The Role of the Unstirred Water Layer in Intestinal Permeation

Abstract

The concept of the unstirred layer (UWL) was originally developed by Noyes and Whitney (1897, quoted by Dainty and House 1966a) and later by Nernst (1904, quoted by Andreoli and Trautman 1979), and extended more recently by Teorell (1936), Dainty (1963), Dainty and House (1966c), and Ginzburg and Kalchalsky (1963). These workers called attention to the potential relevance of unstirred layers in regulating the transport of materials across biologic interfaces. The presence of UWLs at solid-liquid interfaces was demonstrated to be a real physical characteristic of such interfaces by direct microscopic examination by Green and Otori (1970). Since it is virtually impossible to stir a solution so that complete mixing occurs right up to the interface (Dainty 1963), the presence of unstirred layers must be considered in studies of membrane permeation of all but the slowest permeating substances (Lortrup 1963). The actual thickness of the unstirred layer depends partly on the molecular weight of the probe and partly on the rate of stirring (Dainty and Hope 1959 a, b). The fluid in the UWL is not stationary, but is rather a region of slow laminar flow parallel to the membrane in which the only mechanism of transport is by diffusion; the layers are often called “Nernst diffusion layers.” The thickness of the layer is not the actual thickness, but rather an operational value.