Concentration of urine in a central core model of the renal counterflow system

Abstract

Concentration of urine in a central core model of the renal counterflow system. In this model descending Henle's limbs (DHL), ascending Henle's limbs (AHL) and collecting ducts (CD) exchange with a central vascular core (VC) formed by vasa recta loops—assumed so highly permeable that the core functions as a single tube, open at the cortical end and closed at the papillary end. Solute supplied to the VC primarily by AHL increases VC osmolality and so extracts water from DHL and CD, increasing their osmolality while diluting AHL fluid. This single effect multiplied by the counterflow arrangement leads to a high papillary osmolality in all structures. Some of the solute may enter DHL to be recycled. In single solute system energy requirements for transport out of AHL decrease from outer to inner medulla. In two solute systems (e.g. salt and urea) mixing in the central core can supply part of the energy for the final concentration of urine. Urea cycling, regulated by ADH, allows active Na + transport in the outer medulla and cortex to be used for concentration in the inner medulla.