Counter-current multiplication in the loop of Henle
Counter-current multiplication is how the loop of Henle, the hairpin-shaped section of the nephron, makes the medulla (the inner part of the kidney) saltier the deeper you go.
Remember
In the ascending limb (the part rising back towards the cortex, the outer kidney), the thick segment pumps salt out while water cannot follow, so its fluid becomes dilute and the medulla becomes saltier; the descending limb loses water but not salt.
Memory aid
the ascending limb loses salt but not water, and the descending limb loses water but not salt
Use it to tell the two limbs apart by what each one loses.
To concentrate urine, the kidney needs a medulla that becomes saltier the deeper you go. This is the vertical osmotic gradient, and the loop of Henle builds it by counter-current multiplication. The fluid flows in opposite directions in the two limbs, and a small concentration difference at each level is added up along the loop until it becomes a large one.
The descending limb comes first. It is permeable to water (water can cross it) through aquaporin-1 channels, which are protein pores for water that are always open, but it is not permeable to sodium. As the fluid in the tubule travels down into the saltier medulla, water leaves by osmosis, so the fluid inside becomes more concentrated.
The thick ascending limb does the opposite. Its cells use energy-using pumps to move sodium chloride out of the fluid in the tubule into the interstitium (the tissue between the tubules). This limb has no aquaporin channels, so water cannot follow. The fluid inside therefore becomes more dilute, and the interstitium becomes saltier. It is this active pumping that generates the gradient. The water loss from the descending limb follows the gradient rather than creating it.
The long loops of the juxtamedullary nephrons (the nephrons that lie next to the medulla) dip deep into the medulla, and the longer the loop, the further the effect is multiplied. The vasa recta (the capillaries running beside the loops) do not build the gradient. They preserve it.
Key facts
- Descending limb
- it is permeable to water through always-open aquaporin-1 channels, so water leaves and the fluid becomes more concentrated.
- Tip of the loop
- this is where the fluid in the tubule reaches its maximum concentration.
- Thick ascending limb
- it actively reabsorbs sodium chloride and is impermeable to water, so the fluid inside becomes more dilute while the medulla outside becomes saltier.
- If the thick ascending limb's pump is blocked
- as happens with loop diuretics, less salt is moved out, the medullary gradient weakens, and the fluid reaching the distal convoluted tubule (the next segment after the loop) is less dilute than normal.
- Long loops
- they belong to the juxtamedullary nephrons, about 20% of all nephrons; the other 80% are cortical nephrons.
- Vasa recta
- they maintain the gradient by counter-current exchange but do not create it.
- Urea
- it is not the primary mechanism that generates the gradient, but urea reabsorbed from the medullary collecting duct (the part of the tubule that draws water out of the urine) also raises the osmolarity of the inner medulla.
Exam traps
- Passive water reabsorption through aquaporin channels does not describe the thick ascending limb. That limb has no such channels, so salt is pumped out without water following; the descending limb is the water-permeable one.
- Urea does diffuse into the descending limb, but this raises the osmolarity of the fluid inside the tubule rather than of the interstitium, and urea is not the main generator of the gradient: that is the salt pumping of the thick ascending limb.
- The vasa recta are essential to the counter-current mechanism, but their job is to preserve the gradient, not to create it.
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