Q.How is urea formed in the human body?

The loop of Henle sets up the medullary osmotic gradient (via the countercurrent multiplier) that lets the kidney produce concentrated urine.

The nephron has several regions, each with a distinct role:

• The proximal convoluted tubule (PCT) reabsorbs most of the filtrate's water, glucose, amino acids and ions, but is not primarily responsible for fine-tuning urine concentration.
• Bowman's capsule and the glomerular capillary network are involved in ultrafiltration of blood to form the filtrate, not in concentrating urine. …

The loop of Henle, via the countercurrent multiplier mechanism, sets up a high osmotic gradient in the medulla that lets the collecting duct reabsorb extra water and concentrate urine; without it, urine stays dilute — option (d).

The nephron filters blood in the glomerulus and then selectively reabsorbs water and solutes as the filtrate passes through the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and the collecting duct. The loop of Henle, together with the vasa recta, works as a countercurrent multiplier: active transport of NaCl out of the ascending limb (which is impermeable to water) builds up a very high salt concentration deep in the medullary interstitium, while the descending limb (permeable to water) loses water into this concentrated interstitium. This creates a strong osmotic gradient in the medulla.

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