Loop of Henle: limb permeability and the medullary gradient
The hairpin-shaped loop of Henle dips into the medulla (the kidney's inner part), where water leaves one limb and salt the other, building a gradient of increasing saltiness.
Remember
Water leaves on the way down the descending limb while solutes largely stay in, and salt leaves on the way up the thick ascending limb while water stays in; that contrast builds the medullary gradient.
Memory aid
water leaves on the way down, and salt leaves on the way up
Use it to recall that the descending limb gives up water and the ascending limb gives up salt.
The loop of Henle is the hairpin-shaped part of the nephron (the kidney's microscopic filtering tube) that dips into the medulla, the inner part of the kidney. It has a descending limb and an ascending limb, and the two have different properties.
The descending limb is highly permeable to water but largely impermeable to sodium and chloride, so water can leave it while salt largely stays in. The medulla is much saltier than the fluid running down this limb, so water leaves passively through water channels called aquaporins, and the fluid becomes more concentrated as it descends.
The thick ascending limb does the opposite. It is impermeable to water, so water cannot follow the salt, and it actively pumps sodium, potassium and chloride out with the NKCC2 transporter, a carrier protein that moves one sodium, one potassium and two chloride ions into the cell together. Salt therefore leaves without water: the fluid left behind becomes dilute while the medulla becomes saltier.
The two limbs run side by side with fluid flowing in opposite directions. Their contrasting permeabilities act together as countercurrent multiplication, a system in which opposing flows magnify a salt gradient. Pumping in the thick ascending limb builds the gradient, and the descending limb loses water to it. A salty medulla is what later allows the kidney to concentrate urine.
Loop diuretics such as furosemide block NKCC2. Less salt is pumped into the medulla, so the gradient weakens, and the result is a large volume of urine that cannot be concentrated.
Key facts
- Descending limb
- highly permeable to water but largely impermeable to sodium and chloride, so water leaves and the fluid becomes more concentrated.
- Thick ascending limb
- water-tight, with active reabsorption of sodium, potassium and chloride through NKCC2.
- Thin ascending limb
- also impermeable to water, but it moves sodium and chloride passively.
- Diluting segment
- the thick ascending limb is called this because it removes salt and leaves the water behind.
- Medullary gradient
- salt leaves without water, so the medulla becomes saltier and the fluid left behind becomes dilute.
- Loop diuretics
- they block NKCC2 in the thick ascending limb, which weakens the gradient and so reduces water reabsorption.
Exam traps
- The descending limb does not actively reabsorb sodium, because it is the water-permeable segment with few mitochondria and little active transport; being impermeable to water while actively reabsorbing sodium is the pattern of the thick ascending limb.
- Passive movement of sodium and chloride describes the thin ascending limb, which is also impermeable to water; the thick ascending limb is different because it uses the NKCC2 carrier, driven by energy-dependent pumps, to reabsorb sodium, potassium and chloride actively.
- Loop diuretics do not make the thick ascending limb leaky to water: it is the water-impermeable segment, which lets it pump out salt without water and so build the gradient, and what loop diuretics block is its NKCC2 pump.
Read the full notes
Tested in 6 SimpleMed+ questions
Practice it in Urinary System, with the answer and a full explanation after every question.
Practice these questions