Water and Osmotic Regulation
Chapter 8
Water Balance and Concentration
• Internal Environment = aqueous solution
– Volume and composition must be maintained within
narrow limits
• Composition different from external environment
– Composition tends to change towards equilibrium with
the environment
• Organism must control changes in composition of
body fluids
– Overall solute concentration (osmotic concentration)
– Concentration of specific solutes
Control of Fluid Composition
• Limit exchange with environment
– Limit permeability of body surface to different solutes
– Limit concentration gradients between body fluids and
environment
• Must balance movement of materials with equal
countercurrent flow against gradients
– Requires energy
Major Types of Hydric
Environments
• Aquatic – high water availability
– Marine
• High solute concentration
– Fresh Water
• Low solute concentration
• Terrestrial – low water availability
Aquatic Environments
• Sea Water (ca. 3.5% salt, 1 Osm)
– Mainly Na, Cl, Mg, SO4 and Ca
– Generally homogenous throughout oceans
• Fresh Water
– 0.1 mOsm to 10 mOsm
• Brackish Water (0.05% to 3%)
– Possible high variation with tide or flooding
Osmotic Regulation
• Osmoconformers
– Allow body fluid osmotic
concentration to vary with
environmental concentration
• Osmoregulators
– Maintain osmotic
concentration of body fluids
in narrow limits independent
of environmental osmotic
concentrations
Osmotic Tolerance
• Euryhaline
– tolerate wide variations in environmental osmotic
concentrations
• Stenohaline
– tolerate only limited variation in environmental
osmotic concentration.
Marine Invertebrates
• Typically osmoconformers
– Body fluids are isosmotic to sea water
• Often are strict ionic regulators
– Maintain concentrations of specific ions in narrow
ranges, often different from sea water
Marine Invertebrates
• Composition can differ
between different fluids:
– External environment
– Blood & Interstitial fluid
(extracellular fluid)
– Intracellular fluid
Regulation of Intracellular Volume
and Concentration
• Changes in ECF composition
leads to changes in ICF
composition
– Changes in cell volume
• Typically cell volume quickly
corrected in response to ECF
change
– induced by changes in amino acid
concentrations inside the cells
Freshwater Invertebrates
• Typically osmoregulators
– Maintain hyperosmotic body
fluids
• Problems
– Water tends to flow into of
the animal
• Osmotic uptake
– Ions tend to flow out of the
animal
• Diffusion and excretion
Freshwater Invertebrates
• Solutions
– Decrease permeability
• May cause problems with
uptake of other substances
– Active Transport
• Uptake of ions against a
electrochemical gradient
• Requires energy
Brackish Water Invertebrates
• Possible wide
fluctuation in osmotic
environment
• Variety of responses in
osmotic regulation
Marine Vertebrates:
Elasmobranchs
• Isosmotic body fluids
• Strict ionic regulators
– [(Salt]~ 1/3 that of sea water)
– Osmotic concentrations largely due to organic solutes
• Urea (NH2-CO-NH2)
• Trimethylamine oxide (TMAO)
– TMAO counteracts effects of urea on enzymes
Marine Vertebrates:
Elasmobranchs
• Salt levels maintained at low levels
– Kidney – remove many ions
– Rectal gland – excretes fluid with high
NaCl concentration
– Potential active excretion by gills
• Body fluids are slightly
hyperosmotic
– Tends to draw water into the body
– Water used in urine formation and
rectal gland secretion
Marine Vertebrates:
Teleosts
• Hyposmotic blood (~300 Osm)
• Liable to osmotic water loss
– Especially the gills
• Must be able to uptake water to
counter water loss
– Drink sea water
Marine Vertebrates:
Teleosts
• Must excrete salt at higher
concentration than water taken
in
– Urine production
• kidneys cannot produce
hyperosmotic urine, but remove
Ca2+, Mg2+ and SO42-
– Active secretion from the gills
(chloride cells)
• Actively secrete Cl-, Na+ passively
secreted
Fresh Water Teleosts
• Hyperosmotic Blood (~300
mOsm)
– Water enters through the gills
• Excrete dilute urine (2-10 mOsm)
– Lose lots of solutes (high volume)
• Ions tend to be lost from the gills
– Ions taken up in the food
– Active uptake of ions into the gills
“Switch-Hitters”
• Some fish spend part of life cycle both
in sea water and in fresh water
– Anadromous – most of life in sea, spawn
in fresh water (e.g. salmon)
– Catadromous – most of life in fresh
water, spawn in the sea (e.g. eels)
• Must essentially reverse active
transport mechanisms to maintain
solute balance
Terrestrial Organisms
• Advantage
– Easy access to O2
• Disadvantage
– Danger of dehydration
• Only arthropods and vertebrates have largescale terrestrial evolution
– Others largely sequestered in moist microhabitats.
