Q.Hypotonic filtrate is formed in _______

Human kidneys can concentrate urine to nearly four times the osmolarity of blood plasma (up to ~1200 mOsm/L versus blood's ~300 mOsm/L) through a countercurrent mechanism run jointly by the loop of Henle and the vasa recta of juxtamedullary nephrons.

The loop of Henle acts as a countercurrent multiplier: its water-permeable descending limb loses water to the increasingly salty surrounding tissue as filtrate flows down, while its water-impermeable, salt-pumping ascending limb actively removes Na+ and Cl− as filtrate flows up — because flow in the two limbs is in opposite directions, this steadily builds a steep osmotic gradient in the medulla, most concentrated near the tip of the pyramid. The vasa recta run a parallel countercurrent exchanger, picking up solutes and losing water on the way down into the medulla and reversing this exchange on the way back up, so that blood leaves the medulla close to its starting osmolarity without dissipating the gradient the loop of Henle has built. …