A digital computer model of the renal medullary countercurrent system. I

On the solution of equations for renal counterflow models

The results of a comparative study of three discretization techniques and the solution of the resulting algebraic equations by three methods is given. For this study, a four-tube central core model with diffusion in the core was selected and equations were derived for a coherent and efficient implementation. The results of this study show that sparse matrix techniques that take the physiological connectivity of the kidney lead to significant savings in computer storage, running time and overall cost.

Active transport membrane concentration amplifiers

Three different active transport membrane configurations are suggested for achieving concentration amplification in a circulating substrate solution. The steady-state characteristics of the structures are investigated on the basis of a linear approximation of the rate of active transport. An analysis of the effects of system parameters and geometry on the concentration gradient along the flow path is presented. It is seen that the concentration gradient may be synthesized by appropriate choice of membrane arrangement, flow and physico-chemical transport parameters.

A 'bootstrap' model of the renal medulla

The ‘bootstrap’ model of the handling of salt, water and urea by the renal medulla is based on the concepts and data of Stephenson (1972), and Kokko and Rector (1972), and adds support to them. Revision of the traditional counter current hypothesis is necessary since active transport of salt and urea is absent from the inner medulla. This advanced model is structured appropriately and demonstrates how events in the medulla interact and vary with time as change from a diuretic to an antidiuretic state proceeds.