Ionic and Osmotic Balance

Ion & Water Balance Kidneys are main organs of ion & water balance Gills, skin, digestive mucosa all help with ion and water balance

Osmotic Regulation Primarily due to solute movement Animals do not actively pump water

Ionic Regulation Control of ionic composition of body fluids

Nitrogen Excretion Nitrogen waste from protein catabolism Forms of nitrogen waste include ammonia, uric acid, urea

Most Marine Animals Maintain ionic balance by expelling ions (especially salt) against ionic gradient Maintain osmotic balance by obtaining water against osmotic gradient

Most Fresh Water Animals Maintain ionic balance by acquiring ions from ion-poor water Maintain osmotic balance by expelling excess water against osmotic gradient

Terrestrial Animals Constantly challenged with dehydration

Ionoconformers Internal conditions are similar to external conditions even if external conditions change ECF resembles seawater in terms of major cations and anions See examples in blue

Seawater Jelly Fish Starfish Osmolarity(mosM) 1000 Na + (mM) K + (mM) Ca ++ (mM) Mg ++ (mM) Cl - (mM) SO 4 = (mM)

Ionoregulators Controls the internal conditions in their ECF by using ion absorption & excretion Shark, freshwater animals Regulated ECF eases the burden on individual cells to regulate ions

Seawater Shark Flounder Goldfish Osmolarity(mosM) Na + (mM) K + (mM) Ca ++ (mM) Mg ++ (mM) Cl - (mM) SO 4 = (mM) 271.2

Osmoconformer Internal osmolarity is close to that of the external environment even if environment changes Marine invertebrates, primitive vertebrates

Seawater Shark Flounder Goldfish Osmolarity(mosM) Na + (mM) K + (mM) Ca ++ (mM) Mg ++ (mM) Cl - (mM) SO 4 = (mM) 271.2

Osmoregulator Internal osmolarity is maintained within a narrow range regardless of environment Most marine vertebrates Freshwater vertebrates Freshwater invertebrates

Seawater Shark Flounder Goldfish Osmolarity(mosM) Na + (mM) K + (mM) Ca ++ (mM) Mg ++ (mM) Cl - (mM) SO 4 = (mM) 271.2

ECF of most marine invertebrates is similar from seawater. ECF of most marine vertebrates is different from seawater. ECF of most marine vertebrates is similar to mammals.

ICF of most animals is low in Na +, Cl - but high in K +, phosphates and proteins.

Osmoregulatory mechanisms Frog in fresh water (hypo osmotic environment) not only must eliminate excess water but also retain ions that tend to leak though the skin. Since animals live in a wide range of environments many different osmoregulatory mechanisms have evolved.

Water Sources Aquatic environment Diet Metabolism

Solutes – Inorganic & Organic RVI – Regulatory Volume Increase: importing ions resulting in influx of water Na+-K+-2Cl- cotransporter

Regulatory Volume Decrease RVD K+ channels; Cl- channels (separate or as cotransporters) & these exit cell Na+/Ca2+ exchanger followed by Ca2+ ATPase to export calcium Na+/K+ ATPase pump Water follows

Osmotic and Ionic Differences Epithelia surrounding the body maintains both osmotic and ionic differences between the ECF and the external environment. Aquaporins are pores in epithelium for water passage Solutes move by transcellular and paracellular transport

Integument (epithelium plus underlying tissue). Permeability varies among animals. Insects: waxy impermeable cuticle. Amphibians skins: mucus, permeable. Water and ions move by diffusion. Loss of ions compensated by active transport. Fish gills: active transport of ions. Reptiles, birds and mammals: relatively impermeable keratinized skins. Perspiration can lead to substantial water loss.

Gills Transports ions in or out of water depending on salinity of water

Digestive mucosa Solutes move across digestive mucosa Water moves across digestive mucosa

Salt Glands Birds and reptiles Excretes Na+ and Cl- Functions as countercurrent multiplier systems

Rectal Glands Sharks Excrete salt