Explain the role of the Loop of Henle in the absorption of water from the glomerular filtrate

After the filtrate in the nephron undergoes Selective Reabsorption in the Proximal Convoluted Tubule, it then travels to the Loop of Henle. The Loop of Henle is a ‘U’ shaped structure found in the Medullary portion of the Kidneys. It is important to remember that as solutes (such as glucose, sodium etc.) have been filtered from the nephron and reabsorbed into the surrounding capillaries, as a result the filtrate entering the Loop of Henle is now ‘isotonic’ (remember: isotonic means that the tubular fluid and the interstitium fluid have the same tonicity). And so essentially, the function of the Loop of Henle is to achieve ‘hypotonicity’ (remember: HYPOtonic solutions have LOW solute concentrations, and higher water concentrations relatively) within the tubular fluid, it does so by using a countercurrent multiplier system. As a result, it also creates a high solute concentration in the medulla. In order to explain the role of the Loop of Henle in the absorption of water from the glomerular filtrate, it is important to understand its structural features and basic anatomy. The Loop of Henle can be divided into 3 sections: descending, thin ascending and thick ascending … whether it is ‘ascending’ or ‘descending’ is dependent on the direction of filtrate movement within the tubule, and ‘thick’ or ‘thin’ refers to the size of the epithelial cells within the tubule. The 3 different sections carry out different functions, but all help with the overall function of the Loop of Henle:1.      The role of the Loop of Henle is better understood if we begin at the thick, ascending Loop of Henle, despite the fluid entering from the descending Loop of Henle. The Na⁺/K⁺ pumps present (only in the thick ascending Loop of Henle) use ATP to move ions out of the tubule, and combined with its lack of permeability to water (due to its thick epithelium) the fluid has a low tonicity (=hypotonic), and so increasing the solute concentration in the tissue surrounding the Loop of Henle. The concentration gradient established, as a result, helps with the function of the descending Loop of Henle. 2.      Now, the descending Loop of Henle is permeable to water but impermeable to ions. With the help of the concentration gradient established by the thick ascending Loop of Henle, water within the descending Loop of Henle will travel down its osmotic (water potential) gradient … out of the tubule and into the interstitium. This creates a low water potential and high solute concentration (=hypertonic) within the Loop. 3.      Moving on to the thin, ascending Loop of Henle; the thin epithelial cells make it impermeable to water but permeable to the movement of ions. Due to the high solute concentration in the Loop created by the descending Loop of Henle, the solute prefers to move out of the tubule when it enters the thin ascending Loop of Henle. And so, a high solute concentration is created within the medulla. This will then contribute to water leaving the collecting duct/DCT via osmosis, down their concentration gradient. Water reabsorbed from the glomerular filtrate back into the body contributes to a concentrated urine being produced.