Loop of Henle and osmoregulation in the collecting ducts

Organisms (Continuity and change) · Homeostasis · note 6 of 7

Spec D3.3.9, D3.3.10

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Loop of Henle and osmoregulation in the collecting ductsSpec D3.3.9, D3.3.10

In short

The loop of Henle actively transports sodium ions out of the ascending limb, maintaining a high osmotic concentration in the medulla so that water can be reabsorbed from the collecting ducts. Osmoreceptors in the hypothalamus control antidiuretic hormone (ADH) secretion by the pituitary; ADH moves aquaporins from intracellular vesicles into collecting duct cell membranes, increasing water reabsorption.

Role of the loop of Henle

Cells of the ascending limb use ATP to actively transport sodium ions out of the filtrate into the tissue fluid of the medulla.
The ascending limb wall is impermeable to water, so water cannot follow the ions.
This maintains a high osmotic concentration in the medulla. Collecting ducts pass through the medulla, so water can leave them by osmosis whenever their walls are permeable.

Osmoregulation by the collecting ducts

Osmoreceptors in the hypothalamus detect the osmotic concentration of the blood.
If the blood is too concentrated (for example after sweating or not drinking), the pituitary gland increases its secretion of antidiuretic hormone (ADH).
ADH travels in the blood to the collecting duct cells. It causes intracellular vesicles containing aquaporins (water channels) to fuse with the cell membranes facing the filtrate.
With more aquaporins, more water moves by osmosis out of the collecting ducts into the concentrated medulla and back into the blood. A small volume of concentrated urine is produced.
If the blood is too dilute, ADH secretion decreases. Aquaporins are taken back from the membrane into intracellular vesicles, so less water is reabsorbed and a large volume of dilute urine is produced.
Either way, the blood osmotic concentration returns to its set point: negative feedback.

Left: loop of Henle and collecting duct passing through the medulla, shaded darker with increasing osmotic concentration; the ascending limb actively transports Na⁺ out and is impermeable to water, and water leaves the collecting duct. Right: osmoreceptors in the hypothalamus, the pituitary gland and ADH in blood; collecting duct cells with high ADH (aquaporins in the membrane, much water reabsorbed) and low ADH (aquaporins in vesicles, little water reabsorbed). — The ascending limb keeps the medulla concentrated. ADH moves aquaporins into the collecting duct cell membrane, so more water leaves by osmosis.

Exam tip:

Say that ADH changes the location of aquaporins, between intracellular vesicles and the cell membrane. It does not make the duct 'absorb' water actively; water always moves by osmosis.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why does homeostasis use negative feedback rather than positive feedback?

Negative feedback reverses any change from the set point, so it returns a variable to normal whether it rises above or falls below it. Positive feedback amplifies a change away from the set point, which would make an imbalance worse. That is why positive feedback only controls processes that must finish quickly, such as childbirth.

How do insulin and glucagon control blood glucose?

When blood glucose rises, β cells in the pancreatic islets secrete insulin, which makes liver and muscle cells absorb glucose and store it as glycogen. When blood glucose falls, α cells secrete glucagon, which makes liver cells break glycogen down and release glucose. Both hormones travel to their target cells in the blood.

What is the difference between type 1 and type 2 diabetes?

In type 1 diabetes the immune system destroys the insulin-secreting β cells, so little or no insulin is made and insulin must be injected. In type 2 diabetes insulin is still produced but target cells become resistant to it. Type 2 is linked to obesity and inactivity and is managed mainly by diet and exercise.

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