Water follows urea as it is reabsorbed from the collecting duct. This actually increases the concentration of urea in the tubular fluid despite some of it being reabsorbed. This means that urea concentration only drops moderately when the urine concentration falls due to increased reabsorption of water. | Water follows urea as it is reabsorbed from the collecting duct. This actually increases the concentration of urea in the tubular fluid despite some of it being reabsorbed. This means that urea concentration only drops moderately when the urine concentration falls due to increased reabsorption of water. |