Biology 2672a: Comparative Animal Physiology

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Biology 2672a Comparative Animal Physiology

• Osmoregulation in fishes

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Freshwater fish
Water
Inside 300 mOsm High Na Cl-
Outside lt5 mOsm Low Na Cl-
Salts
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Saltwater fish
Salts
Inside 300 mOsm Low Na Cl-
Outside 1000 mOsm High Na Cl-
Water
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Terrestrial fish
Inside Wet High Na Cl-
Outside Dry No Na Cl-
Salts
Water
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Osmoregulation

• Maintenance of water and salt balance in the body
• Why freshwater fishes dont explode, saltwater
  fishes dont dry up and people dont desiccate

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Osmolarity/Osmolality

• The amount of stuff in a solution
• 1 Mole of solutes 1 Osmole
• Cumulative 0.2 M of 5 things 1 Osmole
• Osmolality per kg of solvent
• Osmolarity per litre of solvent

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Osmotic pressure

• Solutes exert pressure that moves water from
  place to place
• Can be a source of hydrostatic pressure

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Osmosis

• Movement of water across a semi-permeable membrane

Net movement of water driven by osmotic pressure
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Osmosis and hydrostatic pressure
Osmotic pressure has caused bulging hydrostatic
pressure
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Osmoconformers and Osmoregulators
Internal Osmolarity (mOsm)
External Osmolarity (mOsm)
Fig. 26.3a,b
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Many different types and combos of osmoregulatory
strategies
Fig. 26.3c
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Strategy and Tolerance are not identical
Euryhaline
Stenohaline
Osmoconformer
Osmoregulator
Internal Osmolarity
External Osmolarity
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Internal Na
Internal Urea
Internal Osmolarity
External Osmolarity
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Inside Outside
Na 286 mM Cl- 246 mM Others 135 mM 667 mOsm
930 mOsm
Na 286 mM Cl- 246 mM Urea 351 mM Others 135
mM 1018 mOsm
From Table 26.5
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Ureo-osmoconformer
Internal Na
Internal Urea
Internal Osmolarity
External Osmolarity
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But Urea is Bad!

• Chaotropic
• Binds strongly to proteins, releasing water and
  disrupts tertiary structure

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Effects of solute concentration on enzyme function
Urea
Km
Concentration
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Trimethylamine oxide(TMAO)
CH3
N
H3C
CH3
O-
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Counteracting Solutes
Fig 26.10
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Inside Outside
Na 286 mM Cl- 246 mM Urea 351 mM TMAO 71
mM Others 64 mM 1018 mOsm
930 mOsm
From Table 26.5
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Ureo-Osmoconformation in sharks

• Urea is used to make up the osmotic gap between
  internal and external concentration
• Requires high protein diet for manufacturing Urea
• TMAO acts as a counteracting solute to preserve
  protein function in high concentrations of urea.
• Why would you soak shark prior to cooking it?

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The situation for a marine teleost
Fig 27.7b
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Gills as exchange organs

• CO2 O2
• Used to remove the salts that are ingested with
  food and water
• (and absorbed through gill surfaces)
• Major site for this in marine teleosts

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How many ions?

• Total daily flux estimated for intertidal
  Xiphister atropurpureus in seawater
• 10-40 g
• Na 110 mM/kg fish/day
• 0.25g for a 10 g fish (2.5 bw)
• Cl- 72 mM / kg fish/day
• 0.25 g
• Water 2480 ml/kg fish/day
• 24.8 g water for a 10 g fish (!)

Evans (1967) J. Exp. Biol. 47 525-534
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Chloride cells
Apical (Mucosa)
Water
Pavement cell
Baso-lateral (serosa)
Blood
Fig. 27.6
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Export of Chloride
Box 27.2
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Export of Chloride is driven by a Na gradient
Box 27.2
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Active removal of Cl- leads to an electrochemical
imbalance that drives Na out of blood via
paracellular channels
Box 27.2
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Chloride cell summary

• Transcellular transport of Cl-
• Driven by Na,K-ATPase (requires energy)
• Paracellular transport of Na
• Ionoregulation accounts for 3-5 of resting MR
  in marine teleosts

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The situation for a freshwater teleost
Fig. 27.7a
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Gills as exchange organs

• CO2 O2
• Used to take up salts from the environment
• Not much NaCl in freshwater, but gills process a
  huge volume

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Chloride cells again
Figs 27.3 27.4
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Exchange of CO2 wastes for NaCl
Fig. 26.2
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Na uptake
Note tight junction
Box 4.1 Fig.A(2)
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Cl- uptake
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NaCl uptake summary

• Exchange for CO2
• Na via electrochemical gradient
• Cl- via HCO3- antiport
• Very dilute urine gets rid of excess water
  without losing too much salt

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Salt Water Fresh Water
Drinking Lots Little
Urine Little, concentrated Copious, dilute
Ion flux Passive into fish active out of fish
Na,K-ATPase Na into bloodstream
Tight junctions Yes
Cl- Transcellular transport driven by Na gradient Transcellular via HCO3- antiporter (driven by H pump)
Na Paracellular driven by electochemical gradient Transcellular driven by electrochemical gradient (set up by H pump and Na,K-ATPase)
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Reading for Thursday

• Water balance in terrestrial organisms
• pp 700-712