Final answer:
The statement that osmolality is greatest in the renal medulla is true; this is due to the function of the loop of Henle, which acts as a countercurrent multiplier, creating high osmotic gradients necessary for urine concentration.
Step-by-step explanation:
True or false: osmolality in the kidney is greatest (highest concentration) in the renal medulla. This statement is true. The osmotic gradient, which contributes to urine concentration in the kidney, reaches its highest level in the renal medulla. This gradient is established through the process involving the countercurrent multiplier and the loop of Henle. Specifically, the descending limb of the loop of Henle, which is permeable to water but not to solutes, helps to increase osmolality as it descends into the medulla. The filtrate becomes more concentrated as water moves out into the higher salt concentration of the interstitial fluid surrounding the loop. At the same time, the ascending limb, which is permeable to ions but not to water, allows ions to leave the filtrate, contributing to the high osmolality of the surrounding interstitial fluid in the renal medulla.