Chapter 44: Maintaining a Constant Internal Environment Homeostasis

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Chapter 44Maintaining a Constant Internal
Environment (Homeostasis)
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Body Temperature

• Enzymes Rxn rates inc. 2-3 times with each 100 C
  temp. inc. (until denatured)
• Each species has an optimal temp. range for
  metabolic rxns to be efficient
• Thermoregulation ? Organisms maintain their body
  temp within optimal range (various methods)

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Heat Gain/Loss
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Heat Gain/Loss (HIGH to LOW)

• Conduction molecule to molecule (ie hot
  concrete and feet in summer)
• Convection heat transfer b/t object and H2O or
  air moving across it
• Radiation electromagnetic waves transferred as
  heat (ie suns rays)
• Evaporative Cooling lowers temp by releasing
  H2O as vapor (ie sweating)

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Ectotherms and Endotherms

• Ectotherms (cold-blooded) maintain a
  temperature close to external temp.
• Low metabolic requirements (little heat
  generated)
• Most fish, reptiles, amphibians
• Endotherms (warm-blooded) maintain a constant
  temp. that may vary significantly from external
  temp (species dependent)
• High metabolic rate (lots of heat!)
• Humans, other mammals, and birds

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Endotherms/Ectotherms
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Endothermic Advantages

• Higher temp allows for inc. metabolic rate
• More energy is generated
• Can perform more vigorous activities for more
  sustained periods
• Allows terrestrial life (more temp. fluctuations
  than H2O)
• Disadvantage Require more frequent meals for
  higher aerobic resp. rate

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Vasodilation and Vasoconstriction

• Vasodilation ? Blood vessels dilate (expand) in
  order to release more heat
• Vasoconstriction ? Blood vessels constrict in
  order to limit heat loss in the cold

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Behavioral Controls

• Basking in the sun to raise body temp
• Hibernation
• Migration to different climates (birds)
• Inc or dec metabolic rate in hot/cold temps
• Certain insects huddle to generate more heat

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Insulation

• Hair
• Feathers
• Blubber, fat
• Reduces the loss of heat
• Allows maintenance of higher body temps

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Insulation
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Thermoregulation in Humans
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Thermoregulation in Humans

• Heat receptors in skin
• Receptors send hot/cold signal to hypothalamus
  (brain)
• Hypothalamus regulates vasodilation and
  vasoconstriction

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Extreme Hot/Cold Environments

• Cryoprotectants ? Certain organisms (some frog
  eggs, arctic fish) have a biologically produced
  antifreeze
• Heat shock proteins ? Produced in response to
  heat. Bind to enzymes and other proteins to
  inhibit denaturization

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Hibernation

• Bears, squirrels go into a deep sleep during
  winter in order to avoid harsh conditions
• Very low energy demands
• Very low metabolic rate

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Hibernation
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Osmoregulation (Water Balance)

• Organisms must balance their water and solute
  concentrations
• Water uptake and loss must remain essentially
  equal
• Cells could swell or shrivel
• Water flows from high water potential (low
  solute) to low water potential (high solute)

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Osmoregulators/Osmoconformors

• Osmoregulators maintain a constant solute
  concentration different from that of ext.
  environment
• Freshwater, terrestrial, some marine
• Costs energy (active transport)
• Osmoconformers maintain solute concentration
  equal to that of surroundings
• Many marine invertebrates

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Osmoregulation
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Waste Disposal

• Elimination of toxic materials is needed to
  maintain homeostasis
• Nitrogenous wastes are very toxic to living cells
• Urea ? Many terrestrial organisms, lowest
  toxicity, high energy requirement (humans)
• Uric Acid ? Birds, insects reptiles, least water
  lost
• Ammonia ? Fish, aquatic organisms, most toxic

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Nitrogenous Waste
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Selective Reabsorption and Secretion

• Organisms will filter their wastes and reabsorb
  anything that may be of use
• Accomplished in tubules (present in human
  kidneys)
• May also secrete more waste materials into urine

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Malpighian Tubules
Remove nitrogenous wastes from open circulatory
system of insects
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Vertebrate Urinary System

• Kidneys
• Function in osmoregulation and reabsorption
• Contain a network of tubules
• Renal Artery ? Blood to kidney
• Renal Vein ? Blood from kidney
• Ureter ? Carries urine to bladder
• Bladder ? Stores urine
• Urethra ? Tube that exits the body

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Vertebrate Kidneys

• Two regions
• Renal cortex and renal medulla
• Contains millions of nephrons
• Microscopic tubules
• Glomerulus? Network of capillaries serving each
  nephron with a blood supply
• Bowmans Capsule ? End of tubule that surrounds
  the glomerulus

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Human Kidney
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Kidneys

• nephron and collecting duct are lined by a
  transport epithelium
• process filtrate to form urine
• reabsorb solutes and water
• sugar, vitamins, and other organic nutrients from
  the initial filtrate and about 99 of the water
• reduce 180 L of initial filtrate to about 1.5 L
  of urine to be voided

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Kidney Function

• Proximal Tubule ? NaCl and H2O reabsorption and
  pH regulation
• Descending Loop of Henle ? H2O reabsorption
• Ascending Loop of Henle ? NaCl reabsorption
• Distal Tubule ? K and NaCl balance, pH
  regulation, some H2O reabsorbed
• Collecting Duct ? NaCl reabsorption, H2O
  reabsorption
• As it moves through the kidney, urine becomes
  more concentrated with unusable waste

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Kidneys

• Kidneys give terrestrial vertebrates the ability
  to regulate their osmotic balance
• Without kidneys, life on land would not be
  possible