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) 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

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

Osmotic Tolerance Euryhaline Stenohaline 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 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 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

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

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 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

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