Isotonic urine is formed at the level of
First, I need to recall the structure of the nephron and the process of urine concentration. The nephron has the glomerulus, proximal convoluted tubule, loop of Henle (with ascending and descending limbs), distal convoluted tubule, and collecting duct.
Urine concentration happens mainly in the loop of Henle. The descending limb is permeable to water, so as the filtrate moves down, water is reabsorbed, making the filtrate more concentrated. The ascending limb is impermeable to water but actively transports NaCl out, which dilutes the filtrate. So by the time the filtrate reaches the end of the ascending limb, it's hypotonic. However, the collecting duct, under the influence of ADH, allows water reabsorption, making the urine isotonic or hypertonic depending on ADH levels.
Wait, isotonic urine would be when the osmolality is similar to plasma. Normally, in the absence of ADH, the collecting duct doesn't reabsorb much water, so the urine is dilute. But isotonic urine might occur in a specific part. Let me think again.
The proximal convoluted tubule reabsorbs about 65% of the filtered water and solutes, so the filtrate here is isotonic. Then, in the loop of Henle, the descending limb starts to concentrate the filtrate, but the ascending limb dilutes it. So isotonic urine would form in the proximal tubule. Wait, the proximal convoluted tubule reabsorbs isosmotically, so the filtrate remains isotonic. Then, the loop of Henle starts to change the osmolality.
So the correct answer is the proximal convoluted tubule. Let me check the options again. The options are A, B, C, D. The correct answer is C, for example. But I need to make sure the options are correct. The question is about where isotonic urine is formed, which is the proximal tubule. Therefore, the answer should be the proximal convoluted tubule.
**Core Concept**
Isotonic urine formation occurs in the **proximal convoluted tubule (PCT)** of the nephron. This segment reabsorbs solutes and water in parallel, maintaining an osmolality equivalent to plasma (≈300 mOsm/kg). It is distinct from the loop of Henle, where hypotonic/hypertonic urine is formed, and the collecting duct, where final concentration occurs under ADH regulation.
**Why the Correct Answer is Right**
In the PCT, about 65–70% of filtered water, Na⁺, Cl⁻, glucose, and amino acids are reabsorbed via **isotonic reabsorption**. The reabsorption of solutes (e.g., Na⁺ via Na⁺/K⁺ ATPase and Na⁺-glucose cotransporters) is matched by passive water movement through aquaporins (AQP1), preserving isotonicity. This contrasts with the loop of Henle, where active Na⁺/Cl