A lecture for Biology/Animal Science students on osmoregulation in vertebrates

1. Osmoregulat
ion
Emma Versteegh

2. Learning goals
By the end of this lecture, you…
• understand how osmoregulation
works in animals in different
environments (osmoconformers,
hyperosmotic and hypoosmotic
regulators)
• can argue which type of nitrogenous
waste is excreted by an animal,
related to:
• osmoregulation and environment
• type of excretory organ

3. This lecture
• Some concepts
• How marine invertebrates maintain salt and water balance
• Salt and water balance in freshwater and marine fishes
• How amniota maintain salt and water balance
• Nitrogen excretion
• Summary

4. Some concepts…
Diffusion: the net movement of molecules or atoms
from a region of high concentration to a region of low
concentration
Osmosis: diffusion of water through a semi-permeable
membrane
Homeostasis: Balancing and rebalancing of
physiological processes that maintain stability and
restore the normal state when it has been disturbed
Osmoregulation: Maintaining internal solute
concentrations within a range that allows cellular
function to proceed

5. Osmolarity (osmotic concentration)
Osmolarity = # particles / L solution
• 1 M glucose = 1 OsM glucose
• 1 M NaCl = 2 OsM NaCl (Na+(aq) + Cl-(aq))
• 1 M MgCl2 = 3 OsM MgCl2 (Mg2+(aq) + 2
Cl-(aq))
Isoosmotic
hyperosmotic
hypoosmotic

6. Principles of osmoregulation
Most invertebrates and some vertebrates have the same
osmolarity as their environment and cannot osmoregulate:
osmoconformers
Most vertebrates are osmoregulators
- Costs energy
+ Independence of environment
Trade-off: physiological optimisation vs. energy
expenditure

7. How marine invertebrates
maintain salt and water balance
Fully marine
Osmotic equilibrium with
seawater
Body surfaces permeable
to salts and water
osmoconformer
stenohaline
(Gr. stenos, narrow, hals,
salt)
Coasts and estuaries
Concentration of salts in
body fluids falls less
rapidly than fall in
seawater
hyperosmotic regulator
euryhaline
(Gr. eurys, broad, + hals,
salt)

8. How marine invertebrates
maintain salt and water balance
Hyperosmotic regulator - 2 problems:
• Water flows in (across gills)
• solved by kidneys (antennal glands)
• excrete dilute (hypotonic) urine
• Salt loss (gills & urine)
• salt-secreting cells in gills move ions
from seawater into blood
• active transport Eriocheir sinensis – Chinese mitten crab

9. Salt and water balance in freshwater fishes
Freshwater much more dilute than estuaries; no salty sanctuary
• Water enters osmotically
• Water pumped out by kidney; very dilute (hypoosmotic) urine;
reabsorbs NaCl
• Salt lost by diffusion
• Salt-absorbing cells in gills transport salt ions from water to
blood
• Salt in food
• Efficient hyperosmotic regulator
• Efficient mechanisms; small part of total energy expenditure

10. 300 mOsM
Freshwater
> 5 mOsM
hyperosmotic regulator

11. Amphibians

12. Salt and water balance in marine fishes
• Descendants of earlier freshwater fishes: ionic concentration in body fluid = 1/3
seawater
• Lose water
• Drink seawater, is absorbed from intestine
• Gain salt
• Specialised salt-secreting cells in gills transport NaCl back into sea
• Mg2+, SO4
2-, Ca2+ remaining in intestine are voided with faeces or excreted by
kidneys, isoosmotic urine
• Hyposmotic regulator

13. hyposmotic regulator
± 350 mOsM
Seawater
1000 mOsM

14. isoosmotic
salts + urea +
TMAO
Kidney
conserves urea
moconformer & ion regulator
CARTILAGINOUS FISH
Slight influx
of salt
Rectal gland
excretes NaCl
No
drinking

15. How amniota maintain salt and water balance
Lose water by:
• evaporation from respiratory and
body surfaces
• excretion in urine
• ectotherm: isoosmotic
• endotherm: hyperosmotic (loop of
Henle)
• elimination in faeces
Replace losses by:
• consuming water in food
• drinking water
• retaining metabolic water

16. Species differences in
maintaining water
balance
Kangaroo rat
• No sweating or panting, pale shiny
coat
• Nocturnal
• Very long loop of Henle for strongly
hyperosmotic urine
• Retains metabolic water, no drinking
Human
• Possibility of sweating and panting
• Cooling through evaporation (much

17. Seawater
1000 mOsM
Body fluids
± 300 mOsM
Marine mammals
Hyperosmotic urine
1200-1500 mOsm
No drinking

18. Nitrogen excretion
• Nitrogenous waste from breakdown
of proteins and nucleic acids
• Ammonia is toxic and soluble in
water
• Liver processes ammonia to less toxic
urea or uric acid
• Ammonotelism, ureotelism,
uricotelism
Often the same organs are used for
excretion and osmoregulation –
functional coupling

19. Salt excretion in marine
birds and reptiles
• Solution for excreting salt eaten with
food
• Salt gland excretes highly
concentrated solution of NaCl
• Important as kidneys cannot produce
concentrated urine
Galápagos marine iguana

20. Seawater
1000 mOsM
Body fluids
± 300 mOsM
Salt gland:
hyperosmotic
NaCl
Isotoosmotic urine 300 m
(slightly hyperosmotic
in some birds)
Drinks
seawater
Marine reptiles & birds

21. Freshwater fish
Strongly
hypoosmotic
Does not drink water
Kidneys reabsorb NaCl
NH3
(some urea)
Marine fish Isoosmotic
Drinks seawater
Salt-secreting cells in gills
NH3
(some urea)
Cartilaginous
fish
Weakly
hypoosmotic
Kidneys reabsorb urea
TMAO counteracts disadvantageous
effects urea
Urea
Amphibian
Strongly
hypoosmotic
Skin absorbs Na+ from water Urea
Freshwater
reptile
Isoosmotic
NH3 (crocodile,
snake), Urea
(turtle)
Marine reptile Isoosmotic
Drinks seawater
Salt gland excretes excess salts
Urea
(some uric acid)
Marine bird
Weakly
hyperosmotic
Drinks seawater
Salt gland excretes excess salts
Uric acid
Terrestrial
reptile
Isoosmotic Little evaporation Uric acid
Terrestrial bird
Weakly
hyperosmotic
Drinks freshwater
Small loop of Henle
Uric acid
Marine mammal
Strongly
hyperosmotic
Does not drink seawater
Obtains water from metabolic processes
Loop of Henle
Urea
Terrestrial Drinks freshwater

22. Achieved?
Now you…
• understand how osmoregulation
works in animals in different
environments (osmoconformers,
hyperosmotic and hypoosmotic
regulators)
• can argue which type of nitrogenous
waste is excreted by an animal,
related to:
• osmoregulation and environment
• type of excretory organ
Thank
you!
Questions
?