The Intestinal Surface Microenvironment Consists of Glycocalyx, Mucus, and an UnstirredWater Layer
Liberally interspersed among the enterocytes are goblet cells, which secrete a rich layer of mucus that covers the mucosa. At the brush border surface, the mucus blends into the glycocalyx, with the two layers forming a viscous coating that tends to trap molecules near the apical membrane.
In addition to the mucus and glycocalyx layers, there is an area near the intestinal surface known as the unstirred water layer. With respect to the unstirred water layer, the intestine can be likened to a large stream or river; that is, the water in the center flows relatively rapidly, whereas the water near the edge or bank is quiet and flows slowly. Because of the same fluidfriction phenomenon that causes the water on the banks of rivers to be less turbulent and to flow at a slower rate than water in center stream, the waler very near the intestinal surface is quiet and flows at a much slower rate than water in the central part of the lumen. The unstirred water layer, mucus, and glycocalyx form an important diffusion barrier through which nutrients must pass before entering the enterocytes.
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