Biology 2672a: Comparative Animal

Physiology

Osmoregulation in fishes

Freshwater fish

Inside:

300 mOsm

High Na+ & Cl-

Outside:

<5 mOsm

Low Na+ & Cl-

Water

Salts

Saltwater fish

Inside:

300 mOsm

Low Na+ & Cl-

Outside:

1000 mOsm

High Na+ & Cl-

Salts

Water

Terrestrial fish

Inside:

Wet

High Na+ & Cl-

Outside:

Dry

No Na+ & Cl-

SaltsWater

OsmoregulationMaintenance of water and salt

balance in the bodyWhy freshwater fishes don’t

explode, saltwater fishes don’t dry up and people don’t desiccate

Osmolarity/OsmolalityThe amount of ‘stuff’ in a

solution1 Mole of solutes = 1 OsmoleCumulative: 0.2 M of 5 things = 1

OsmoleOsmolality – per kg of solventOsmolarity – per litre of solvent

Osmotic pressureSolutes exert pressure that

moves water from place to placeCan be a source of hydrostatic

pressure…

OsmosisMovement of water across a

semi-permeable membrane

Net movement of water driven by osmotic pressure

Osmosis and hydrostatic pressure

Osmotic pressure has caused bulging – hydrostatic pressure

Osmoconformers and Osmoregulators

External Osmolarity(mOsm)

Inte

rnal O

sm

ola

rity

(mO

sm

)

Fig. 26.3a,b

Many different types and combos of osmoregulatory strategies

Fig. 26.3c

Strategy and Tolerance are not identical

External Osmolarity

Inte

rnal

Osm

ola

rity Osmoregulator

Osmoconformer

Euryhaline

Stenohaline

External Osmolarity

Inte

rnal O

sm

ola

rity

Inte

rnal [N

a+

]In

tern

al [U

rea]

Inside OutsideNa+ 286 mMCl- 246 mMOthers 135 mM

667 mOsm

Na+ 286 mMCl- 246 mMUrea 351 mMOthers 135 mM

1018 mOsm93

0 m

Osm

From Table 26.5

Ureo-osmoconformer

External Osmolarity

Inte

rnal O

sm

ola

rity

Inte

rnal [N

a+

]In

tern

al [U

rea]

But Urea is Bad!Chaotropic

Binds strongly to proteins, releasing water and disrupts tertiary structure

Km

Concentration

Urea

Effects of solute concentration on enzyme function

Trimethylamine oxide(TMAO)

N+

CH3

H3C CH3

O-

Counteracting Solutes

Fig 26.10

Inside Outside

Na+ 286 mMCl- 246 mMUrea 351 mMTMAO 71 mMOthers 64 mM

1018 mOsm

930

mOsm

From Table 26.5

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?

The situation for a marine teleost

Fig 27.7b

Gills as exchange organsCO2 & 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

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

Chloride cellsWater

Blood

Apical(Mucosa)

Baso-lateral(serosa)

Pavement cell

Fig. 27.6

Export of Chloride

Box 27.2

Export of Chloride is driven by a Na+ gradient

Box 27.2

Active removal of Cl- leads to an electrochemical imbalance that drives Na+ out of blood via paracellular channels

Box 27.2

Chloride cell summaryTranscellular 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

The situation for a freshwater teleost

Fig. 27.7a

Gills as exchange organsCO2 & O2

Used to take up salts from the environment Not much NaCl in freshwater, but

gills process a huge volume

Chloride cells again

Figs 27.3 & 27.4

Exchange of CO2 wastes for NaCl

Fig. 26.2

Na+ uptake

Box 4.1 Fig.A(2)Note tight junction

Cl- uptake

NaCl uptake summaryExchange for CO2

Na+ via electrochemical gradient Cl- via HCO3

- antiport

Very dilute urine gets rid of excess water without losing too much salt

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)

Reading for ThursdayWater balance in

terrestrial organisms

pp 700-712