The nephron loop is a U-shaped tubular segment of the nephron located between the proximal and distal convoluted tubules. It consists of three main segments: the thin descending limb, the thin ascending limb, and the thick ascending limb, the latter of which is also referred to as the diluting segment.  

 

As filtrate enters the nephron loop, it contains water, electrolytes, and metabolic waste products.[1] In the thin descending limb, the tubular epithelium is highly permeable to water, allowing water to exit the tubule into the interstitial fluid of the renal medulla.[1] This water is absorbed into the vasa recta, a specialized network of capillaries that runs parallel to the nephron loop, contributing to the concentration of salt and urea within the tubular fluid.[1] During the ascent through the thin ascending limb, sodium chloride passively diffuses out of the tubule into the surrounding medullary interstitium. In the thick ascending limb, sodium chloride is actively transported out of the tubular lumen against its concentration gradient, a process requiring energy in the form of adenosine triphosphate (ATP).[1] This segment is impermeable to water, further contributing to the establishment of a medullary osmotic gradient.