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Chapter 51

Chapter 51 Study Guide

Biological Sciences
Course Code
Mary Olaveson

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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
-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
-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
-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
-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
-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
-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
-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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