Passive movement of water and sodium across the human small intestinal mucosa.

Abstract

The permeability of the intact human small intestinal mucosa to H2O, Na, urea, and mannitol was studied by continuous intestinal perfusion. A method is presented for determining H2O diffusion and unidirectional Na flux based on the rate of tracer disappearance from the intestinal lumen. From an estimate of the effective osmotic pressure generated by urea, and of the filtration flow caused by hypertonic mannitol solutions, Renkin''s formulas yielded equivalent pore radii of 7.9 A in jejunum and of 2.9 A in the ileum. Compared with H2O filtration, the resistance to H2O diffusion was higher than expected for a membrane penetrated by uniform pores of less than 30 A radius. The human intestinal mucosa behaves as a serial membrane where the resistance to filtration flow is provided by the luminal cell wall, penetrated by pores of 2.9-8 A radius, while diffusion is restricted by 1 or more additional barriers which do not restrict filtration flow. The unidirectional Na flux was similar in magnitude to the rate of H2O diffusion.