Homeostasis

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Homeostasis

• Biology 2 Form and Function

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Overview

• Homeostasis maintenance of constant internal
  environment
• Physiological controls
• Negative feedback loops
• Positive feedback loops
• Behavioral controls

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Osmoregulation

• Water is vital to the chemistry of life
• Therefore, must attain a water balance within the
  body
• Water balance systems are based on three
  processes
• Diffusion
• Osmosis
• Active transport
• Osmoregulation processes often tied to excretion

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• Diffusion
• The spread of molecules along a concentration
  gradient by brownian motion, towards a state of
  entropy
• Osmosis (the diffusion of water)
• Water moves from a low solute concentration
  (hypotonic) to a high solute concentration
  (hypertonic)
• Active transport
• The movement of molecules across a membrane,
  usually against a gradient, involving the
  expenditure of energy

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Osmoregulation in invertebrates

• Since most invertebrate phyla evolved in water,
  no shortage
• However, differences in concentration between the
  cell and the solution surrounding it may cause
  problems
• e.g., Amoeba in freshwater hypertonic cell in
  hypotonic solution
• Result movement of water into cell
• Defense Contractile vacuole pumps out water

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• Multicellular organisms use transport epithelia
  to control water loss and excretion
• Platyhelminthes
• Protonephridia (flame cells) collect excess water
  in addition to nitrogenous wastes, empty into
  nephridiopore, excretes NH3
• Annelida
• Metanephridia organized on a per segment basis
  collect waste from coelom via the nephrostome,
  counters water uptake by epidermis, excretes NH3
• Insecta
• Malpigian tubules collect nitrogenous wastes from
  haemocoel, excretes Uric Acid

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Osmoregulation in fish

• Depends on environment
• Freshwater
• Cells are hypertonic to environment, must defend
  against water uptake
• Excretion of dilute urine
• Mucous covering of epidermis
• Marine
• Cells are hypotonic to environment, must defend
  against water loss
• Water gain through food uptake and drinking
• Concentration of urine

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Filtration

• Occurs in Bowmans capule
• Afferent arteriole from renal artery enters
  glomerulus, exits via efferent arteriole
• Blood filtered by capsule all non-cellular
  products pushed into nephron (proximal tubule)
• Filtrate includes products that must be retained
  blood sugars, salts and vitamins

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Secretion

• Occurs in proximal and distal tubule
• Secretion is initially active, although certain
  molecular transport occurs passively as a result
• e.g., NaCl actively pumped out, H2O follows

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Reabsorption

• Materials that must be retained are brought back
  by active transport or passive diffusion
• Result of absorption/secretion in Loop of Henle
  is highly concentrated urine
• Nephron tubule is lined by transport epithelia
• Amount of water retained is controlled by
  hormones that control activity of transport
  epithelia

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The Loop of Henle

• Descending limb is permeable to water but not
  NaCl
• H2O moves by osmosis to high salt concentration
  in interstitial fluid
• Thin segment of ascending limb is permeable to
  NaCl which moves passively by diffusion to
  equalize gradient
• Thick segment of ascending limb actively
  transports NaCl

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