Final answer:
The blood supply to the renal medulla by the vasa recta maintains the NaCl concentration gradient through countercurrent diffusion exchange, integral to the body's osmotic balance and urine concentration.
Step-by-step explanation:
In mammals, blood supply to the renal medulla is by the looped vasa recta, which preserves the NaCl concentration gradient by countercurrent diffusion exchange of solutes and gains of water. The vasa recta works in conjunction with the loop of Henle to form a countercurrent multiplier system, which is crucial for conserving water and maintaining the osmotic balance in the body. This system allows the solute and water recovery essential to reduce the volume and increase the concentration of urine.
As the blood descends into the vasa recta, Na+ and urea can freely enter the capillary, while water exits into the renal medulla. Conversely, as the blood ascends, Na+ and urea are secreted back into the medulla, and water is reabsorbed. This process is carefully balanced to prevent rapid removal of Na+ and urea which would disrupt the osmolar gradient needed for the recovery of water and solutes back into the circulation.