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Water and Osmotic Regulation

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

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Water and Osmotic Regulation

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  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) • 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 • 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 • 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 Ways of losing water: • Evaporation • Body surface • Respiratory surface • Excretion/secretion • Feces • Urine • Other secretions Over time, water gain must equal water loss 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 • 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 • 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 • 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

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