Loop Of Henle ( Absorption and Secretion ): Renal Physiology USMLE Step 1

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Loop Of Henle ( Absorption and Secretion ): Renal Physiology USMLE Step 1
What Is the Loop of Henle?
The loop of Henle (LOH) is the next major tubular segment. It helps reabsorb the water, sodium, and chloride that make it beyond the PT and is key to regulating total body water and serum osmolality. The LOH allows the kidney to concentrate the urine by creating a concentration gradient from the cortex to inner medulla. Thus, as water flows down the CD, it can be reabsorbed as it flows past an increasingly concentrated interstitium. You will read more about this in the brick on Water Homeostasis.
The LOH consists of several portions: a thin descending limb, a thin ascending limb, and a thick ascending limb
The thin descending limb has low permeability to sodium but is highly permeable to water. Here, water reabsorption occurs both transcellularly through aquaporins and paracellularly through tight junctions, which are “leaky” to water but not ions.
Conversely, the loop’s ascending limb (thin and thick) has water-impermeable tight junctions, making it impermeable to water. However, there are many transport proteins here, making this segment highly permeable to sodium. You will see how the body takes advantage of this in another brick when you read about how the LOH generates high osmolality in the interstitium by means of a countercurrent multiplier system. This process is crucial for water reabsorption.
The channels and transporters in the thick ascending limb, which is the most complex segment of the LOH
Absorption
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The thick ascending limb is notable for the presence of the apical Na+/K+/2Cl- cotransporter, (Figure 4, upper left) which reabsorbs sodium, potassium, and chloride ions.
Chloride reabsorption creates a positive luminal charge that also drives the reabsorption of calcium and magnesium. The thick ascending limb reabsorbs calcium (about 25%) and most of the filtered magnesium (70%). Both are reabsorbed via paracellular transport. Interestingly, this segment also houses a calcium sensor that regulates calcium absorption by detecting the calcium concentration in tubular fluid.
Secretion
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Although the loop does not engage in any net secretion (except for urea secretion in states of low-volume status), it does contain an apical potassium ion channel. Recall how the Na+/K+/2Cl- transporter reabsorbed potassium? The apical potassium channel counteracts this, causing a reverse flux of potassium from the tubular cells back into the lumen. The potassium channel contributes to a charge and concentration gradient that “recycles” potassium to increase reabsorption of other cations like calcium and magnesium. This also means that in the LOH, there is no significant net gain or loss of potassium into the tubular fluid under normal circumstances.
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