Respiratory epithelium mucus layer

Airway cross-sections have the nominal anatomy shown in Fig. 5.16. Airway surface liquid (AST), primarily composed of mucus gel and water, surrounds the airway lumen with a thickness thought to vary from 5 to 10 mm. AST lies on the apical surface of airway epithelial cells (mostly columnar ciliated epithelium). This layer of cells, roughly two to three cells thick in proximal airways and eventually thinning to a single cell thickness in distal airways, rests along a basement membrane on its basal surface. Connective tissue (collagen fibers, basement membranes, elastin, and water) lies between the basement membrane and airway smooth muscle. Edema occurs when the volume of water within the connective tissue increases considerably. Interspersed within the smooth muscle are respiratory supply vessels (capillaries, arteriovenous anastomoses), nerves, and lymphatic vessels. 

The pharynx, larynx, trachea and bronchi are lined with pseudostratified, ciliated columnar epithelium that contain at least eight cell types, including mucous secretory goblet and Clara cells, which produce a protective mucus layer of 5-10 jum thickness (see Table 9.2). Subepithelial secretory glands, present in the bronchial submucosa, also contribute to the mucus blanket [9]. Through coordinated ciliary movement a propulsive wave is created, which continuously moves the mucus layer up towards the larynx. Consequently, the mucosal surface of trachea and bronchi is constantly swept to remove inhaled materials. As the bronchi divide into bronchioli, the ciliated columnar respiratory epithelium is much thinner and changes to a simpler non-ciliated cuboidal epithelium. The epithelium in the terminal and respiratory bronchioles consists of ciliated, cuboidal cells and a small number of Clara cells. However, Clara cells become the most predominant type in the most distal part of the respiratory bronchioles [10]. 

MRT mean residence time (drug turnover time), mucosa The lining of the gut. consisting of three layers the inner epithelium lamina propria muscularis mucosae, mucous membrane (mucosa) The moist membrane lining internal structures, e.g. respiratory tract, mucus The fluid secreted by mucous membranes, multideterminant Antigen carrying more than one antigenic determinant. 

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. 

The superficial layer is used to classify the stratification into simple, which consists of a single layer of cells, and stratified, which consists of more than one layer of cells. Stratified epithelium is usually composed of three layers. Cell division takes place in the basal cell layer (layer 1). Cells migrate and differentiate in the intermediate cell layer (layer 2) until they reach the superficial cell layer (layer 3). Specializations include keratinized and ciliated cells. Keratinized epithelium, which contains the cytoskeletal protein keratin and provides a tough impermeable barrier, occurs mainly in the skin. Ciliated cells have apical plasma membrane extensions composed of microtubules capable of beating rhythmically to move mucus or other substances through a duct. Cfiia are common in the respiratory system. 

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