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Homeostasis. Biology 2: Form and Function. Overview. Homeostasis = maintenance of constant internal environment Physiological controls Negative feedback loops Positive feedback loops Behavioral controls. Osmoregulation. Water is vital to the chemistry of life

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Biology 2: Form and Function

  • Homeostasis = maintenance of constant internal environment
  • Physiological controls
    • Negative feedback loops
    • Positive feedback loops
  • Behavioral controls
  • 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
    • 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
osmoregulation in invertebrates
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
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
osmoregulation in fish
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
  • 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
  • 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
  • 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
the loop of henle
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