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Salt and water absorption in the human colon: a modern appraisal
  1. Molecular Medicine Unit,
  2. St James’s University Hospital,
  3. Beckett Street, Leeds LS9 7TF, UK.

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The past 20 years have seen many advances in all aspects of colonic physiology, and the unrelenting appearance of new information is daunting to clinicians and scientists alike. Nevertheless, we should not lose sight of the fact that the main function of the human colon is to absorb about 90% of the 1.5–2 litres of ileal effluent which passes daily through the ileocaecal valve.1 ,2 In mammalian species, the key determinant of colonic water absorption is the rate of Na+ absorption. We now know that Na+ transport processes are not distributed uniformly throughout the human colon, a concept which has important clinical implications. This review provides an update on the basic mechanisms underlying salt and water transport in the human colon in health and disease, and highlights several interesting areas for future research.

General description of Na+ absorptive processes

The human colon has a nominal mucosal surface area of about 2000 cm2,3 but in reality the total absorptive area is even greater because colonic crypt cells are capable of absorption as well as secretion.4 Although it is well established that the rates of colonic salt (Na+ plus Cl) and water absorption are directly related,5 it is only recently that we have begun to appreciate the array of Na+absorptive processes present in human colon. These show considerable intrinsic segmental heterogeneity.6-10 This explains, at least in part, why the colon’s capacity for sodium and water absorption in vivo is greater in the proximal (ascending) segment than in the distal (descending and sigmoid colon/rectum) segment.11-15 Several different active (transcellular) Na+ absorptive processes exist in human colon. It will become clear that segmental differences in the distribution and regulation of these processes play an important role in colonic Na+ salvage during periods of …

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