Osmoregulation File

• The kidney is responsible for
osmoregulation – ‘the control of the water
content in the body fluids’
• reabsorbs more or less water from the renal
• 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
– ADH is released
– makes the collecting duct cells permeable to water
– enables water to be withdrawn from the filtrate by
– 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 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
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
• 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
Q “Why does coffee make people
wee more?”
• Diuretics increase the production of
– Caffeine is a diuretic and is present in
tea, coffee and many soft drinks
– Diuretics are also present in dandelion
• 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 1.2dm3 min-1, from which they produce
125cm3 filtrate min-1 (renal filtration rate). Typical
production of urine is 1.5dm3 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 cm3 = 180,000 cm3 = 180 dm3
Answer = 180 dm3 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
• 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
– 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
Body fluids
Osmoreceptors in
hypothalamus detect
changes in concentration
of body fluids
Body fluids
ADH is secreted by the
posterior pituitary and
transported to the
kidney in the blood
Body fluids
Osmoreceptors in
hypothalamus detect
changes in concentration
of body fluids
Collecting duct walls
impermeable (less
permeable) so less
H2O is absorbed
Body fluids
Less ADH is secreted by
the posterior pituitary so
less is transported to the
Collecting duct walls
more permeable so
more H2O is

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