Osmoregulation

• Osmoregulation

  The process of maintaining salt and water balance (osmotic balance) across membranes within the body

  
• Optimal function of an animal

  • Requires a well-defined, relatively constant composition of its body fluids, & substantial deviations are usually incompatible with life
• Problem of osmoregulation

  The concentration of body fluids invariably differ from those of the environment; animals must thus maintain appropriate concentration

  
• Animals minimize the difficulties of osmoregulation by

  • Decreasing their permeabilities
  • Decreasing the concentration gradients between body fluids & the environment
• Problems of keeping water & solute concentration constant vary with the environment & are entirely different in sea water, fresh water & on land
• Iso-osmotic (isosmotic)

  Most marine invertebrates have body fluids with the same osmotic pressure as the sea water
• Osmoconformer

  When there is a change in concentration of medium, the animal may change the osmotic concentration of its body fluids to conform with that of the medium

  
• Osmoregulator

  When there is a change in concentration of medium, the animal may maintain or regulate its osmotic concentration despite external concentration changes

  
• Hyperosmotic

  Fresh water animals have body fluids that are osmotically more concentrated than the medium

  
• Hypo-osmotic (hyposmotic)

  If an animal has a lower concentration than the medium, as does a teleost fish, it is said to be hypo-osmotic

  
• Concentrations of the various individual solutes in the body fluids of an animal usually differ substantially from those in the medium, even if the animal is isosmotic with the medium
• Ionic regulation

  The differences in solute concentrations between body fluids and the medium are carefully regulated, and occurs in all living organisms both in osmoregulators & in osmoconformers
• Euryhaline animals

  Some aquatic animals can tolerate wide variations in the salt concentration of the medium

  
• Stenohaline

  Others have a limited tolerance to variations in the concentration of the medium
• Problem of osmoregulation in the marine environment

  • Animals face problem of water loss because of hyperosmotic state of the environment
  • Exosmosis is unavoidable as water from gill membranes, skin, etc. will pass out of the body; these animals face osmotic dehydration
  • As far as the solutes are concerned, problem is exactly the opposite: concentration of Na+, K+, Cl-, etc. are higher outside than within the animal. Animal gains salts by diffusion through permeable surfaces

  
• How marine forms maintain body water constant

  • Reducing exosmosis (outward movement of H2O) through gills, skin
  • Drinking the medium to compensate for water loss
  • Renal retention of water & production of hyper-osmotic urine

  
• How marine forms maintain the concentration of various salts

  • Eliminating excess salts (taken in by passive diffusion, with food or during drinking)
  • Maintaining body fluids isosmotic to check passive diffusion of various salts (elasmobranchs)
  • Excreting concentrated urine containing large amounts of salts

  
• Problem of osmoregulation in the freshwater environment

  • Animals have their body fluids at a higher osmotic concentration than the environment; they tend to lose salts by diffusion & gain water by endosmosis
  • Salt loss & water gain occurs through intestinal, renal & respiratory membranes & these losses & gains must be compensated for by other regulatory mechanisms
  • Excess water gained by endosmosis is removed by the kidney by excretion of dilute urine that is usually hyposmotic to the blood, while diffusion of salts is checked by reducing permeable surfaces
  • The gills & kidney tubules are capable of gaining & retaining salts by active absorption
  • The animal also gains salts in the diet

  
• Problem of osmoregulation in the terrestrial environment

  • Animals tend to lose water (desiccation) because of high temperature rather than by osmotic loss & it does not involve a simultaneous gain of salts by diffusion
  • Animals obtain their salts & water predominantly through diet
  • To maintain a balance, animals have developed a variety of mechanisms
  • Unlike other environments, the osmoregulation problem on land is not fixed, & may arise from excess or scarcity of fluid constituents
  • Accordingly, various osmoregulation devices employed by land animals regulate body fluid concentration both when constituents are available in excess or when they are scarce

  
• How marine elasmobranchs maintain osmotic equilibrium of the body fluids

  • They maintain salt concentration of body fluids at ~ 1/3 the level in seawater, but they still maintain osmotic equilibrium by adding to body fluids large amounts of organic compounds, primarily urea, so total osmotic concentration of blood equals or slightly exceeds that of sea water
  • Blood urea concentration in marine elasmobranchs is more than 100X as high as in mammals
  • Urea is excreted by the mammalian kidney, but elasmobranch kidney actively reabsorbs it
  • Urea, however, is known to destabilize proteins & enzymes, so the inhibitory effect of urea is counteracted by trimethylamine oxide (TMAO) & by two other compounds (methylated amines), betaine & sarcosine
  • Excess salts are actively excreted by the rectal glands

  
• Osmoregulation in freshwater elasmobranchs


  • Their blood concentration is lower than those of strictly marine forms; in particular, the urea is decreased to < 1/3 the value for marine sharks, although it remains far above the normal level for other vertebrates
  • Low level of solutes in blood reduces problems of osmotic regulation, for osmotic inflow of water is reduced & lower salt concentrations are easier to maintain
  • Low osmotic inflow of water leaves less water to be eliminated by the kidney. Since urine contains solutes, a low urine flow reduces the urinary salt losses

  
• Osmoregulation in marine teleosts


  • They maintain their osmotic concentration at about 1/4 to 1/3 the level in sea water
  • They are hyposmotic, so face exosmotic water loss & salt gain by diffusion through permeable surfaces of the body (e.g. gills)
  • To accommodate for osmotic loss of water, they drink sea water; but this results in ingestion & absorption of large amounts of salt
  • Excess salt is eliminated by the gills (functions in both osmotic regulation & gas exchange) by active transport
  • Specific acidophilic cells (chloride secreting cells) at the base of the gills are responsible for the salt transfer
  • Kidney plays a major role in excretion of divalent ions, magnesium & sulphate, which make up approx. one-tenth of the salts in sea water

  
• Osmoregulation in freshwater teleosts


  • Their osmotic concentration in blood (approx. 300 mOsm l-1) is much higher than surrounding freshwater
  • Major problem is osmotic water inflow, which can be as much as 30% of body weight (BW) taken in daily
  • They lose some salts by diffusion through exposed permeable surfaces, so need to compensate for salts lost by diffusion
  • They drink little or no water, and excess water is excreted as urine (very dilute: hypotonic) which may be produced in quantities up to 1/3 of BW per day
  • Although urine contains no more than 2-10 mmol l-1 solutes, the large urine volume causes a substantial loss of solutes that must be replaced, mainly by active transport in gills