OSMOREGULATION

Osmoregulation...• The kidney is responsible for

osmoregulation – ‘the control of the water content in the body fluids’

• reabsorbs more or less water from the renal filtrate

• 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concentrated, – ADH is released – makes the collecting duct cells permeable to water– enables water to be withdrawn from the filtrate by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

Blood transports ADH to the kidney, making the collecting ducts permeable to water.

ADH is secreted from nerve endings into blood.

Posterior lobe of the pituitary gland.

Osmoreceptors in the hypothalamus lose water if body fluids are concentrated.

Cell body of sensory neurone (osmoreceptor) produces ADH.

• 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

Q “Why does coffee make people wee more?”• Diuretics increase the production of

urine. – 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 24125 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 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

Osmoreceptors in hypothalamus detect changes in concentration of body fluids

Body fluids concentrated

Body fluids dilute

Complete to show

mechanisms

ADH is secreted by the posterior pituitary and transported to the kidney in the blood

Collecting duct walls more permeable so more H2O is absorbed

Collecting duct walls impermeable (less permeable) so less H2O is absorbed

Osmoreceptors in hypothalamus detect changes in concentration of body fluids

Body fluids concentrated

Body fluids dilute

Less ADH is secreted by the posterior pituitary so less is transported to the kidney