bioa02 chapter 51

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Published on 26 Jun 2011
School
UTSC
Department
Biological Sciences
Course
BIOA02H3
Professor
Chapter 51
51.1 – What roles do excretory organs play in maintaining homeostasis?
-Excretory organs control the volume, concentration, and composition of the
extracellular fluids of animals
-The movement of water across cell plasma membranes depends on differences in
solute concentration
oIf the solute concentrations are different on two sides of a membrane
permeable to water but not the solutes, the water will flow from the side
with the lower concentration to the side with a higher concentration of
solute
-Animal physiologists use the term osmolarity in discussing osmosis
oThe osmolarity of a solution is the number of moles of osmotically active
solutes per liter of solvent
-Excretory organs control the osmolarity and the volume of the extracellular fluids
(blood and interstitial fluid) by excreting solutes that are present in excess (such
as NaCl when we eat lots of salty food) and conserving solutes that are valuable
or in short supply (such as glucose and amino acids)
-Excretory organs also eliminate the waste products of nitrogen metabolism
oThe output is called urine
-The excretory organs filters extracellular fluid to product a filtrate that contains no
cells or large molecules, such as proteins
oThe composition of the filtrate is then modified to produce urine
-In animals with closed circulatory system, the blood plasma is filtered across the
walls of capillaries
oThe filtration is driven by blood pressure
oThe filtrate (water and small molecules) then flows through tubules
oThe cells of the tubules change the composition of the filtrate by active
secretion and reabsroption of specific solute molecules
-In the excretory system, water must be moved either be a pressure difference or
by a difference in osmolarity
-Animals that live in marine, freshwater, or terrestrial environments face different
salt and water balance problems
oIn the terrestrial environment salts and water can be scarce and usually
must be conserved by excretory system
oIn the freshwater environment waster is plentiful, but salts are scarce
They have to conserve salts and excrete the water that continuously
invades their bodies through osmosis
-Most marine invertebrates equilibrate their extracellular fluid osmolarity with the
ocean water and are therefore called osmoconformers
-Other marine animals maintain extracellular fluid osmolarities much lower than
seawater and are therefore called osmoregulators
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-Even animals that can osmoconform over a wide range of osmolarities must
osmoregulate in extreme environments
oNo animal could survive if its extracellular fluid had the osmolarity of
fresh water, because that would mean there were too few solutes in the
extracellular fluid, including nutrients and ions necessary for cell functions
oNor could animals survive with internal osmolarities as high as those that
may be reached in an evaporating tide pool
-High solute concentrations can cause proteins to denature
-Artemia (brine shrimp), could survive in almost any osmolarity
oFound in huge numbers in the most salty environments known
oThe osmolarity of such water reaches 2,500 mosm/l
oMaintains its tissue fluid osmolarity considerably below that of the
environment
oIt’s mechanism of osmoregulation is the active transport of Cl- from its
extracellular fluid out across its gill membranes to the environment
oCannot survive in fresh water, but it can live in dilute seawater, in which it
maintains the osmolarity of its extracellular fluid above that of the
environment
Under this, Artemia reverses the direction of Cl- transport across
its gill membranes
-Osmoconformaers can also be ionic conformers, allowing the ionic composition,
as well as the osmolarity, of their extracellular fluid to match that of the
environment
-Most osmoconformers are ionic regulators to some degree
oThe employ active transport mechanisms to excrete some ions and to
maintain other ions in their extracellular fluid at optimal concentration
-Terrestrial animals obtain their salts from food and regulate the ionic composition
of their extracellular fluids by conserving some ions and excreting others
oHerbivores have to conserve Na+ because the plants they eat have low
concentration of Na+
oBirds that feed on marine animals must excrete the excess of Na+ they
ingest with their food
Their nasal salt glands excrete a concentrated solution of NaCl via
a duct that empties into the nasal cavity
Penguins and seagulls have nasal salt glands can be seen frequently
sneezing or shaking their heads to get rid of the very salty droplets
excreted from their nasal salt glands
51.2 – How do animals excrete toxic wastes from nitrogen metabolism?
-the end products of the metabolism of carbohydrates and fats are water and
carbon dioxide, which are not difficult to eliminate
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-proteins and nucleic acids, however, contain nitrogen, so their metabolism
produces nitrogenous wastes in addition to waster and carbon dioxide
-the most common nitrogenous waste is ammonia
ois highly toxic, so it must either be excreted continuously to prevent its
accumulation, or it must be detoxified by conversion into other molecules
-Ammonia is highly soluble in waster and diffuses rapidly, so the continuous
excretion of ammonia is relatively simple for aquatic animals
oAnimals that breathe water continuously lose ammonia from their blood to
the environment by diffusion across their gill membranes
oAnimals that excrete ammonia, such as aquatic invertebrates and bony
fishes are ammonotelic
oIf ammonia builds up in the extracellular fluids, it becomes toxic at rather
low levels and is a dangerous metabolite for terrestrial animals
oTherefore, terrestrial animals convert ammonia into either urea or uric acid
-Urea ureotelic animals such as mammals, amphibians, and cartilaginous fishes
(sharks and rays), excrete urea as their principal nitrogenous waste product
oIs quite soluble in water, but excretion of urea can result in a large loss of
water
oThe sharks and rays keep their extracellular fluids almost isomotic (same
osmotic concentration) to the marine environment by retaining high
concentration of urea
-Uric acid animals that conserve water by excreting nitrogenous wastes mostly
as uric acid are uricotelic
oInsect, repiles, birds, and some amphibians are uricotelic
oIs very insoluble in water, so it can precipitate out of the urine and by
excreted as a semisolid
oA uricotelic animal loses very little water as it disposes of its nitrogenous
wastes
-Humans are ureotelic, but we also excrete uric acid
oThe uric acid comes largely from the metabolism of nucleic acid and
caffeine
-Humans can also excrete ammonia, which is an important mechanism for
regulating the pH of the extracellular fluids
oExcreted ammonia buffers the urine and enables the secretion of more
hydrogen ions
-The tadpoles of frogs and toads excrete ammonia across their gill membranes, but
when they develop into adult frogs or toads, they generally excrete urea
-Some adult amphibians that live in dry habitats excrete uric acid
51.3 – How do inveterate excretory systems work?
-many flatworms, such as Planaria, live in fresh water
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Document Summary

Excretory organs  control the volume, concentration, and composition of the extracellular fluids of animals. Animal physiologists use the term osmolarity in discussing osmosis: the osmolarity of a solution is the number of moles of osmotically active solutes per liter of solvent. Excretory organs also eliminate the waste products of nitrogen metabolism: the output is called  urine. The excretory organs filters extracellular fluid to product a filtrate that contains no cells or large molecules, such as proteins: the composition of the filtrate is then modified to produce urine. In the excretory system, water must be moved either be a pressure difference or by a difference in osmolarity.  they have to conserve salts and excrete the water that continuously invades their bodies through osmosis. Most marine invertebrates equilibrate their extracellular fluid osmolarity with the ocean water and are therefore called  osmoconformers.