Water Balance in Living Organisms

Water balance and blood pressureOsmoregulation 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 pressure

KidneysOrgans 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 nephronsEach 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

ReninBlood 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 AmoebaSingled 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 water

Waste products are diffused across the membrane

Water balance in fishFreshwater 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 MgSO4

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 birdsWater, 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 noseBirds with high salt diets have larger salt

glands

Excrete nitrogenous waste a uric acid

Water balance in reptilesAquatic reptiles

Turtles, crocodiles, alligators etcLots of water, nitrogenous waste is ammonia or

urea

Terrestrial reptilesLizards, snakes, goannas

Need to conserve water

Large number of kidney tubulesMore are active when hydrated and less when

dehydrated

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 amphibiansFrogs, 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

Biozone

Read page 261 “Managing fluid balance on land” and complete the questions of page 262.

Due: Monday 5 September