8th February 2024

Loop of Henle

Overview

Sodium is actively reabsorbed via Na+/K+/2Cl- cotransporters in the ascending limb. 90% of sodium is actively reabsorbed in between the proximal tubule and the ascending limb of the loop of Henle, creating a high osmolality within the renal medulla. Water then moves passively out of the descending limb of the loop of Henle to counteract this until an equilibrium is reached. The thick ascending limb is impermeable to water, creating a medullary concentration gradient and reducing the tubular osmolality when it reaches the distal convoluted tubule, allowing urine concentration. The counter-current mechanisms, i.e. the flow of tubular fluid through the nephron in one direction and the flow of blood through the vasa rectus in the opposite direction, along with the opposing flow of the tubular fluid in the descending and ascending limbs, also contribute to creating the osmotic gradient.  

Loop diuretics compete for the chloride binding sites on these cotransporters. As they are larger than chloride ions, they cannot be transported across the membrane, thus inhibiting the transporter. Reduced sodium reabsorption reduces the osmolality within the medulla, and therefore, less water moves passively out of the tubule, resulting in a greater urine volume.

Image Source: Wikimedia Commons (2011). Kidney nephron molar transport diagram, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/2/26/Kidney_nephron_molar_transport_diagram.svg  (Accessed: 4 January 2024).

References

Kumar, P. and Clark, M., 2012. Kumar & Clark’s clinical medicine. 8th ed.

Fairweather, J., Findlay, M., & Isles, C. (2020). Clinical companion in nephrology. Springer International Publishing. 

Tortora, G. J., & Derrickson, B. (2017). Tortora’s principles of anatomy and physiology. John Wiley and Sons.