LE 44-8Nitrogenous basesNucleic acidsAmino acidsProteins—NH2Amino groupsMost aquaticanimals,including mostbony fishesMammals, mostamphibians,sharks, some bonyfishesMany reptiles(includingbirds), insects,land snailsAmmonia Urea Uric acidAmmonia• Animals that excrete nitrogenouswastes as ammonia need lots of water• They release ammonia across thewhole body surface or through gillsUrea• The liver of mammals and most adultamphibians converts ammonia to lesstoxic urea• The circulatory system carries urea tothe kidneys, where it is excretedUric Acid• Insects, land snails, and many reptiles, includingbirds, mainly excrete uric acid• Uric acid is largely insoluble in water and can besecreted as a paste with little water lossThe Influence of Evolution andEnvironment on NitrogenousWastes• The kinds of nitrogenous wastesexcreted depend on an animal’sevolutionary history and habitat• The amount of nitrogenous waste iscoupled to the animal’s energy budgetExcretory Processes• Most excretory systems produce urine byrefining a filtrate derived from body fluids• Key functions of most excretory systems:– Filtration: pressure-filtering of body fluids– Reabsorption: reclaiming valuable solutes– Secretion: adding toxins and other solutes fromthe body fluids to the filtrate– Excretion: removing the filtrate from the systemLE 44-9FiltrationReabsorptionSecretionExcretionExcretorytubuleCapillaryFiltrateUrineMalpighian Tubules• In insects and other terrestrial arthropods,Malpighian tubules remove nitrogenouswastes from hemolymph and function inosmoregulation• Insects produce a relatively dry waste matter,an important adaptation to terrestrial lifeLE 44-12Salt, water, andnitrogenouswastesDigestive tractMidgut(stomach)MalpighiantubulesRectumIntestineHindgutReabsorption of H2O,ions, and valuableorganic moleculesMalpighiantubuleHEMOLYMPHAnusRectumFeces and urineVertebrate Kidneys• Kidneys, the excretory organs of vertebrates,function in both excretion and osmoregulationConcept 44.4: Nephrons andassociated blood vessels are thefunctional unit of the mammaliankidney• The mammalian excretory system centers onpaired kidneys, which are also the principal site ofwater balance and salt regulation• Each kidney is supplied with blood by a renalartery and drained by a renal vein• Urine exits each kidney through a duct called theureter• Both ureters drain into a common urinary bladderLE 44-13Excretory organs and major associated blood vesselsRenalmedullaRenalcortexRenalpelvisSection of kidney from a ratKidney structureUreterKidneyGlomerulusBowman’s capsuleProximal tubulePeritubular capillariesAfferentarteriolefrom renalarteryEfferentarteriole from glomerulusDistaltubuleCollectingductSEM20 µmBranch ofrenal veinFiltrate and blood flowVasarectaDescendinglimbAscendinglimbLoopofHenleRenalmedullaNephronTorenalpelvisRenalcortexCollectingductJuxta-medullarynephronCorticalnephronPosterior vena cavaRenal artery and veinAortaUreterUrinary bladderUrethraLE 44-14FiltrateH2OSalts (NaCl and others)HCO3–H+UreaGlucose; amino acidsSome drugsKeyActive transportPassive transportINNERMEDULLAOUTERMEDULLANaClH2OCORTEXDescending limbof loop ofHenleProximal tubuleNaClNutrientsHCO3–H+K+NH3H2ODistal tubuleNaClHCO3–H+K+H2OThick segmentof ascendinglimbNaClNaClThin segmentof ascendinglimbCollectingductUreaH2OConcept 44.5: The mammaliankidney’s ability to conserve wateris a key terrestrial adaptation• The mammalian kidney conserveswater by producing urine that is muchmore concentrated than body
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