A patient produces very dilute urine despite being severely dehydrated. Assuming the glomerulus and proximal tubule are functioning normally, which is the most likely explanation?
AADH is absent or non-functional, so the collecting duct cannot reabsorb water
BThe glomerulus is over-filtering, producing too much filtrate to concentrate
CGlucose reabsorption in the proximal tubule has failed, drawing water into the urine
DUrea accumulation in the medulla has disrupted the osmotic gradient
The collecting duct's ability to reabsorb water depends entirely on ADH inserting aquaporin channels into collecting duct cells. Without ADH, water cannot leave the tubule regardless of the medullary osmotic gradient, producing large volumes of dilute urine — the classic picture of diabetes insipidus. The other options describe different pathologies but would not produce dilute urine despite an intact osmotic gradient.
Question 2 Multiple Choice
The ascending limb of the loop of Henle actively pumps sodium and chloride into the medullary interstitium but is impermeable to water. Why is this impermeability essential to the concentrating mechanism?
AIt prevents the concentrated solutes from being diluted back into the tubule fluid before they can act on the collecting duct
BIt forces urea to remain in the tubule so it can be excreted
CIt ensures glucose is reabsorbed in the proximal tubule rather than the distal tubule
DIt prevents the Bowman's capsule from being overwhelmed by returning water
If the ascending limb were permeable to water, water would follow the sodium and chloride back out, collapsing the osmotic gradient. By pumping solutes out while keeping water inside, the ascending limb builds a hyperosmotic medullary interstitium. This steep gradient is what pulls water out of the collecting duct (under ADH) to concentrate urine. The impermeability is the structural requirement for the counter-current multiplier to work.
Question 3 True / False
The glomerulus filters approximately 180 liters of fluid per day into the nephron, of which only about 1–2 liters becomes urine.
TTrue
FFalse
Answer: True
This is correct and represents one of the most striking facts about kidney function. The glomerular filtration rate is enormous — the kidney dumps nearly everything from the blood into the tubule, then selectively reabsorbs about 99% of the filtrate. This strategy allows the kidney to excrete waste molecules even at low concentrations by first capturing them in the large filtrate volume and then failing to reabsorb them.
Question 4 True / False
Glucose is a normal constituent of urine because it is a metabolic waste product that the kidney is designed to excrete.
TTrue
FFalse
Answer: False
Glucose is actively reabsorbed in the proximal tubule and is absent from healthy urine. The kidney treats glucose as a valuable molecule, not a waste. Glucose only appears in urine (glucosuria) when blood glucose levels exceed the tubular reabsorption threshold — as in uncontrolled diabetes mellitus — because the active transport carriers become saturated. Its presence in urine is a sign of pathology, not normal function.
Question 5 Short Answer
Why does the loop of Henle need to build a high-osmolarity gradient in the medullary interstitium, and how does that gradient allow the collecting duct to concentrate urine?
Think about your answer, then reveal below.
Model answer: The loop of Henle builds the gradient through counter-current multiplication: the descending limb loses water to the already-concentrated interstitium, and the ascending limb actively pumps sodium and chloride out while retaining water, deepening the gradient with each pass. This creates a hyperosmotic zone in the medulla. When ADH is present, it inserts aquaporin water channels into the collecting duct wall. As dilute filtrate flows through the collecting duct surrounded by hyperosmotic interstitium, water moves out osmotically, concentrating the final urine. Without the medullary gradient, there would be no osmotic driving force to pull water from the collecting duct regardless of ADH levels.
The key is that the collecting duct itself does not create the gradient — it only exploits it. The loop of Henle does the work of building osmolarity in the interstitium; ADH controls whether the collecting duct wall is permeable enough to let water respond to that gradient. Both pieces are required: gradient + permeability = concentrated urine.