By the end of this lesson
- Explain collecting-duct water reabsorption.
- Relate the medullary gradient to osmosis.
- Connect permeability to urine concentration.
BIG QUESTION
How can the collecting duct recover water from fluid flowing toward the bladder?
A medullary water-potential gradient and regulated duct permeability allow osmosis into surrounding tissue and capillaries.
The medulla provides a gradient
The kidney medulla has a lower water potential than fluid entering the collecting duct, partly because ion movement in the loop of Henle establishes a concentration gradient.
This creates a driving force for water to leave the duct.
Water crosses by osmosis
When the collecting-duct epithelium is permeable, water moves through aquaporin channels by osmosisThe net movement of water molecules, through a partially permeable membrane, from a region of higher water potential to a region of lower water potential; a dilute solution has a higher water potential than a concentrated one. into medullary tissue and then surrounding capillaries.
Blood flow carries reabsorbed water away, helping maintain the gradient.
Permeability controls concentration
More permeable collecting ducts recover more water and produce a smaller volume of concentrated urine. Less permeable ducts recover less water and produce a larger volume of dilute urine.
ADH controls this permeability rather than directly pulling water.
Collecting-duct states
Water movement depends on both a gradient and membrane permeability.
| Duct condition | Water reabsorption | Urine outcome |
|---|---|---|
| Many aquaporins/high permeability | More. | Low volume, concentrated. |
| Few aquaporins/low permeability | Less. | High volume, dilute. |
| No water-potential gradient | Little net osmosis. | Permeability alone is insufficient. |
Language that earns precision
- collecting duct
- Final nephron-associated tube where variable water reabsorption occurs.
- aquaporin
- Membrane channel that permits rapid water movement.
- medulla
- Inner kidney region containing loops of Henle and collecting ducts.
Common misconceptions
Claim: ADH carries water back to blood.
Correction: ADH changes duct permeability; water itself moves by osmosis.
Claim: Water is reabsorbed by active transport.
Correction: It moves passively down a water-potential gradient.
Claim: Concentrated urine contains more water.
Correction: It has more solute relative to water and usually a smaller volume.
Match the answer to the command word
Gradient + aquaporins/permeability + osmosis + capillaries.
More reabsorption means smaller-volume, more-concentrated urine.
Recall, apply, then justify
Explain how water is reabsorbed from a collecting duct.
Hint
Use water potential and permeability.
Suggested marking points
- The medulla has a lower water potential than duct fluid.
- The duct wall is permeable through aquaporins.
- Water moves out by osmosis.
- Water enters surrounding capillaries and returns to blood.
Explain how increased collecting-duct permeability changes urine.
Hint
Recover more water.
Suggested marking points
- More water leaves the collecting duct.
- Urine volume decreases.
- Urine becomes more concentrated.
Explain why increasing aquaporins would have little effect if the medullary gradient collapsed.
Planning hint
A channel does not create a direction.
What a strong design includes
- Aquaporins increase water permeability.
- Osmosis still requires a water-potential difference.
- Without the medullary gradient there is little driving force.
- Little net water leaves the collecting duct.
- Urine remains relatively dilute despite high permeability.