Abstract
The flows of sodium, potassium, and chloride under electrical and chemical gradients and of salt and water in the presence of osmotic pressure gradients are described by phenomenological equations based on the thermodynamics of irreversible processes. The aim was to give the simplest possible description, that is to postulate the least number of active transport processes and the least number of separate pathways across the intestine. On this basis, the results were consistent with the following picture of the intestine: Two channels exist across this tissue, one allowing only passive transport of ions and the other only active. In the passive channel, the predominant resistance to ion flow is friction with the water in the channel. The electroosmotic flow indicates that the passive channel is lined with negative fixed charged groups having a surface charge density of 3000 esu cm-2. The values of the ion-water frictional coefficients, and the relationship between ionic concentrations and flows indicate that the passive channel is extracellular. The active channel behaves as two membranes in series, the first membrane being semipermeable but allowing active transport of sodium, and the second membrane being similar to the passive channel. Friction with the ions in the second "membrane" is the predominant resistance to water flow.
Highlights
Published reports indicate a strong interdependence between the flows of salts and water across the ileum
The other point of view postulates that water flow is dependent on solute flow (Curran and Solomon (3); Clarkson and Rothstein, (4)) and that water may be transported against its activity gradient by active transport of solutes (Curran, 5)
The results indicate a property of the active transport system hitherto unsuspected; namely, that the rate-determining resistance to water is the frictional interaction with the ions present in the second membrane
Summary
Published reports indicate a strong interdependence between the flows of salts (or solutes generally) and water across the ileum. First put forward by Ingraham and Visscher (1) and recently reaffirmed by Grim (2) for transport across gall bladder, maintains that salts are transported by entrainment in the water flow. The other point of view postulates that water flow is dependent on solute flow (Curran and Solomon (3); Clarkson and Rothstein, (4)) and that water may be transported against its activity gradient by active transport of solutes (Curran, 5). THE JOURNAL OF GENERAL PHYSIOLOGY · VOLUME 50 · 1967. Kedem (8) has demonstrated that this model offers a simple method for quantitative treatment of water flow in the presence of active transport of solutes
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