, pulmonary edema develops (Ware and Matthay 2001;Schuster and Marklin 1986;Fronius, Clauss, and Althaus 2012;Ware et al 1999;Matthay, Folkesson, and Clerici 2002). It has been shown that the degree of impaired alveolar fluid clearance in human ARDS patients is a predictor of hospital mortality (Ware and Matthay 2001;Sartori and Matthay 2002).…”

Hydrostatic pressure activates ATP‐sensitive K+channels in lung epithelium by ATP release through pannexin and connexin hemichannels

“…Further, the apical side of epithelial cells is entirely covered by a thin fluid layer (42). This fluid layer can be found in lungs of all air‐breathing vertebrates (43) and represents a fourth ply of the air–blood barrier (42). The height and composition of this fluid layer are strictly balanced to guarantee proper lung functions such as gas exchange and immune defense (42).…”

“…This fluid layer can be found in lungs of all air‐breathing vertebrates (43) and represents a fourth ply of the air–blood barrier (42). The height and composition of this fluid layer are strictly balanced to guarantee proper lung functions such as gas exchange and immune defense (42). This balance is realized by the active ion transport mechanisms of pulmonary epithelial cells.…”