Q.Describe countercurrent mechanism. OR Discuss the role of lungs, liver and skin in excretion.

A longer loop of Henle enables a steeper countercurrent osmotic gradient in the kidney medulla, so more water can be reabsorbed from urine, essential for water-scarce desert mammals.

The loop of Henle works together with the vasa recta as a countercurrent multiplier system to establish a gradient of increasing osmotic concentration from the cortex toward the inner medulla of the kidney. The longer the loop of Henle (i.e. the deeper it dips into the medulla), the steeper and more extensive this osmotic gradient becomes.

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Henle's loop, working as a countercurrent multiplier with the vasa recta, builds up a high salt concentration deep in the kidney medulla; this gradient later pulls water out of the collecting duct, producing concentrated (hypertonic) urine.

Henle's loop is a long, hairpin-shaped part of the nephron with a descending limb (permeable to water, relatively impermeable to salts) and an ascending limb (impermeable to water, but actively transports NaCl out into the surrounding interstitial tissue).

Mechanism (countercurrent multiplier):

1. As the filtrate flows down the descending limb, water passively moves out into the surrounding medullary interstitium (which is already salty), so the filtrate becomes progressively more concentrated toward the tip of the loop.
2. As the (now concentrated) filtrate flows up the ascending limb, NaCl is actively pumped out into the interstitium, while water cannot follow (the ascending limb is impermeable to water). This makes the filtrate more dilute as it moves up, and simultaneously keeps adding salt to the medullary interstitium.
3. Because the descending and ascending limbs carry fluid in opposite directions right next to each other, this arrangement ("countercurrent") progressively multiplies and maintains a steep, increasing osmotic gradient from the cortex (less concentrated) down to the inner medulla (highly concentrated) — this is why it's called the countercurrent multiplier. …