The fluid that covers the surface of conducting airways (airway surface fluid, ASF) is a critical component of one of the first defense mechanisms of the lung against microbial and other environmental insults. Despite its physiologic importance, ASF is one of the only fluids in the human body whose composition remains poorly defined and understood. Attempts to analyze ASF have been hampered greatly by the fact that it exists only as a very thin layer covering the mucosal surface of airway epithelia. Toovercome some of these limitations, we have applied ultramicroanalytic techniques to microsamples collected in human airways in vivo. In contrast to previous thinking from studies on sputum samples, ASF collected from healthy airways contains much less Na and CI (approximately 450/0 less) and much more K (around 6000/0 more) than extracellular fluid or plasma (ECF), which shows that steep ion gradients exist across normal airway epithelia. These differences also show that ASF composition must be regulated and maintained by active electrolyte transport processes of airway epithelia and that it is not merely the evaporated residue of isotonic secretions or extracellular fluid exudate. However, in patients with sustained airway irritation, infection, or cystic fibrosis, we find that ASF composition appears to become more isotonic with respect to plasma and much more hypotonic in patients with asthma. Joris L, Dab I, Quinton PM. Elemental composition of human airway surface fluid in healthy and diseased airways. Am Rev Respir Dis 1993; 148: 1633-7.

Airway surface fluid (ASF) is an extremely thin (approximately 15 to 30 J. 1m) low-viscosity fluid layer that covers the mucosal surface of airway epithelia lining the conducting airways. Until recently, the major and probably only function attributed to this fluid was to serve as the medium needed for ciliary beating, so that the more viscous mucus layer with its entrapped debris" floating" above it could be effectively propelled out of the lung (1). Consequently, it is assumed that for effective mucociliary clearance, ASF requires precise volume regulation, because effective coupling of ciliary beat energy to mucus propulsion requires the depth of the low-viscosity medium to be closely matched to the height of the cilia. Much less attention has been directed to the role of the airway epithelial ion transport in regulating ASF composition, even though it has become apparent over the last decade that not only the volume but also the composition of ASF is important for maintaining pulmonary homeostasis under normal conditions as well as for defense of the lung against exogenous and endogenous insults.