Concentration of urine at the end of collecting duct is

The concentration of urine at the end of the collecting duct is hypertonic compared to blood plasma. This is a result of the complex processes that occur within the nephrons of the kidney.

In the final stages of urine formation, the filtrate reaches the collecting ducts. The collecting ducts are permeable to water and urea but less permeable to solutes like sodium and other ions. The concentration of the urine is determined by the osmotic gradient established by the loop of Henle, specifically the descending and ascending limbs. The loop of Henle creates a counterflow exchange system, where the descending limb of the loop is permeable to water but not to ions. As the filtrate descends into the medulla of the kidney, water is reabsorbed into the surrounding interstitial fluid, which becomes increasingly hypertonic. In the ascending limb of the loop, ions like sodium are actively transported out of the filtrate, further increasing the osmolarity of the interstitial fluid. This sets up a concentration gradient that allows water to move out of the collecting ducts when they pass through the hypertonic medullary region. As water is reabsorbed from the filtrate, the urine becomes more concentrated and hypertonic to blood plasma.