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Chapter 51 Salt and Water Balance and Nitrogen Excretion
Blood, sweat, and tears
-blood, sweat, tears taste salty because they have similar ionic concentrations as interstitial fluids
-excretory system is responsible for maintaining homeostasis of interstitial fluid
-all animals derive water from metabolism of food
-desert animals must conserve water
-they excrete wastes that are extremely concentrated, urine that is so concentrated that it contains
crystals of solute
-insects excrete semisolid wastes
-animals that live in fresh water must constantly bail themselves out by producing copious
amounts of dilute urine while they conserve solute
-vampire bats feed on blood of unsuspecting (usually sleeping) mammal
-blood is a high-protein, liquid food
-to maximize volume of blood it can ingest, it eliminates water from its food by producing a lot
of very dilute urine
-when feeding ends, high rate of water loss must stop
-produce small amounts of concentrated urine
What Roles do Excretory Organs Play in Maintaining Homeostasis?
-excretory organs control volume, concentration, and composition of extracellular fluids
-concentration of solutes in extracellular fluid determines water balance of its cells
Water enters or leaves cells by osmosis
-movement of water across cell plasma membranes depends on differences in solute
concentration
-water flow from high to low solvent concentration across a water permeable membrane
-solute concentration of extracellular fluid affects both volume and solute concentration of cells
-osmolarity of a solution is the number of moles of osmotically active solutes per L of solvent
-glucose is a one osmolar solution
-NaCl is a 2 osmolar solution because NaCl dissociates into 2 osmotically active ions
Excretory organs control extracellular fluid osmolarity by filtration, secretion, and reabsorption
-excretory organs control osmolarity and volume of extracellular fluids by excreting solutes that
are in excess (NaCl) and conserve solutes that are in short supply (glucose and amino acid)
-eliminate waste products of nitrogen metabolism
-output of excretory organ is urine
-filters extracellular fluid to produce a filtrate that contains no cells or large molecules, like
protein
-in closed circulatory system, blood plasma is filtered across walls of capillaries
-filtration is driven by blood pressure
-water and small molecular weight solutes can cross capillary wall
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-large molecules and cells remain in blood
-flitrate (water and small molecules) flows through tubules
-tubules change composition of filtrate by active secretion and reabsorption of specific solute
molecules
-3 mechanisms: filtration, secretion, reabsorption are used in excretory system
-no active transport of water
-water must be moved either by pressure difference or difference in osmolarity
Animals can be osmoconformers or osmoregulators
-osmolarity of ocean water is high
-osmoconformers are marine invertebrates that equilibrate extracellular fluid osmolarity with
ocean water
-osmoregulators are marine animals that maintain extracellular fluid osmolarities much lower
than seawater
-all marine vertebrates, except for shark and ray are osmoregulator
-no animal can survive if its extracellular fluid has osmolarity of fresh water
too few solutes in extracellular fluid, nutrients and ions necessary for cell functions
-high solute concentrations can cause proteins to denature
-brine shrimp Artemia can survive in environment of any osmolarity
-insolute concentrated environment, it maintains tissue fluid osmolarity below that of
environment
-its mechanism of osmoregulation is active transport of Cl- from extracellular fluid to
surrounding environment, followed by Na+ ions
-cannot survive in fresh water, but dilute seawater
-maintains osmolarity of extracellular fluid above environment
-reverse direction of Cl- transport across gill membranes
Animals can be ionic conformers or ionic regulators
-osmoconformers can be ionic conformers, ionic composition and osmolarity of extracellular
fluid match that of environment
-most osmoconformers are ionic regulators
-employ active transport mechanism to excrete ions and maintain other ions in extracellular fluid
at optimal concentrations
-herbivores conserve Na+ because plants contain low concentration of Na+
-birds that feed on marine animals must excrete excessive sodium they ingest with their food
-nasal salt glands excrete concentrated solution of NaCl through a duct that empties into nasal
cavity
-ex. penguin and seagull
How Do Animals Excrete Toxic Wastes from Nitrogen Metabolism?
-end products of metabolism of carbohydrates and fats are water and carbon dioxide
-protein and nuclei acids contain nitrogen
-their metabolism produces nitrogenous wastes, water, and carbon dioxide
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Animals excrete nitrogen in a number of forms
-ammonia (NH3) is the most common nitrogenous waste
-is highly toxic
-must be excreted to prevent accumulation or detoxified by conversion into other molecules
Ammonia
-highly soluble in water
-diffuses rapidly
-excretion of ammonia is simple for aquatic animals
-lose ammonia from blood to environment by diffusion
-ammonotelic are animals that excrete ammonia, i.e. aquatic invertebrates and bony fish
-terrestrial animals convert ammonia to urea or uric acid
Urea
-ureotelic animal excrete urea as principal nitrogenous waste product
-ex. mammal, amphibian, cartilaginous fish (shark and ray)
-urea is soluble in water
-excretion of urea solution results in large loss of water
-marine animal keeps extracellular fluid almost isosmotic (same osmotic concentration) to
marine environment by retaining high concentrations of urea
Uric Acid
-uricotelic animal conserves water by excreting nitrogenous wastes as uric acid
-ex. insect, reptile, bird, some amphibian
-uric acid is insoluble in water
-precipitate out of urine
-excreted as semisolid
-lose very little water as it disposes its nitrogenous wastes
Most species produce more than one nitrogenous waste
-humans are ureotelic
-also excrete uric acid
-uric acid in human urine comes largely from metabolism of nucleic acids and caffeine
-can excrete ammonia to regulate pH of extracellular fluids
-excreted ammonia buffers urine and enables secretion of more hydrogen ions
-tadpole excretes ammonia
-adult toad and frog excrete urea
-adult amphibians in arid habitat excrete uric acid
How Do Invertebrate Excretory Systems Work?
The protonephridia of flatworms excrete water and conserve salts
-flatworms live in fresh water
-excrete water through tubules that run throughout bodies
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Document Summary

Chapter 51 salt and water balance and nitrogen excretion. Blood, sweat, tears taste salty because they have similar ionic concentrations as interstitial fluids. Excretory system is responsible for maintaining homeostasis of interstitial fluid. All animals derive water from metabolism of food. They excrete wastes that are extremely concentrated, urine that is so concentrated that it contains crystals of solute. Animals that live in fresh water must constantly bail themselves out by producing copious amounts of dilute urine while they conserve solute. Vampire bats feed on blood of unsuspecting (usually sleeping) mammal. To maximize volume of blood it can ingest, it eliminates water from its food by producing a lot of very dilute urine. When feeding ends, high rate of water loss must stop. Excretory organs control volume, concentration, and composition of extracellular fluids. Concentration of solutes in extracellular fluid determines water balance of its cells. Movement of water across cell plasma membranes depends on differences in solute concentration.

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