Airway mucosa plasma leakage

There are several consequences of an increase in the permeability of the blood vessels of the airway mucosa. Plasma leakage could result in mucosal edema and the movement of fluid into the airway lumen, both of which could contribute to airflow obstruction. In addition, plasma-derived inflammatory mediators could form in the mucosa and airway lumen, and extravasated plasma proteins could increase the viscosity of sputum. 

Whether an increase in vascular permeability results in mucosal edema depends on the balance between the amount of leakage into the mucosa and the rate of clearance from the mucosa, either through the lymphatics or across the epithelium into the airway lumen. The increase of vascular permeability produced by inflammatory stimuli can result in the bulk flow of plasma into the airway mucosa (Renkin, 1992). The amount of plasma leakage depends upon the number of gaps that form in the endothelium of the leaky vessels, the duration of the gaps and the intravascular pressure that drives the extravasation (Clough, 1991 Taylor and Ballard, 1992). The movement of plasma proteins and other osmotically active solutes into the mucosa can increase the interstitial oncotic pressure, which favors the net movement of fluid out of vessels and further increases the amount of leakage (Taylor and Ballard, 1992). 

As discussed in Section 3.3, few histopathological studies of asthma have focused specifically on the microvasculature. Nonetheless, there is evidence that neovascularization is one of the changes in the airways of asthmatics (Kuwano et al., 1991). This observation in potentially important because newly formed blood vessels may be abnormally leaky (Schoefl, 1967) or abnormally responsive to inflammatory mediators (McDonald, 1992). The plasma leakage that occurs in the airway mucosa of asthmatics could result from the heightened response of the mucosal blood vessels to inflammatory mediators. 

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