Water Balance, Waste Disposal and Excretory Systems

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Water Balance, Waste Disposal and Excretory
Systems

• AP Biology Chapter 44

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Homeostasis AGAIN

• Maintaining balance between uptake of water from
  the environment and water loss from the body is
  critical to homeostasis
• Keeping toxins that are by-products of metabolism
  from building up in the body fluids is also
  critical to homeostasis
• Both of these require movements of solutes into
  and out of the body.

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Transport epithelium

• Layer or layers of specialized epithelial cells
  that regulate solute movements
• Cells joined by tight junctions keep the correct
  side of the cell facing the correct environment
  (internal or external)
• Form a selectively permeable barrier at the
  tissue/environment boundary
• Form fits function

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Nitrogenous wastes

• Toxic by-products of metabolism
• Result from breakdown of proteins and nucleic
  acids
• Ammonia the nitrogenous waste product formed
• Excreting it directly is very efficient no
  energy expended
• Very toxic
• Urea and Uric Acid
• Requires energy to convert ammonia to these forms
• Less toxic
• Type of waste formed depends on the animal

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Ammonia

• Excreted by most aquatic animals
• Very soluble in water and easily pass through
  membranes
• Inverts ammonia diffuses across whole body
  surface
• Fish
• Most ammonia lost as ammonium ions across the
  membranes of the gills.
• Conversion to less toxic forms unnecessary
• Fresh water fish constantly take in fresh water
• Water loss not an issue
• Dilute ammonia using large amounts of water and
  excrete constantly
• Only tiny amounts excreted by kidneys

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Urea

• Ammonia must be converted to urea in land animals
• Terrestrial animals cannot dilute ammonia enough
  to make it nontoxic. Too much water loss would
  result
• Urea is 100,000 times less toxic than ammonia
• Ammonia is converted to urea in the liver
• Most animals can tolerate high concentrations of
  urea
• Secretion of high concentrations of urea allows
  for conservation of water
• Amphibians, marine fish, some turtles, mammals
• Marine fish face many of the same water loss
  issues that land animals do. Salt water causes
  water loss from tissues.
• Tadpoles, however secrete ammonia

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Uric Acid

• Excreted by land snails, insects, birds and many
  reptiles
• MUCH less soluble in water than ammonia OR urea
• Excreted in paste like form
• Another type of adaptation to land life
• Used by terrestrial egg layers SHELLED EGGS
• Can be excreted by developing embryos in shelled
  eggs and STORED without poisoning embryos
• It precipitates out of water in solid form.

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Water Balance

• Balance must be maintained between water gain and
  loss in cells in all environments
• Fresh water water will enter cells
• Salt water water will leave cells
• Terrestrial water will leave cells

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Two main solutions to problem of water balance

• Osmoconformer
• Conform your bodys internal conditions to be
  like those of your environment
• Some marine animals are isoosmotic with their
  saltwater environment
• Internal solute concentrations same as external
• Do not actively adjust internal osmolarity
• Energetically cheap

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Two main solutions to problem of water balance

• Osmoregulator
• Internal conditions are different from that of
  the environment
• Animal must adjust its internal osmolarity
  because body fluids are not isoosmotic with the
  outside environment
• Must continually discharge excess water in
  hyposmotic environment
• Must continually take in water if in a
  hyperosmotic environment
• Energetically expensive pump solutes in or
  out
• Energetic cost depends on how different internal
  is from external

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Osmoregulation in salt water

• ALL about getting rid of excess salts while
  retaining water

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Osmoregulation in fresh water

• ALL about getting rid of excess water while
  maintaining salt concentrations

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Maintaining osmotic balance on land

• Threat of dessication is perhaps the most
  important problem facing terrestrial life.
• Only two animal groups (arthropods and
  vertebrates have colonized land with great
  success.

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Terrestrial adaptations

• Body coverings to prevent water loss
• Waxy exoskeletons of insects
• Shells of land snails
• Multiple layers of dead keratinized cells in
  terrestrial vertebrates
• Nocturnal activity
• Water supply replenished by drinking, but some
  are so well adapted that they can survive in
  deserts without drinking
• Kangaroo rat recovers 90 of water lost by
  using metabolic water

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Kangaroo rat
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Despite adaptations, water loss in land animals
still a problem

• Water lost from
• Moist surfaces of gas exchange organs
• urine

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Excretory systems

• Central to homeostasis
• Dispose of metabolic wastes
• Respond to imbalances in body fluids
• Excrete more or less of some particular ion
• Common theme to all solutions for water balance
  problem
• Regulation of solute movement
• if you regulate solute movement, you regulate
  water movement.

