Permeability characteristics of the human small intestine.

Abstract

Water movement into the small intestine in response to osmotic gradients created by non-lipid soluble solutes of graded molecular size (urea, 2.3 A; erythritol, 3.2 A; and mannitol, 4.0 A) and by sodium chloride was measured in normal subjects. Mannitol was not absorbed to a measurable extent, and this solute was therefore assumed to be capable of exerting its full theoretic osmotic pressure. Water movement in response to osmotic gradients created by mannitol was, therefore, a reasonably accurate measure of the filtration coefficient of the small intestinal mucosa. It was found that there was a progressive fall in bulk water movement from proximal to distal segments of the small intestine in response to hypertonic mannitol, the upper intestine showing a 9-fold higher permeability to water flow than the lower small intestine. The ability of urea, erythritol and sodium chloride to exert osmotic pressure, relative to that exerted by mannitol, was found to vary at different levels of the small intsstine. In the jejunum, urea, erythritol and sodium chloride were only 45 to 60% as effective as mannitol in inducing bulk water flow, whereas in the ileum they were almost as effective as mannitol. These data indicate a basic difference in the permeability of the upper and lower small intestine that cannot be explained by differences in surface area or blood flow. The observations can be explained by differences in the radius of pores in the membranes of the small intestine. With the formulas and concepts of Renkin and Solomon the effective pore radius of the jejunum was calculated to be at least 2 times larger than that of the ileum.