The thin segment of the nephron loop's descending limb

Urine is formed by the process of ultrafiltration, and collected in the proximal tubule, from where it passes to the loop of Henle, the distal tubule and the collecting ducts, undergoing the processes of reabsorption and secretion to form the urine that is ultimately passed out. The concentration of urine so formed is governed by the water and elecrolyte balance of the body, and the required concentration is achieved by the processes mentioned above in the nephron

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.