Physiological Pharmaceutics

112 Physiological PharmaceuticsFigure 6.3 Structure of a villus. Note blood and lymph flowThe major features of a villus are illustrated in Figure 6.3. Each villus contains anarteriole and a venule, and also a blind-ending lymphatic vessel called a lacteal. The arterioleand venule do not anastomose in the small intestine as they do in the gastric mucosa. Smallmolecules absorbed through the villus pass into the descending loop of the villus capillaryand diffuse into the ascending vessel. This creates a counter-current exchange system in eachvillus, which is relatively inefficient since it decreases the concentration gradient for passivediffusion. The efficiency of this process has been estimated to be around 15% and has thenet effect of slowing the rate of absorption.MicrovilliEach enterocyte has about 1000 minute processes or microvilli which project into theintestinal lumen. Multiple actin filaments extending into the interior of each microvillus arebelieved to be responsible for contraction of the microvilli and thus the movement of thefluid immediately in contact with the surface.The membrane which forms the microvilli on the outer surface of the absorptive cellsis rich in protein, cholesterol and glycolipids and contains enzymes, disaccharidases andpeptidases which are localised within the surface membrane. Specific receptor proteins arelocated on the microvillus membrane-surface coat complex which selectively bindsubstances prior to their absorption, including the intrinsic factor-vitamin B 12complex inthe ileum and conjugated bile salts in the distal intestine.EpitheliumThe epithelium which covers the intestinal villi is composed of absorptive cells, goblet cells,a few endocrine cells and tuft or calveolated cells. The goblet and endocrine cells closelyresemble those found in the crypts. The tuft or calveolated cells are characterized by long,broad apical microvilli and an intra-cytoplasmic system of tubules and vesicles. These cells