water and osmotic regulation chapter 8 water balance and concentration internal environment =...
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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 large-scale 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
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
Over time, water gain must equal water loss
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
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