Lecture_15_Comparative Anatomy_The Urogenital System

The kidneys regulate the constant volume and

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Unformatted text preview: Metabolism of proteins and nucleic acids produces nitrogen, usually in the reduced form of ammonia (NH3). Ammonia is highly toxic and therefore must be removed from the body quickly, sequestered, or converted into a nontoxic form to prevent accumulation in tissues. Three routes for eliminating ammonia: o ammonotelism – direct excretion of ammonia; most common in animals living in water; NH3 is very water soluble o uricotelism – excretion of nitrogen in the form of uric acid; common in birds and lizards; uric acid is not very water soluble; uric acid is transported via the ureters to the cloaca where it precipitates out to form a salt (K, NA, or ammonium) o ureotelism – excretion of nitrogen in the form of urea; common in mammals; urea is much less toxic than NH3 and more water soluble; urea is concentraited in urine and excreted. o Note: in some species, the route of ammonia elimination can change depending on the amount of water available. For example, turtles excrete primarily ammonia in aquatic habitats, but eliminate urea or uric acid when on land. Osmoregulation: Regulating Water and Salt Balance • General Points o Osmoregulation involves the maintenance of water and salt levels. The kidneys regulate the constant volume and composition of blood and lymph in terrestrial vertebrates. In aquatic vertebrates, the gill epithelium and digestive tract are as important as the kidneys in addressing the problem of salt balance. • Water Balance o General Points  On land, the kidneys, cloaca, and possibly the urinary bladder are water conservers, meaning that they recover water before nitrogen is eliminated from the body.  Organisms in water must deal with water fluxes, movement of water into or out of the body. In freshwater fishes...
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