Chapter 44 Osmoregulation and Excretion 44 1 OSMOREGULATION BALANCES THE UPTAKE AND LOSS OF WATER AND SOLUTES Regulating the chemical composition of body fluids depends on balancing the uptake and loss of water and solutes The driving force for the movement of both water and solutes in animals is a concentration gradient OSMOSIS AND OSMOLARITY Water enters and leaves through osmosis which occurs when 2 solutions separated by a membrane differ in total solute concentration The unit of measurement of solute concentration is osmolarity 2 solutions with the same osmolarity are isotonic A hyperosmotic solution has a higher concentration of solutes while a hypo osmotic solution has a lower concentration of solutes OSMOREGULATORY CHALLENGES AND MECHANISMS An animal can maintain a water balance by being an osmo conformer being isosmotic with its surroundings or by being an osmo regulatory control internal osmolarity independent of external environment All osmo conformers are marine animals Osmoregulation allows animals to live in environments that are uninhabitable for osmo conformers such as freshwater and terrestrial habitats In a hypo osmotic environment an organism must discharge excess water while in a hyperosmotic environment water must be taken in Stenohalines cannot tolerate great changes in external osmolarity Euryhaline animals can survive large fluctuation in external osmolarity Marine Animals Most marine invertebrates are osmo conformers and have the same osmolarity as that of seawater However they have different concentrations of specific solutes from the seawater and they must actively transport them to maintain homeostasis Many vertebrates and some marine invertebrates are osmo regulators due to the ocean being a dehydrating environment Marine fishes can overcome this by drinking seawater and excreting excess salt through gills and kidneys A distinct osmo regulatory strategy evolved in marine sharks and most other chondrichthyans They have lower salt concentrations than the seawater and salt diffuses across gills However they are not hypo osmotic to seawater They maintain an osmolarity close to seawater by having salts urea TMAO protects proteins from urea and other compounds in the body fluids This makes Sharks osmo conformers The excess water than enters by osmosis is secreted as urine through a specialized gland Freshwater Animals Freshwater animals have the opposite problem of marine animals The body fluids of freshwater animals must be hyperosmotic because animal cells cannot tolerate salt concentrations as low as that of freshwater They have the problem of gaining water by osmosis and losing salts by diffusion This problem is overcome by drinking low amounts of water and excreting large amounts of very dilute urine Salts lost by diffusion and in urine are replenished by eating or uptake across gills Fish that migrate between freshwater and seawater undergo dramatic changes in osmo regulatory status For example Salmon act like freshwater fish in freshwater excrete urine and acclimatize in the ocean by making cortisol which increases the size of salt secreting chloride cells This causes them to excrete excess salt from their gills and produce low amounts of urine Animals That Live In Temporary Waters Animals that live in temporary ponds and films of water are prone to desiccation but overcome this by entering a dormant state when things dry out called ANHYDROBIOSIS This adaptation requires leaving cell membranes intact Tardigrades survive drying out by having trehalose disaccharide protects the cell by replacing water associated with proteins and membrane lipids Land Animals Land animals must overcome the threat of dehydration Just like a waxy cuticle in plants animals have many coverings such as the waxy layers of insect exoskeletons the shells of snails and layers of dead keratinized skin cells covering most terrestrial vertebrates Many desert dwellers are also nocturnal Land animals lose water through urine and feces but replace it by eating moist foods and drinking Energetics of Osmoregulation Marinating an osmolarity difference between an animal s body and its external environment carries an energy cost Marinating osmotic gradients sometimes requires active transportation to manipulate solute concentrations Minimizing the osmotic difference between fluids and the surrounding environment decreases the energy cost of osmoregulation TRANSPORT EPITHELIA IN OSMOREGULATION The ultimate function of osmoregulation is to control solute concentrations in cells but most animals do this indirectly by managing the solute concentration of an internal body fluid that bathes cells hemolymph blood In most animals osmoregulation and metabolic waste disposal rely on transport epithelia epithelial cells that move particular solutes in certain amounts and in specific directions Transport Epithelia are usually arranged in complex tubular networks with extensive surface areas The albatross has salt glands to discharge a concentrated NACL solution 44 2 AN ANIMALS NITROGENOUS WASTES REGLECT ITS PHYLOGENY AND HABITAT FORMS OF NITROGENOUS WASTE Different waste products have different levels of toxins and energy costs Ammonia Need lots of water to dilute ammonia because it can only be tolerated in very low concentrations It is common in aquatic species Many invertebrates release ammonia across whole body surface Urea Common in land animals because Ammonia is very dangerous if not very dilute In vertebrates Urea is the product of a metabolic cycle that combines ammonia with carbon dioxide in the liver Its advantage is its low toxicity but its main disadvantage is its energy cost Uric Acid Insects land snails and many reptiles including birds excrete uric acid as their primary nitrogenous waste It is relatively nontoxic and does not readily dissolve in water It can be therefore excreted as a semisolid paste with very low water loss However it is more energetically expensive than urea THE INFLUENCE OF EVOLTUON AND ENVIRONMENT ON NITROGENOUS WASTES Type of nitrogenous waste depends on phylogeny and habitat availability of water Another major factor that determines the type of waste produced is the type of animal egg Amphibian eggs and mammalian embryo can produce soluble waste due to water and blood while animals with hard shelled eggs can t Animals with shelled eggs have eggs that are permeable to gases but not liquids More waste is produced in animals with higher energy budgets endotherms 44