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3 main processes of Excretory Systems

• Filtration
• Blood exposed to filter made of selectively
  permeable membranes
• Retain proteins and other large molecules
• Pressure forces water and small solutes into the
  excretory system
• Salts
• Sugars
• Amino acids
• Nitrogenous wastes
• Called the FILTRATE

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3 main processes of Excretory Systems

• Reabsorption
• Active transport used
• Water and valuable solutes are moved from
  filtrate back to body fluids
• Required because filtration is not terribly
  selective
• Secretion
• Solutes are removed from the animals body and
  added to the filtrate
• Excess salts
• Toxins

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Excretory System Diversity

• Phylum platyhelminthes
• Protnephridium
• Tubular excretory system
• Closed tubules lacking internal openings
• Branch throughout body
• Capped by flame bulb
• Beating of cilia draws water and solutes from
  body fluid through flame bulb and to tubule
  system
• Urine from system empties into environment
  through pores.
• Excreted fluid is very dilute in freshwater
  flatworms
• Functions mainly in osmoregulation
• Nitrogenous Wastes diffuse through body surface

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Excretory System Diversity

• Phylum Annelida
• Metanephridia
• Tubular excretory system
• Internal openings collect fluids
• Ciliated funnel (nephrostome) collects fluid
• Each segment of warm has a pair
• Tubules immersed in coelomic fluid
• Wrapped in capillaries
• Both excretory and osmoregulatory functions

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Excretory System Diversity

• Insects
• Malpighian tubules
• Remove nitrogenous wastes and function in
  osmoregulation

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Excretory System Diversity

• Vertebrates
• Originated as segmentally arranged tubules
• Modern kidneys are compact and not segmentally
  arranged
• Contain lots of tubules and capillaries
• In osmoregulators, kidneys have both osmotic and
  excretory funciton

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Human Excretory System

• Unfiltered blood enters the kidney by the renal
  artery
• Filtered blood leaves by the renal vein
• Urine exits the kidney through a tube called the
  ureter
• Ureters drain into the urinary bladder
• Urine exits the bladder through the urethra

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Mammalian Kidney

• Two regions
• Renal cortex outer
• Renal medulla inner
• Both regions packed with nephrons
• Nephron
• the functional unit of the kidney
• Single long tubule
• ball of capillaries glomerulus
• Cup shaped swelling at beginning of tubule called
  Bowmans capsule
• Surrounds the glomerulus like a funnel

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Filtration of the blood

• Blood pressure forces water, urea, salts, etc.
  from blood in glomerulus into Bowmans capsule.
• Blood cells and larger molecules are left behind
  in capillaries
• NONselective with regard to SMALL molecules
• Good stuff AND wastes all go into FILTRATE

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After Bowmans Capsule

• Filtrate passes through regions of nephron
• FIRST proximal tubule
• Remove bicarbonate ions from filtrate and send
  back to body
• Put salt back into body tissues
• Nutrients back into body tissues
• Potassium into body tissues
• Take up hydrogen ions and ammonia from
  surrounding tissue to keep pH balance
• Toxins are secreted into the proximal tubule from
  surrounding tissue

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Proximal tubule and reabsorption of salt (NaCl)

• Salt diffuses out of proximal tubule
• Water follows with concentration gradient
• Gets water out of the tubule and back into body.

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The Loop of Henle

• A hairpin turn of the nephron
• Descending limb
• Nearest the proximal tubule
• Ascending limb
• Empties into collecting duct

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Descending limb of Loop of Henle

• Reabsorption of water
• Water leaves the descending loop and is reclaimed
  by body
• Permeable to water, but NOT salt
• PASSIVE transport water moves by osmosis
• Salt concentration is higher in tissues
  surrounding descending limb so water flows out by
  osmosis. NO ENERGY.
• WHY is salt concentration higher in surrounding
  tissues???

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Ascending limb of loop of Henle

• Permeable to SALT but NOT water
• Salt moves out of tubule by diffusion
• Concentration of salt is higher in the tubule
  than in surrounding tissues
• Salt became concentrated in the descending limb
  as water diffused OUT
• Now it diffuses out as fluid moves up the
  ascending limb
• In upper, thick portion of ascending limb salt is
  actively transported out
• By losing salt while retaining water, the
  filtrate becomes MORE DILUTE
• THIS LOSS OF SALT FROM THE NEPHRON CONTRIBUTES TO
  THE HYPERTONIC NATURE OF THE BODY FLUID IN THE
  MEDULLA OF THE KIDNEY.
• Makes the preceding loss of water from descending
  limb possible.

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Distal tubule

• Regulates K and salt concentration of body
  fluids
• Regulates pH

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Collecting Duct

• Carries filtrate back in direction of medulla and
  renal pelvis (to ureter)
• Actively removes salt from filtrate and sends
  back medulla tissue
• Filtrate is yet again more dilute
• Collecting duct is permeable to water, so MORE
  water flows out of duct as it passes into medulla
• Also permeable to urea at the bottom of the duct
• Creates (with the salt) the environment that
  pulls water out of tubule in the medulla regions
• High osmolarity enables kidney to conserve water
  by excreting urine that is MORE concentrated in
  solutes than the body fluids.

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