Evaporation
• Transition of water into gaseous
state from ice or liquid
• Driven by vapor pressure
difference between air at the
body surface and surrounding air
– Increases with increased
temperature
– Decreases with increased humidity
Evaporation
Additional factors influencing evaporation:
• Convection – increases rate of evaporation
• Evaporative cooling – lowers temperature
– Affects diffusion rate
• Barometric pressure -  rate w/ pressure
• Orientation
– air flow created by density changes due to
evaporative cooling
– Orientation to convection
Water Budgets
Over time, water gain must equal water loss
Ways of losing water:
• Evaporation
– Body surface
– Respiratory surface
• Excretion/secretion
– Feces
– Urine
– Other secretions
Ways of gaining water:
• Drinking/Eating
– Imbibing water
– Water in food
• Integumental Uptake
– From water
– From air
• Metabolic Water
Approaches for Terrestrial Animals
• Vapor-limited system
– Animals have permeable integuments
– Rate of water loss determined by transfer of water to
surrounding air
• Difference in vapor pressure, convection, etc.
• Membrane-limited system
– Surface provides resistance to evaporation
– Rate of evaporation altered by changing membrane
permeability
• Vapor pressure differences, convection, etc. are minor
Earthworms
• Highly permeable integument
– Readily gains/loses water
• Strict osmoregulator and ion regulator
– Much like a fresh water animal
• Live in moist habitats
– Vapor saturated soil, soil particles with layer of
free liquid water around them
Amphibians
• Highly permeable
integument
– Readily gains/loses water
• Typically live in moist
habitats
– Near water, fossorial, under
leaf litter, etc.
• Some desert species
– Numerous special adaptations
Arid Amphibians
• Estivation
– Estivate during dry periods
– Emerge with rains to breed,
replenish water, then return
– May form “cocoons” around
them ( EWL)
– Store large amounts of water
in bladder
– Tolerate high urea
concentrations (~ 500 mM)
• Reduced Integumental
Permeability
– Phyllomedusa - secretes
waxy coating
Crustaceans
• Crabs
– Most semi-terrestrial (intertidal)
• Need moist microhabitat
(burrows, sea weed, etc)
• Isopods
– Some fully terrestrial
• Live in humid habits, nocturnal
• Relatively high rates of EWL
Insects and Arachnids
• Evaporative Water Loss
Countermeasures
– Highly impermeable
integument
• Waxy cuticle prevents
excessive EWL
– Discontinuous
ventilation
• Intermittent opening of
spiracles reduces EWL
Insects and Arachnids
• Excretory Water Loss
Countermeasures
– Active reclamation of water from
urine and feces from rectum
– Uric acid formation
• Insoluble nitrogenous waste product
• Requires little water to excrete
• May be retained in fat and cuticle
Reptiles
• Generally impermeable
integument
– 1/10th to 1/100th that of an
amphibian
– Become more impermeable in
spp. from drier habitats
• Excrete uric acid
– Insoluble in water
– Requires less water to excrete
than urea
Mammals
• May need to use water to
regulate body temperature
– trade off between
temperature regulation and
water balance
• Desert mammals
– Little opportunity to drink
– Gain most water from food
Kangaroo Rats
• Never drink, survive on diet of
dry seeds
• Obtain most water from aerobic
metabolism
• Possess kidneys that produce
concentrated urine
• Spends considerable time in
burrows to reduce respiratory
EWL
• Cooling system in nasal passages
reduces respiratory water loss
Marine Mammals, Birds and
Reptiles
• Body surfaces do not exchange
water/solutes
• Must drink to replenish water stores
– Sea water 3x osm. conc. of body fluids
– Salts imbibed or ingested must be secreted at
high concentration
Marine Reptiles and Birds
• Kidneys produce urine with
[Osm] less than sea water
• Salt glands
– Produce highly concentrated
saline fluid (mostly NaCl)
• More concentrated than sea water
– Respond to increased salt load in
plasma
Marine Mammals
• Efficient kidneys
– Produce hyperosmotic urine
• Produce concentrated milk during lactation
– High fat + protein