Periciliary fluid

Cilia are thin cylindrical hair-like structures with a cross-sectional radius of 0.1 gm projecting from the apical epithelial surface of ciliated columnar cells. Ciliary length is thought to correspond to periciliary fluid depth and range from approximately 7 gm in proximal airways to roughly 5 gm in more distal airways.- Each ciliated epithelial cell supports approximately 200 cilia at a density of eight cilia/gm. Short microvilli, possibly associated with secretory functions, are interspersed among the cilia. 

Airway surface liquid (ASL) A mixture of periciliary fluid and submucosal... 

Periciliary fluid Transepithelial secretion along the conducting airways con-... 

Mucus is cleared continuously by cilia, which extend through the periciliary fluid to hook the mucus layer. The claw-like tips of the cilia engage with the mucus layer and transport the latter toward the nasopharynx [8,9]. Cilia are complex motile structures that extend from the surface of columnar ciliated cells the number of cilia per cell is approximately 200 with a cell density of 6-8 cilia per pm2. These hair-like protrusions range in length between 5 and 10 pm and width from 0.1 to 0.3 pm. 

Major determinants of the efficiency of mucociliary clearance are cilia density, periciliary fluid, and composition of mucus. Some drugs and excipients, such as preservatives in drug formulations, may diminish the ciliary movement in the nasal cavity and trachea. A suggested adverse effect of ciliostasis (permanently or momentarily arrest or impairment of ciliary activity) is lower respiratory tract infection as a result of impaired nasal microbiological defense. 

Periciliary fluid is a watery, ionic solution, maintained by transepithelial ion transport, that provides an environment within which the cilia are able to beat. It also provides a reservoir of fluid for the humidification of inspired air. The depth of the periciliary fluid dictates whether the overlying mucus layer is at the ciliary tips and thus available for clearance ... 

Should the periciliary fluid become too deep, it is hypothesized that the ciliary tips would be unable to reach the mucus layer and that clearance would therefore be compromised. 

If the periciliary fluid were too shallow, the cilia would become entangled in the mucus gel and mucus transport would again be compromised. 

The cilium unrolls within the periciliary fluid ready for the next effective stroke. Undergoing the recovery stroke beneath the mucus layer prevents retrograde mucus transport (Figure 9.5). 

The structure and function of the epithehal ciha have been the subject of excellent reviews (19,20). The periciliary fluid appears to be regulated by various mechanisms, the most important of which are active water and ion transport across the airway surface epithelium (21,22). If the depth of the periciliary fluid is too shallow, the cilia are unable to beat effectively and may become entangled in the mucous gel layer. On the other hand, a fluid layer too deep may not allow the cilia to make contact with the mucous gel layer, thereby decreasing mucociliary clearance. The depth of the periciliary layer may be regulated by homeostatic mechanisms. 

The transport velocity of mucus-simulant gels is directly related to mucus s elasticity and the depth of the periciliary fluid, and it is inversely related to mucus viscosity (1). An ideal viscoelastic ratio may exist for optimal mucociliaiy interaction an increase in viscosity or a decrease in elasticity would result in a reduced transport rate. Transport by cough or airflow interaction depends inversely on viscosity, elasticity (spinnability), and adhesivity (1). Mucus that is elastic, rather than viscous is transported well by ciliary action, but less well by coughing (2). 

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