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bioa02 chapter 51

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Biological Sciences
Mary Olaveson

Chapter 51 51.1 What roles do excretory organs play in maintaining homeostasis? - Excretory organs control the volume, concentration, and composition of the extracellular fluids of animals - The movement of water across cell plasma membranes depends on differences in solute concentration o If the solute concentrations are different on two sides of a membrane permeable to water but not the solutes, the water will flow from the side with the lower concentration to the side with a higher concentration of solute - Animal physiologists use the term osmolarity in discussing osmosis o The osmolarity of a solution is the number of moles of osmotically active solutes per liter of solvent - Excretory organs control the osmolarity and the volume of the extracellular fluids (blood and interstitial fluid) by excreting solutes that are present in excess (such as NaCl when we eat lots of salty food) and conserving solutes that are valuable or in short supply (such as glucose and amino acids) - Excretory organs also eliminate the waste products of nitrogen metabolism o The output is called urine - The excretory organs filters extracellular fluid to product a filtrate that contains no cells or large molecules, such as proteins o The composition of the filtrate is then modified to produce urine - In animals with closed circulatory system, the blood plasma is filtered across the walls of capillaries o The filtration is driven by blood pressure o The filtrate (water and small molecules) then flows through tubules o The cells of the tubules change the composition of the filtrate by active secretion and reabsroption of specific solute molecules - In the excretory system, water must be moved either be a pressure difference or by a difference in osmolarity - Animals that live in marine, freshwater, or terrestrial environments face different salt and water balance problems o In the terrestrial environment salts and water can be scarce and usually must be conserved by excretory system o In the freshwater environment waster is plentiful, but salts are scarce They have to conserve salts and excrete the water that continuously invades their bodies through osmosis - Most marine invertebrates equilibrate their extracellular fluid osmolarity with the ocean water and are therefore called osmoconformers - Other marine animals maintain extracellular fluid osmolarities much lower than seawater and are therefore called osmoregulators - Even animals that can osmoconform over a wide range of osmolarities must osmoregulate in extreme environments o No animal could survive if its extracellular fluid had the osmolarity of fresh water, because that would mean there were too few solutes in the extracellular fluid, including nutrients and ions necessary for cell functions o Nor could animals survive with internal osmolarities as high as those that may be reached in an evaporating tide pool - High solute concentrations can cause proteins to denature - Artemia (brine shrimp), could survive in almost any osmolarity o Found in huge numbers in the most salty environments known o The osmolarity of such water reaches 2,500 mosml o Maintains its tissue fluid osmolarity considerably below that of the environment o Its mechanism of osmoregulation is the active transport of Cl- from its extracellular fluid out across its gill membranes to the environment o Cannot survive in fresh water, but it can live in dilute seawater, in which it maintains the osmolarity of its extracellular fluid above that of the environment Under this, Artemia reverses the direction of Cl- transport across its gill membranes - Osmoconformaers can also be ionic conformers, allowing the ionic composition, as well as the osmolarity, of their extracellular fluid to match that of the environment - Most osmoconformers are ionic regulators to some degree o The employ active transport mechanisms to excrete some ions and to maintain other ions in their extracellular fluid at optimal concentration - Terrestrial animals obtain their salts from food and regulate the ionic composition of their extracellular fluids by conserving some ions and excreting others o Herbivores have to conserve Na+ because the plants they eat have low concentration of Na+ o Birds that feed on marine animals must excrete the excess of Na+ they ingest with their food Their nasal salt glands excrete a concentrated solution of NaCl via a duct that empties into the nasal cavity Penguins and seagulls have nasal salt glands can be seen frequently sneezing or shaking their heads to get rid of the very salty droplets excreted from their nasal salt glands 51.2 How do animals excrete toxic wastes from nitrogen metabolism? - the end products of the metabolism of carbohydrates and fats are water and carbon dioxide, which are not difficult to eliminate
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