International A Level Biology / Control of the internal environment
The loop of Henle as a countercurrent multiplier
Edexcel International A Level Biology (YBI11), chapter 22: Control of the internal environment, specification point 7.20. Free lesson.
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The loop of Henle builds a low water potential in the kidney medulla, allowing water to be reabsorbed from the collecting duct. Longer loops build a bigger gradient, helping desert animals produce a small volume of concentrated urine.
How the loop and collecting duct pass through the kidney
The loop of Henle comes after the proximal tubule. It starts and ends in the cortex, the outer region of the kidney, and dips like a hairpin into the medulla, the inner region. Filtrate flows down the descending limb, turns at the tip, then flows up the ascending limb. It later travels down the collecting duct through the same medulla, towards the ureter. The medulla's water potential therefore affects water loss from both the loop and the collecting duct. The descending limb carries filtrate deeper into the medulla. The ascending limb carries filtrate back towards the cortex.
What each limb allows or moves
The two limbs have different properties. As filtrate travels down the descending limb, water leaves by osmosis, so the filtrate becomes more concentrated. The ascending limb is impermeable to water, but it actively moves sodium and chloride ions out into the medulla. It uses ATP for this, so its cells have many mitochondria. As the filtrate rises, it loses ions but keeps its water, becoming more dilute. Descending limb: permeable to water; does not pump out Na⁺ and Cl⁻ ions. Ascending limb: impermeable to water; actively pumps out Na⁺ and Cl⁻ ions using ATP.
How the countercurrent multiplier builds a gradient
Countercurrent means the filtrate flows in opposite directions in the two limbs. The ascending limb pumps ions into the medulla, but water cannot follow there. The ions lower the medulla's water potential, so water moves out of the descending limb by osmosis and is carried away by the blood. Filtrate becomes most concentrated at the bottom of the loop. The pumps build up the effect down the loop, making the medulla's water potential lowest at the tip. This increasing difference is the water-potential gradient. Water moves by osmosis from a higher water potential to a lower one. Adding…
How the collecting duct helps concentrate urine
The collecting duct passes down through the salty medulla. Because the medulla has a lower water potential than the filtrate, water leaves the duct by osmosis and is reabsorbed into the blood. A stronger gradient means more water can leave, so less water remains in the urine. Water leaves the descending limb and the collecting duct by osmosis. The ascending limb must not let water follow the ions out. Otherwise the medulla would not become as concentrated and its water potential would not fall as much.
Why desert animals have longer loops
A kangaroo rat has very long loops relative to the size of its kidney. They reach deep into a thick medulla, while short loops reach only part way. A longer ascending limb pumps out more ions, producing a bigger gradient and a very low water potential deep in the medulla. More water is then reabsorbed from the collecting duct into the blood, leaving a small volume of very concentrated urine. Longer loops: bigger gradient, more water reabsorbed, more concentrated urine. Shorter loops: smaller gradient, less water reabsorbed, urine cannot be as concentrated.