1. Water and Osmotic Regulation
Chapter 8

2. 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

3. 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

4. Major Types of Hydric Environments
Aquatic – high water availability
Marine
High solute concentration
Fresh Water
Low solute concentration
Terrestrial – low water availability

5. Aquatic Environments Sea Water (ca. 3.5% salt, 1 Osm) Fresh Water
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

6. Osmotic Regulation Osmoconformers Osmoregulators
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

7. Osmotic Tolerance Euryhaline Stenohaline
tolerate wide variations in environmental osmotic concentrations
Stenohaline
tolerate only limited variation in environmental osmotic concentration.

8. 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

9. Marine Invertebrates Composition can differ between different fluids:
External environment
Blood & Interstitial fluid (extracellular fluid)
Intracellular fluid

10. 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

11. 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

12. Freshwater Invertebrates
Solutions
Decrease permeability
May cause problems with uptake of other substances
Active Transport
Uptake of ions against a electrochemical gradient
Requires energy

13. Brackish Water Invertebrates
Possible wide fluctuation in osmotic environment
Variety of responses in osmotic regulation

14. 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

15. 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

16. 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

17. 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

18. 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

19. “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

20. 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.

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. Arid Amphibians Estivation Reduced Integumental Permeability
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

28. Crustaceans Crabs Isopods 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

29. Insects and Arachnids Evaporative Water Loss Countermeasures
Highly impermeable integument
Waxy cuticle prevents excessive EWL
Discontinuous ventilation
Intermittent opening of spiracles reduces EWL

30. 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

31. Reptiles Generally impermeable integument Excrete uric acid
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

32. 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

33. 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

34. 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

35. 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

36. Marine Mammals Efficient kidneys
Produce hyperosmotic urine
Produce concentrated milk during lactation
High fat + protein