• depends upon the needs of the body
• determined by the permeability of the collecting duct to water
• controlled by anti-diuretic hormone (ADH)

• ADH is secreted by the posterior pituitary.
• When water is in short supply and body fluids are concentrated,
– ADH is released
– makes the collecting duct cells permeable to water
– enables water to be withdrawn from the filtrate by osmosis
– water moves down the water potential gradient
• In contrast, if body fluids are dilute,
– less ADH is released
– collecting ducts impermeable to water
– larger volume of more dilute urine is produced

• In the brain there are sensory neurones which act as osmoreceptors
– osmoreceptors are in the hypothalamus
• The cell bodies of these nerve cells make ADH and are sensitive to the concentration of the blood
• When the water content of the blood is low (water potential is low) water leaves the cell
– causes impulses to travel from the hypothalamus to
– the posterior lobe of the pituitary
– ADH is secreted from the nerve endings into the blood
• In contrast, when water content is high (water potential high) water enters the osmoreceptors
– prevents them firing off impulses to the pituitary

Cell body of sensory neurone
(osmoreceptor) produces ADH.
Osmoreceptors in the hypothalamus lose water if body fluids are concentrated.
Posterior lobe of the pituitary gland.
ADH is secreted from nerve endings into blood.
Blood transports ADH to the kidney, making the collecting ducts permeable to water.

• ADH makes the walls of the collecting ducts more permeable to water
– cytoplasm contains vesicles with ready-made water channels ( aquaporins )
– ADH binds to receptor sites in the collecting duct cell membrane
• get a cascade of enzyme-controlled reactions that produces an active phosphorylase
• makes the vesicles move to and fuse with the cell membrane
• This puts water channels ( aquaporins ) into the membrane-making them more permeable
– enables absorption of water from the fluid in the collecting ducts as it travels through the salty medulla
• water travels down its water potential gradient

• probably explains the old wives’ tale that picking dandelion makes you wet the bed, and hence names for it like piss-a-bed (old
English) and pissenlie (French)

TASK: The human kidneys filter large volumes of blood, approximately 1.2dm
3 min -1 , from which they produce
125cm 3 filtrate min -1 (renal filtration rate). Typical production of urine is 1.5dm
3 day -1 . How much filtrate is produced per day? What is the percentage reabsorption of water from this filtrate?
Filtrate produced per day = renal filtration rate x 60 x 24
125 x 60 x 24 cm 3 = 180,000 cm 3 = 180 dm 3
Answer = 180 dm 3 day -1 percentage reabsorption = reabsorbed/daily filtrate x 100
(180-1.5) x 100 / 180 = 178.5 x 100 / 180 = 99.16666667
Answer = 99.2% reabsorption

Q: Using water potential terminology, explain how water is reabsorbed from the fluid in the collecting duct as it passes through the medulla.
• Medulla has a low (more negative) water potential compared to the fluid in the collecting duct. If CD walls are permeable, water travels down water potential gradient from CD into the medulla
Q: What effect will the removal of water from the fluid in the collecting duct have upon the volume of urine produced and its concentration?
• Smaller volume of more concentrated urine

Q: What impact will this have on the water potential of body fluids?
• It will raise the water potential, making it less negative
Q: When the water content of the body is high, osmoreceptors are no longer stimulated and the neuro-secretion of ADH slows.
Suggest how this results in reduced permeability of the collecting ducts to water.
• The reabsorbed water dilutes the blood and raises its water potential
– water is no longer withdrawn from the osmoreceptors and they stop neuro-secretion of ADH
– causes water channels to be removed from the collecting duct membranes
• membranes become less permeable
• produce greater quantity of more dilute urine

Complete to show mechanisms Body fluids concentrated
Osmoreceptors in hypothalamus detect changes in concentration of body fluids
Body fluids dilute

ADH is secreted by the posterior pituitary and transported to the kidney in the blood
Body fluids concentrated
Collecting duct walls impermeable (less permeable) so less
H
2
O is absorbed
Osmoreceptors in hypothalamus detect changes in concentration of body fluids
Body fluids dilute
Less ADH is secreted by the posterior pituitary so less is transported to the kidney
Collecting duct walls more permeable so more H
2
O is absorbed