Osmoregulation
• Diffusion and Osmosis
• Osmoregulation
• Osmoregulation in bony fish
Diffusion
• The movement of molecules from areas of high
concentration to an area of low concentration.
Osmosis
• The diffusion of molecules across a
semipermeable membrane.
Osmosis
Hypo-osmotic – more H20
Hyperosmotic – less H20
Isosmotic – same concentration
osmosis #1
Osmoregulation
• Osmoregulation = biological processes
involved in controlling the levels of water
and salt in the body fluids and cells.
Living in freshwater:
Freshwater is 100% H2O
Fish cells are 98.6% H2O
Freshwater fish are hyperosmotic (less water)
Problem:
1. lose salt
2. gains too much H2O by osmosis.
Solution:
1. does not drink
2. salt absorbed by gills
3. large volume of dilute urine
Living in the Ocean:
Seawater is 96.5% H2O
Fish cells are 98.6% H2O
Marine fish cells are hypo-osmotic (more H2O)
Problem:
1. Loses H2O by osmosis
2. gains too much salt.
Solution:
1. drink lots of water
2. secrete salt through gills
3. excrete small amount of concentrated salty urine
Living in saltwater:
Alternative #1 = Osmoconformers - organism that
allows internal salt concentration to change with the
salinity of the water (conform to the environment).
Cells are isosmotic. Total salts in body balance salts
in sea water (3.5% or 35 0/00 marine algae and
most invertebrates).
Alternative #2 = Osmoregulator - an organism that
controls internal salt concentration different from the
environment. In fish, mammals, birds and reptiles
blood is hypoosmotic so they tend to lose water.
(sharks save urea). May expend 5-30% of energy
maintaining osmotic balance.
• Cartilagenous fish (ex. sharks) can be
osmoconformers
Kidneys- The shark has two kidneys on either side of
the midline. The shark osmoregulates in a unique way
compared to most other vertebrates. The shark kidney
extracts urea from urine and returns the urea to the
blood, whereby it concentrates urea in the blood. In this
way the osmotic pressure of the sharks body fluids are
maintained as high as that of sea water. With this
system the shark does not lose water or gain salts
through osmosis.
Rectal glands- This is a tube-like extensions of the
rectum. This gland helps the kidney control the salt
(NaCl) concentration within the body. Excess salt is
released into the rectum for expulsion
Salinity Tolerance in Fish
Stenohaline Fish: tolerate a narrow range of salinities
Most marine and fresh water fish
Euryhaline Fish: tolerate a wide range of salinities
Diadromous (migrating) fish
Anadromous: fw-sw-fw (ex. salmon)
catadromous: sw-fw-sw (ex. eel)
Air breathing marine vertebrates:
Mammals and some birds use their kidneys to
remove salt from blood then release in urine.
Some birds and reptiles salt is removed by
glands under the eyes, salty tears.
Salt Excreting Glands
•
Birds, although they have loops of
Henle, cannot make a very
concentrated urine - their loops are
fairly short.
•
Marine birds and reptiles (which
cannot make a concentrated urine)
have evolved extrarenal routes of salt
excretion.
•
Birds use nasal glands that release salt
excretions into the nasal passages.
•
Sea turtles have modified tear ducts
that secrete salt into the orbit of the
eye.
Osmoregulation
Advantages
Osmoregulators can live in a wide variety of habitats:
marine, estuaries, freshwater, land.
Disadvantages
Osmoregulation is energetically costly, depending on
how different the animal’s internal osmolarity is from
the environment, how permeable the animal’s surfaces
are to water and ion movement, and how costly it is to
pump ions across membranes.
Osmoregulation in marine fish
Marine fish face two problems: they tend to lose
water and gain ions.
Osmoregulation in freshwater fish
Freshwater fish face two problems: they tend to
lose ions and gain water.
Salt Excreting Glands
• Birds, although they have loops of
Henle, cannot make a very
concentrated urine - their loops
are fairly short.
• Marine birds and reptiles (which
cannot make a concentrated urine)
have evolved extrarenal routes of
salt excretion.
• Birds use nasal glands that release
salt excretions into the nasal
passages.
• Sea turtles have modified tear
ducts that secrete salt into the
orbit of the eye.
Osmoconformers: some marine animals
Animals whose body fluids are isotonic to their
environment. They do not actively adjust the internal
osmolarity.
Osmoregulators: terrestrial animal, freshwater animals,
many marine animals
Animals whose body fluids are hypotonic. They will
gain water from the environment and must continuously
eliminate excess water.
Animals whose body fluids are hypertonic. They will
lose water to the environment and must continuously
take in excess water.
Osmoregulators must expend energy to maintain osmotic
balance (5% to 30% of metabolic rate).
Marine Animals:
1. Osmoconformers – most
marine invertebrates
May still need to regulate
internal composition of
specific ions. Usually slight,
but may be significant.
The external environment determines the mechanism of
maintaining water balance:
2. Osmoregulators – most marine vertebrates
a. Cartilagenous fish (ex, sharks)
b. Bony fish
Cartilagenous fish (ex. shark)
Maintain a lower osmolarity for salt
than seawater.
Salt diffuses through gills.
Excrete salt
a. kidneys excrete some salt
b. rectal glands (salt excretory glands) excrete NaCl.
Animal fluids still slightly hypertonic to seawater
Due to accumulation of urea and trimethlyamine oxide
(TMAO) in fluids.
Water slowly enters body by osmosis.
Excretory organs (kidneys) eliminate excess water as urine.
Bony fishes: ex. cod
Body fluids are hypotonic to
seawater.
Constant loss of water to
surroundings by osmosis.
Constant gain of salt by diffusion
and in food.
Compensate by drinking large
quantities of seawater and
pumping out excess salt through
gills and skin.
Chloride cells in gills pump
chloride ions out, sodium
follows passively.
Kidneys excrete only small
amounts of water.
Freshwater Animals: ex. perch
Body fluids are hypertonic to fresh
water.
Constantly taking water in by
osmosis.
Constantly losing salt by diffusion.
Compensate by excrete large
volumes of dilute urine and
regaining lost salts.
Chloride cells in gills pump
chloride ions in, sodium
follows passively.
Ingest salt in food.
Body fluid osmotic concentrations
Types of Osmoregulators
1600
Body Fluid, mOsm/Kg
1400
Hyperosmotic regulator
1200
Shore Crab
1000
Decorator crab
Isosmotic Line
800
Freshwater Fish
Osmoconformer
Marine Fish
600
Fiddler Crab
400
Hypo-osmotic regulator
200
0
0
200
400
600
800
1000
1200
External Medium, mOsm/Kg
1400
1600
Reptiles, Birds, Mammals
Amphibia
Fish FW
Fish SW
Sharks FW
Sharks SW
Invertebrates FW
Invertebrates SW
0
200
400
600
800 1000 1200
Body Fluid, mOsm/Kg
Ions
Urea & TMAO
• Freshwater animals must regulate their internal osmolality above
ambient levels.
• Marine inverts are often osmoconformers since they experience little
environmental change (they are isoosmotic to their environment).
Reptiles, Birds, Mammals
Amphibia
Fish FW
Fish SW
Sharks FW
Sharks SW
Invertebrates FW
Invertebrates SW
0
200
400
600
800 1000 1200
Body Fluid, mOsm/Kg
Ions
Urea & TMAO
• Marine sharks are isoosmotic but not isoionic.
• Marine teleost fish are hypoosmotic and hypoionic compared to
seawater.