Urea recycling and the medullary osmotic gradient
Urea recycling keeps urea in the medulla (the inner part of the kidney), raising its osmolarity (the concentration of dissolved particles) so the kidney can concentrate urine.
Remember
With ADH (antidiuretic hormone, released by the brain to conserve water) present, urea that is recycled raises medullary osmolarity and helps the kidney concentrate urine.
The collecting ducts (the final drainage tubes) run through the medulla, and the kidney concentrates urine by drawing water out of them into a medulla that becomes more concentrated towards its tip. When ADH is present, water is drawn out of the duct by osmosis.
Sodium chloride pumped out of the loop of Henle (the hairpin-shaped tubule) supplies most of the saltiness of the medulla. Urea adds to that gradient.
Urea is a small waste molecule that comes from the breakdown of proteins, and the glomerulus filters it freely. In the medulla it is more than waste: it works as an effective osmole (a particle that holds water), because it adds to the concentration of the medullary tissue.
As water leaves the outer part of the collecting duct, which is largely impermeable to urea (little urea can cross it), the urea left behind in the tubular fluid becomes more concentrated. In the inner medullary collecting duct, ADH makes urea transporters (proteins that carry urea across the cell membrane) more active. Urea then moves down its concentration gradient, out of the duct and across to the interstitium (the fluid between the tubules).
This raises the osmolarity of the medulla and makes the corticomedullary gradient (the rise in concentration running from the cortex, the outer part, to the inner medulla) steeper, so more water is drawn out of the duct and the urine becomes more concentrated.
Some urea also diffuses into the loop of Henle, passes round the nephron and returns to the collecting duct. This process is called urea recycling, and it keeps urea trapped in the medulla.
Urea plays a smaller supporting part: it adds to the gradient built by sodium chloride transport, and urea does not replace it.
Key facts
- Trigger
- ADH increases the activity of urea transporters in the inner medullary collecting duct.
- Direction
- urea moves down its concentration gradient out of the collecting duct and into the medullary interstitium.
- Recycling
- urea can pass from the interstitium back into the loop of Henle and then round the nephron to the collecting duct.
- Role of urea
- urea supports the gradient that sodium chloride transport builds and does not replace it.
- Result
- when ADH is present and medullary osmolarity is higher, more water is drawn out of the collecting duct and the urine is more concentrated.
- In dehydration
- with high ADH, more urea is reabsorbed from the inner medullary collecting duct and the medullary osmotic gradient becomes steeper.
Exam traps
- Urea does not reduce medullary osmolarity or produce dilute urine, because it increases the osmolarity of the medullary tissue. Dilute urine is produced when the medullary gradient is low or ADH is absent.
- In the inner medullary collecting duct, urea is not a solute that stays in the duct and holds water there, because it crosses the wall through transporters, and this movement adds to the gradient.
- Urea does not trigger the release of ADH, because ADH release is triggered by osmoreceptors (sensors that detect the concentration of the blood, found in the hypothalamus) noticing a small rise in blood concentration, not by urea. Once ADH has been released, it promotes urea movement into the medullary interstitium.
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