Water and Osmotic Regulation Chapter 8 Water Balance and Concentration Internal Environment = aqueous solution –Volume and composition must be maintained.

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 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

Marine Mammals

• Efficient kidneys– Produce hyperosmotic urine

• Produce concentrated milk during lactation– High fat + protein