how does Osmotic pressure/salinity tend to change
by location
- External environment (prone to wide fluctuation)
- Intracellular environment (allows for no variation; HOMEOSTASIS)
- Extracellular environment maintains balance between the two (blood, lymph fluid etc.)
what is homeostasis
Maintaining a steady state equilibrium in the internal environment of an organism
- Much of homeostasis is involuntary by action of hormones, enzymes and osmoregulatory processes
- Although occasionally fish do just “pick up and move” if environmental conditions are unfavourable
what is osmoregulation
the active regulation of the osmotic pressure of an organism’s fluids to maintain the homeostasis of the organism’s water content
what’s the problem with homeostasis in fish
- Homeostasis requires the concentrations of internal water and solutes to be maintained within fairly narrow limits, however…
- Physiological systems of fishes operate in an internal fluid environment that may not match their external fluid environment
what is Molarity
the amount of substance (1 mol) per volume (1 litre) of solution
what is Molality
the amount of substance (1 mol) per weight (1 kg) of solution
what is osmosis
the movement of water across a semi-permeable membrane as a result of varying concentrations of dissociated molecules (salts, proteins, ions)
what determines wether water will move across the membrane or not
osmolarity (osmotic pressure)
- Greater osmolarity = lower osmotic pressure
-Something very salty = low osmotic pressure
-Pure water = high osmotic pressure
- Water flows from high to low pressure
what is Isosmotic
2 solutions that exert the same osmotic pressure
- Intracellular osmolarity = External osmolarity
- No water lost or gained: OSMOCONFORMER
- Many marine invertebrates
what is Hyperosmotic
a solution that exerts a lower osmotic pressure and so attracts water
- Intracellular osmolarity > External osmolarity (saltier than water around you)
- Tissues gain water: OSMOREGULATOR
- Freshwater fish
what is Hyposmotic
a solution that exerts a greater osmotic pressure and so loses water
- Intracellular osmolarity < External osmolarity (less salty than water around you)
- Tissues lose water: OSMOREGULATOR
- Marine teleosts
what are the chief organs of excretion/osmoregulation
gills
**Kidneys first evolved as osmoregulatory organs in fishes to remove water
4 osmoregulatory functions in fish
- Isosmotic (nearly isoionic; osmoconformers)
- Hyperosmotic with regulation of specific ions – has strategy to be hypersmotic to live in that environment (elasmobranchs)
- Hyposmotic (marine fish)
- Hyperosmotic (freshwater fish)
explain Isosmotic function
Osmoconformers (no strategy)
- Hagfishes
- Internal salt concentration = seawater
- However, since they live IN the ocean….no regulation required!
- Only vertebrate that is isotonic to seawater - much like many marine invertebrates
explain Hyperosmotic with regulation of specific ions
- Elasmobranchs
- Internal salt concentration ~ 1/2 seawater (hyposmotic)
- BUT additional 1/2 of internal osmolarity made up of urea
- So total internal osmotic concentration is slightly greater than seawater (hyperosmotic)
- Gill membrane has low permeability to urea so it is retained within the fish
- Because internal inorganic and organic salt concentrations mimic that of their environment, passive water influx or efflux is minimized
explain Ionic and osmoregulation in marine elasmobranchs
- Body fluids = 1100 mOsm/kg
- External environment = 1000 mOsm/kg
- Slightly hyperosmotic – gain water
- Less energy required than freshwater fish to maintain this balance
- Maintain high solutes
- Slightly hyperosmotic body fluids by retaining some nitrogenous solutes - especially Urea, Betaine, Sarcosine and some amino acids (Taurine and β-alanine)
- Skin and gills of elasmobranchs impervious to urea (don’t excrete it)
- Monovalent ions enter the body and are excreted by the RECTAL GLAND (very similar function to chloride cells)
impact of urea in elasmobranchs
- Urea concentration 0.4 M in blood of elasmobranchs - 100x concentration that most vertebrates would die from!!!!
- Some proteins and enzymes actually need a high urea concentration to function efficiently, others are resistant to effects of urea
- Trimethylamine oxide (TMAO) and other methylamine substances protect proteins from urea
- THE ENTIRE METABOLISM OF SHARKS IS ADAPTED TO THE PRESENCE OF UREA
- Freshwater species have less conc of salt ions + especially urea
explain Hyposmotic
- Marine teleosts
- Intracellular osmolarity < External osmolarity (Tissues lose water)
- Ionic conc. ~ 1/3 of seawater
- Drink copiously to gain water
- Drinking rates in marine fish = very high - But drinking saline water causes dehydration due to increased salt loading – need to excrude/excrete these salts
- Chloride cells eliminate Na+ and Cl-
- Kidneys eliminate Mg++ and SO4–
what’s the issue with fish drinking to gain water and what’s the solution
absorption in the gut is still against an osmotic gradient
- MECHANISM = Solute-linked water transport - water can be absorbed if linked to monovalent ions (single charge Na+, K+ and Cl-)
- Not all solutes are absorbed by the alimentary canal
- Absorbed water has a solute loading ½ seawater (½ osmolarity) - got rid of half the ions in the seawater
but still have an excess of monovalent ions in the fish
monovalent ions vs Divalent ions
- monovalent ions (single charge Na+, K+ and Cl-)
- divalent ions = double charged e.g. Mg++, Ca++
explain Ion exchange and osmoregulation in a marine teleost
- Body fluids = 400 mOsm/kg
- Sea environment = 1000 ~mOsm/kg
- fish loses water through skin and gills
- Fish drinks water to compensate for water loss – but full of salt
- Divalent ions get excreted in faeces
- Active excretion of monovalent ions via chloride cells (Solute-linked water transport)
explain the process that goes on in Saltwater teleosts
- Loss of water across gills – osmosis
- Salts diffuse into blood across gill epethelium
- Fish drinks seawater – comes with loads of salt ions but also divalent ions (magnesium + sulfate)
- A lot of divalent ions will pass through alimentary canal and get excreted as feaces (not part of Solute-linked transport process) - ones with a diffusion gradient will get passed through kidneys
- Salt ions removed actively by chloride cells (contain concentration gradient)
explain how Salt ions get removed actively by chloride cells in marine teleosts
- Carrier protein allows salt ions to enter the chloride cell (passive)
- Sodium-potassium pump (active) swaps the positive ions – pumps out Na, replaces with K
- Loads of K now is in the chloride cell + little outside
- K diffuses out of the cell passively (concentration + electrochemical gradient)
- Net negative charge is left in the cell (Cl) - leaves electrochemical gradient for chloride to diffuse out
- Abundance of Na outside chloride cell will diffuse back into seawater – higher conc of Na than seawater
explain Hyperosmotic
- Freshwater fish
- intracellular osmolarity > External osmolarity (Tissues gain water )
- Freshwater animals constantly take in water by osmosis from their environment
- They lose salts by diffusion and maintain water balance by excreting large amounts of dilute urine
- Salts lost by diffusion are replaced in foods and by active uptake across the gills
