A Better Explanation of Countercurrent Multiplication in the Formation of the Corticopapillary Osmotic Gradient in the Outer Medulla

Countercurrent multiplication (CCM) is widely accepted as the mechanism for the generation of the corticopapillary osmotic gradient in the outer medulla of mammalian kidneys. However, several issues in the literature cause the current explanations of CCM to be inefficient and incomplete. As a result, it is challenging to clearly explain CCM in physiology education. The goal of this article is to share a modified version of CCM with more understandable explanation in the hopes of motivating peer discussion, further improvement, and future research. To reach this goal, the logical processes leading to CCM are first analyzed, which results in a set of formulas that serve as the principles governing CCM. Next, the cessation of CCM is addressed to provide a complete picture of the modified version of CCM. Throughout these two steps, the issues mentioned above are identified and addressed so that how the modified version of CCM eliminates these issues becomes clear. The formulas mentioned above are provided in the Tables S1, S2, and S3 (all Supplemental material is available in the Supplemental Excel File at https://doi.org/10.6084/m9.figshare.23515614) to explain how the interstitial and intrathick ascending limb osmotic concentration (OC) values used in the figures in this article are simulated and how alternative OC values can be generated from Tables S1 and S2 to illustrate CCM.