I General principles of drug absorption Body membranes are generally classified as three main types those composed of several layers of cells, as the skin those composed of a single laver of cells, as the intestinal epithelium those of less than one cell in thickness, as the membrane of a single cellI. General principles of drug absorption Body membranes are generally classified as three main types: - those composed of several layers of cells, as the skin, - those composed of a single layer of cells, as the intestinal epithelium, - those of less than one cell in thickness, as the membrane of a single cell