Chapter 52 Kidneys (1)

Chapter 52 Kidneys (1) - Give or take (salt, water)...

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Excretory systems control volume , osmolarity , and ionic composition of the extracellular fluid, and excrete wastes . The problems of salt and water balance that animals face depend on their environments, but in all animal excretory systems, there is no active transport of water . As you know, water moves by osmosis , from regions of low solute concentration to regions of high, expressed as osmolarity . The osmolarity of a solution is the number of moles of osmotically active solutes per liter of solvent. Thus, a solution of 150 mM (millimolar) NaCl is 150 mOsm (milliOsmolar) of Na + and 150 mOsm of Cl - , giving an total osmolarity of 300 mOsm. Give or take (salt, water)
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Water transport The extracellular fluid (tissue fluid) of the simplest marine animals is seawater. More complex marine animals have extracellular fluids ( tissue fluids ) isolated from, but similar to, seawater. Most marine vertebrates, all freshwater vertebrates, and terrestrial animals maintain extracellular fluids that differ substantially from seawater. The salt and water regulation of animals that live in different environments makes sense. Examples: Freshwater animals must continually excrete water and conserve salts . Simple marine animals may just conform to external osmotic pressure. Animals that live in an evaporating salt pond must hold onto water and excrete salts. On land, water conservation is essential , and diet determines whether salts must be conserved or excreted. For example, marine birds (who eat salty fish) excrete excess salt through nasal salt glands.
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Excreting nitrogen Aquatic animals can eliminate nitrogenous wastes such as ammonia by diffusion across their gill membranes. Terrestrial animals must detoxify nitrogenous waste by converting it to (water-soluble) urea (with attendant loss of water) or (water- insoluble) uric acid (with little loss of water). Humans also excrete uric acid (from nucleic acids and caffeine) and ammonia to regulate the pH of extracellular fluid by buffering urine.
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The systems are similar across species For example, metanephridia in earthworms (left figure) look similar to vertebrate nephrons (right figure). All excretory systems involve filtration , secretion , and reabsorption .
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All adaptations for maintaining salt and water balance and for excreting nitrogen wastes employ filtration of body fluids and active secretion and reabsorption of specific ions . Remember: water move by OSMOSIS. The
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Chapter 52 Kidneys (1) - Give or take (salt, water)...

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