Juxtamedullary

Juxtamedullary is an anatomical term used to describe structures that are adjacent to or located near the medulla of an organ, most commonly the kidney. The term derives from Latin, with juxta meaning near and medullaris meaning medullary. In the context of renal anatomy, it is applied to a subset of nephrons that lie near the corticomedullary junction.

Juxtamedullary nephrons are distinguished from cortical nephrons by their location and structure. About 15–20% of human nephrons are juxtamedullary. Their glomeruli sit close to the boundary between the cortex and the outer medulla, and their long loops of Henle extend deep into the outer and sometimes the inner medulla. This contrasts with cortical nephrons, which have short or relatively shallow loops that mostly reside in the cortex.

The extended loop of Henle in juxtamedullary nephrons, along with the surrounding vasa recta, underpins the kidney’s ability to concentrate urine. The long loops create and maintain a hyperosmotic medullary interstitium through the countercurrent multiplier mechanism, while the vasa recta preserves the osmotic gradient as blood flows through the medulla. Together, these features enable more efficient reabsorption of water and production of concentrated urine when needed.

In clinical and physiological discussions, the term juxtamedullary highlights the functional distinction from cortical components of the nephron and is important in understanding renal water handling, fluid balance, and certain pathophysiological conditions affecting urine concentration.