The Transport of Salt and Water across Isolated Rat Ileum

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Abstract
The flows of Na, K, 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 ihe other only active. In the passive channel, the predominant resis tance to ion flow is friction with the water in the channel. The electro-osmotic 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 the passive channel is extracellular. The active channel behaves as two mem -branes in series, the first membrane being semipermeable but allowing active transport of Na, and the second membrane being similar to the passive channel. Friction with the ions in the second membranes the predominant resistance to water flow.