Osmoregulation & Excretion Chapter 44 Regulating Chemicals • All animals regulate the chemical composition of their bodies by balancing uptake and loss of water and fluids Osmoregulation • Osmoregulation is the management of the body’s water content – By controlling movements of solutes between interstitial fluids and the external environment – Water movement follows solute movement by osmosis – The main goal is to regulate the composition of the body’s cells • But is primarily accomplished by regulating the composition of the blood and, subsequently, the interstitial fluids that bathe the cells • Specialized organs maintain fluid composition • Example: vertebrate kidneys Transport Epithelia • Excretory Organs have 2 main functions: – Water balance – Waste disposal (nitrogenous waste) • Both depend on transport epithelia – Moves particular solutes in controlled amounts in specific directions – Some transport epithelia directly face the external environment while others line channels that lead to the outside of the body surface – Most are arranged in complex tubular networks • Provides a large surface area Waste Removal • Nitrogenous waste accumulates from the breakdown of proteins and nucleic acids • If not removed, these wastes become toxic • Nitrogenous wastes are the most important in terms of osmoregulationNitrogenous Waste • Enzymes breakdown proteins and remove nitrogen in the form of ammonia • Ammonia is a very toxic molecule • Animals that can quickly get rid of ammonia usually do so – These are typically aquatic animals (animals that live in water) • Those that can’t usually convert ammonia into a less toxic form – Terrestrial (land) animals must do this – Generally ammonia is converted into Urea or Uric Acid – There is an energetic cost in doing this Nitrogenous Waste • Ammonia (a liquid) • Requires large amounts of water to excrete • Very toxic • Small energetic cost • Typical for aquatic animals • Urea (a liquid) • Requires medium amounts of water to excrete • Mildly toxic • Medium energetic cost • Synthesized in the liver by combining ammonia with carbon dioxide • Excreted by kidneys • Typical for mammals and many other vertebrates • Uric Acid (a semi-solid paste) • Requires minimal amounts of water to excrete • Insoluble in water • Least toxic • Large energetic cost • Typical for insects and reptiles (including birds) Osmotic Balance • Water Loss and Gain • Loss must equal gain over time • Cells will burst and die or shrivel and die is water balance is not maintained • Water moves in both directions by osmosis• If two solutions separated by a membrane have different osmotic pressures (osmolarity), then water will move • Water always moves passively—never actively Osmolarity • A measure of osmotic pressure • (solute concentration in water solutions) • Molarity is the number of moles solute per liter of solution • Osmolarity measures solutes dissolved in water • Measured in milliosmoles per liter (mOsm/L) • A solution with a total solute concentration of 10-3 M = 1 mOsm/L • Human Blood = 300 mOsm/L • Seawater = 1000 mOsm/Liter • When two solutions differ in osmolarity… • The one with the greater solute concentration is said to be hyperosmotic with respect to the other • The one with the lower solute concentration is said to be hypoosmotic with respect to the other • If the two solutions have the same osmolarity • They are both isoosmotic with respect to each other Osmoconformers • An osmoconformer is an animal that has an internal environment isoosmotic with respect to the external environment – These do not regulate their bodies in response to external osmolarity changes – They usually live in very stable environments • Marine invertebrates are generally osmoconformers, so are hagfishes Osmoregulators • An osmoregulator is an animal that maintains its internal environment in a state that is not isoosmotic with respect to the external environment – Their bodies may be • Hypoosmotic – They must take in water • Hyperosmotic – They must discharge water – Allows them to live in unstable environments, such as freshwaters and terrestrial habitats • This is energetically expensive – Marine vertebrates and freshwater vertebrates and invertebrates are generally osmoregulatorsFreshwater Fish • Constantly gain water through the gills and skin while losing ions to the water • Thus, they do not need to drink water • They actively uptake ions across the gills from the surroundings • They produce lots of urine Saltwater Fish • Constantly loses water through the gills and skin while taking in excess ions from seawater • Thus, they need to drink lots of water • They actively excrete large amounts of ions across the gills • They produce very little urine Sharks (and other chondrichthyes) are an exception • Maintain a high level of urea and trimethylamine oxide (TMAO) in their body fluids • Raises osmolarity above seawater • Water slowly enters the shark by osmosis and in food • They do not need to drink water Excretory Systems • Protonephridia – Very simple system – Found in flatworms – Branching network of dead-end tubules – Specialized flame cells draw in water and solutes and excretes it through openings called nephridiopores • Metanephridia – Found in most annelids – Internal openings collect body fluids from the coelom through a ciliated funnel (nephrostome) and release the fluid through nephridiopores – Metanephrida often have both excretory and osmoregulatory functions • Malpighian Tubules – Found in most insects and other terrestrial arthropods – Function in removal of nitrogenous waste and water balance – They open into the digestive system – The other end is blind-ended, surrounded by hemolymph – Very efficient at removing nitrogenous waste without too much water loss – Uric acid is main waste – A key adaptation allowing successful invasion of land • Kidneys (be sure to study how the kidneys work in the textbook) • Found in vertebrates • Function in osmoregulation and excretion • Come in pairs • Output is urine, containing mostly urea and
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