For an individual with highly concentrated urine, filtrate entering the loop of Henle is likely to be more concentrated than filtrate exiting the loop of Henle. Why? If the descending loop of Henle allows water to passively diffuse OUT of the loop into the interstititum, wouldn't you want filtrate entering the loop of Henle to be less concentrated? That way, more water can passively leave in the descending loop and you could concentrate the urine more in the ascending loop. Also I found these statements online and everything sounds correct. If anything looks wrong please post! Thanks! If you have a long loop of Henle (ie desert rat), you can make your interstitium really salty and reabsorb almost all your water. This will concentrate the urine. The thick ascending limb is impermeable to water allowing the area outside of the tubules (interstitium) to become concentrated and the area inside the tubule to become dilute. The ascending part is necessary to concentrate the interstitium with salt/ions, creating a concentration gradient so that WATER can be absorbed in the descending part. Anti-Diuretic Hormone (ADH) works on the collecting duct of the kidney. My question: only on the collecting duct?