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NCERT calls it "a hairpin shaped Henle's loop". Students memorise which limb lets water out and which pumps salt, often without a picture of why a loop should exist at all. The man who first described it did not know either. This post covers Jacob Henle, what his 1862 paper actually claimed, how its purpose was worked out almost a century later, and a worked example that shows why the loop matters.
Who was Jacob Henle?
Henle lived from 1809 to 1885, according to a 2026 article by Kun Hwang in the Journal of Craniofacial Surgery. Hwang calls him a pivotal figure among the anatomists whose work fed evolutionary thinking, and shows that his detailed maps of the facial muscles supplied evidence that Charles Darwin later used in The Expression of the Emotions in Man and Animals (1872). An anatomy museum study from Heidelberg describes a specimen he prepared there between 1844 and 1852.
He also worked on disease. A 2022 paper by Jutta Schickore in the Bulletin of the History of Medicine examines his essay on miasma and contagious diseases, a medical classic often read as foreshadowing Robert Koch's postulates for identifying germs. Schickore argues that both men belonged to a wider experimental tradition rather than one simply anticipating the other.
His name already appears in William Bowman's 1842 kidney paper, which cites the observation, made by Purkinje and Henle, that glands are lined with large quantities of epithelium. Our post on William Bowman and the capsule covers that paper.
What did Henle find in 1862?
His paper Zur Anatomie der Niere (On the anatomy of the kidney) was printed in Göttingen in 1862, taken from the tenth volume of the treatises of the Royal Society of Sciences there. It opens with a striking remark. In our translation of its first lines, Henle says his observations show again how obvious anatomical facts can escape notice once our picture of an organ's structure fits our idea of its purpose. He adds that kidney anatomy had made no notable progress since Bowman's discovery.
Everyone, he explained, knew about the ducts that open on the renal papilla and the convoluted tubules in the cortex, and assumed they simply joined. But most of the fine tubules in the medulla do not open on the papilla. They bend round in loops, higher or lower in the medulla, and return towards the cortex they came from. Asking whether all the fine medullary tubules end in loops, he wrote that he could answer yes without hesitation.
Did Henle get everything right?
No. In the same opening, in our reading of the German text, he argued that the convoluted tubules starting at the glomerular capsules are not connected to the ducts that open on the papilla. Today's NCERT Class 11 Biology chapter on excretion shows the full path: Bowman's capsule, proximal convoluted tubule, Henle's loop, distal convoluted tubule, then collecting duct, with many distal tubules opening into one collecting duct.
Why did it take so long to understand the loop?
According to a historical review by Koulouridis and Koulouridis (2014), Henle's discovery was largely neglected during the long argument between Bowman's and Ludwig's theories of urine formation. In 1942 Werner Kuhn, a physical chemist, proposed that the loop works like a hairpin countercurrent multiplier. In 1951 Kuhn, Hargitay and Wirz showed experimentally that the loop is central to concentrating urine. An English translation of Hargitay and Kuhn's 1951 paper, "The multiplication principle as the basis for concentrating urine in the kidney", was published in 2001.
English-speaking physiologists were persuaded by micropuncture: Gottschalk and Mylle sampled fluid from loops of Henle, collecting ducts and vasa recta in rodent kidneys, and their results, published in 1959, supported the countercurrent hypothesis.
How does the loop work, in NCERT's terms?
NCERT explains that the descending limb is permeable to water but almost impermeable to electrolytes, so the filtrate concentrates as it moves down. The ascending limb is impermeable to water but moves electrolytes out, so the filtrate becomes dilute as it rises. Because fluid flows in opposite directions in the two limbs, and blood does the same in the U-shaped vasa recta, the medulla holds a gradient from about 300 mOsmol per litre in the cortex to about 1200 mOsmol per litre in the inner medulla, built mainly by NaCl and urea. Water leaving the collecting duct into this salty medulla makes urine concentrated.
Worked example: why the gradient matters
Suppose your body must get rid of 600 mOsmol of dissolved waste in a day (a sample number for practice).
Without a loop, if urine could be no stronger than blood, about 300 mOsmol per litre: volume = 600 ÷ 300 = 2.0 litres a day.
With the loop, urine can approach the inner medulla's 1200 mOsmol per litre: volume = 600 ÷ 1200 = 0.5 litres a day.
Same waste, a quarter of the water. That saving of 1.5 litres is what the loop buys.
Common mistakes:
Swapping the limbs. Descending lets water out; ascending moves salt out but keeps water in.
Saying the loop makes urine concentrated by itself. It builds the gradient; the final concentrating happens as water leaves the collecting duct.
Forgetting the vasa recta. NCERT says the loop and the vasa recta together maintain the gradient.
If you can picture the two limbs but not the countercurrent idea, the Fermi Simulation Lab has interactive simulations for ideas easier to see than read, so you can explore how opposite flows behave. Our step-by-step guide to urine formation covers filtration and reabsorption around the loop.
What can a student take from Henle?
Look again at the familiar. He found the loop by not accepting the standard picture.
Describe carefully even when you cannot explain. His description waited 80 years for its explanation, and it held up.
Expect to be partly wrong. He missed a connection; others fixed it. Science works by correction.
Next step: draw the loop with arrows for water and salt in each limb, then redo the worked example for 900 mOsmol of waste.
