Water Balance in Living Organisms

Water balance and blood pressure
 Osmoregulation is the maintenance of internal salt
and water concentrations in internal fluids despite
different concentrations in the external environment
 Addition of water in the bloodstream = increased
blood pressure
 Less water in the bloodstream = decreased blood
 Organs for water balance control in vertebrates
 Eliminate nitrogenous waste
 Kidneys are the blood’s filtration system
 Vital for maintaining homeostasis and without
kidneys we would only survive a few days
 Glomerulus
 Bowman’s capsule
 Loop of Henle
Function of nephrons
 Each kidney contains approx 1 million nephrons!
 Maintain pH, ion and water balance by excreting
excess ions, water, vitamins and hormones in the form
of urine
 Use active transport and passive transport
Antidiuretic Hormone (ADH)
 Vasopression is an antidiuretic produced by
neurosecretory cells in the hypothalamus
 Secreted when osmoreceptors in the hypothalamus
detect a rise in blood solutes (and therefore a decrease
in water concentration)
 Vasopression initiates a feeling of thirst and increases
the permeability of the distal tubules in the kidney
which helps to re-absorb water into the bloostream
 Blood volume decreases with dehydration
 Pressure sensitive cells detect this this triggers the
secretion of renin
 Renin initiates a chemical reaction that releases
aldosterone from the adrenal glands
 Aldosterone acts on the distal tubules of the nephrons to
actively re-absorb sodium ions and increase blood pressure
Nitrogenous waste
Water balance in Amoeba
 Singled celled organism, relatively simple to maintain
water balance
 Live in aqueous environment with higher
concentration of water outside the cell than inside
 Water permeates the cell membrane via osmosis
 Contractile vacuoles accumulate and expel the excess
 Waste products are diffused across the membrane
Water balance in fish
Freshwater fish
Marine (saltwater) fish
Tissues hypertonic to surroundings
Tissues hypotonic to surroundings
Concentration gradient results in a loss
of salts and an uptake of water
Concentration gradient results in a loss
of water and an uptake of salts
Fish must counter these changes to
maintain homeostasis
Fish must counter these changes to
maintain homeostasis
1. Does not drink
1. Drinks sea water
2. Kidney contains glomeruli and
secretes copious amounts of very dilute
urine. Tubules actively re-absorb NaCl
2. Minimal urine produced. Kidneys
lack glomeruli. Tubules actively secrete
3. Gill membranes permeable to water
3. Gill membranes are relatively
impermeable to water
4. Gills actively absorb ions. Some
ammonia leaves gills at the same time
4. Gills actively secrete sodium from
chloride cells; chloride ions follow
 Marine
Fresh water 
Water balance in sea birds
 Water, water everywhere and not a drop to drink
 Salt glands above the eye excrete a salt solution twice
the concentration of sea water out the birds nose
 Birds with high salt diets have larger salt glands
 Excrete nitrogenous waste a uric acid
Water balance in reptiles
 Aquatic reptiles
 Turtles, crocodiles, alligators etc
 Lots of water, nitrogenous waste is ammonia or urea
 Terrestrial reptiles
 Lizards, snakes, goannas
 Need to conserve water
 Large number of kidney tubules
 More are active when hydrated and less when
 Highly convoluted coprodaeum (region between
rectum and cloacal opening) to re-absorb water
 Can also have salt glands to excrete excess salts
Water balance in amphibians
 Frogs, toads etc
 Permeable skin allows diffusion of water and salts
 Produce large quatities of dilute urine and ammonia
 Actively transport Na and Cl across skin into the body
 Read page 261 “Managing fluid balance on land” and
complete the questions of page 262.
 Due: Monday 5 September

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