1. Osmoregulation and Excretion
Chapter 44
Osmoregulation and Excretion

2. Overview: A Balancing Act
Physiological systems of animals operate in a fluid environment
Relative concentrations of water and solutes must be maintained within fairly narrow limits

3. Freshwater animals show adaptations that reduce water uptake and conserve solutes
Desert and marine animals face desiccating (dehydrating) environments that can quickly deplete body water

4. Osmoregulation regulates solute concentrations and balances the gain and loss of water
Excretion gets rid of metabolic wastes

5. Concept 44.2: An animal’s nitrogenous wastes reflect its phylogeny and habitat
The type and quantity of an animal’s waste products may greatly affect its water balance
Among the most important wastes are nitrogenous breakdown products of proteins and nucleic acids

6. This is dependent on their habitat!
Proteins
Nucleic acids
HOW these waste products are excreted depends on how much water an organism needs to maintain homeostasis
This is dependent on their habitat!
Amino acids
Nitrogenous bases
—NH2
Amino groups
(nitrogenous waste)
Most aquatic animals, including most bony fishes
Mammals, most amphibians, sharks, some bony fishes
Many reptiles (including birds), insects, land snails
Which organism consumes more energy to get rid of their waste?
Which organism needs more water to get rid of their waste?
More H2O needed to excrete
Ammonia
Urea
Uric acid
More energy needed to excrete

7. Forms of Nitrogenous Wastes: Ammonia
Animals that excrete nitrogenous wastes as ammonia need lots of water
So… they live in an environment where water is abundant.
They release ammonia across the whole body surface or through gills
Freshwater animals
Marine invertebrates

8. Forms of Nitrogenous Wastes: Urea
Animals that produce urea have a limited source of water
The liver of mammals and most adult amphibians converts ammonia to less toxic urea
The circulatory system carries urea to the kidneys, where it is excreted

9. If liver can’t get rid of waste, there is a buildup of uric acid in the blood.
It can crystallize in the joints and cause inflammation of the joints.
Condition is known as gout

10. Forms of Nitrogenous Wastes: Uric Acid
Animals that excrete uric acid have a critical problem with getting water some time in their lifetime
Insects, land snails, and many reptiles, including birds, mainly excrete uric acid
Lay eggs with limited supply of water inside the egg.
Note: eggshells are impermeable to water so it won’t lose any water, but...
Uric acid
NH3
NH3
Uric acid
Why reptiles/birds evolved to release uric acid instead of ammonia…
These organisms share the same orifice for their reproductive, excretory and digestive wastes. In other words, they use the same hole for their pee, poop, and eggs. This means that their eggs were exposed to their nitrogenous waste products. Reptiles/birds that evolved to release ammonia probably contaminated the eggs since the shells are permeable to ammonia. Reptiles/birds that evolved to excrete uric acid did not contaminate the eggs. Non-contaminated egg meant healthy viable offspring, which would survive, and reproduce. 
NH3
Uric acid
H2O
H2O
NH3
Uric acid
Contamination
No Contamination

11. Forms of Nitrogenous Wastes: Uric Acid
Uric acid is largely insoluble in water and can be secreted as a paste with little water loss

12. The Influence of Evolution and Environment on Nitrogenous Wastes
The kinds of nitrogenous wastes excreted depend on an animal’s evolutionary history and habitat
The amount of nitrogenous waste is coupled to the animal’s energy budget

13. The Influence of Evolution and Environment on Nitrogenous Wastes
Remember, there are 3 general wastes products
Special cases:
Lungfish: make ammonia when there’s a lot of water, urea when pond dries out
Aquatic mollusks make ammonia, terrestrial snails make urea
ammonia
urea
uric acid
Increasing amount of water

14. Concept 44.1: Osmoregulation balances the uptake and loss of water and solutes
Osmoregulation is based largely on controlled movement of solutes between internal fluids and the external environment
Cells require a balance between osmotic gain and loss of water
Various mechanisms of osmoregulation in different environments balance water uptake and loss
Terms to remember:
Osmotic pressure: solute “pull”
Osmotic potential: water concentration
Hyper- , hypo-, iso- tonic solutions

15. Osmotic Challenges
Osmoconformers, consisting only of some marine animals, are isoosmotic (isotonic) with their surroundings and do not regulate their osmolarity
Osmoregulators expend energy to control water uptake and loss in a hyperosmotic (hypertonic) or hypoosmotic (hypotonic) environment

16. Most animals are stenohaline; they cannot tolerate substantial changes in external osmolarity
Euryhaline animals can survive large fluctuations in external osmolarity

17. Marine Animals Most marine invertebrates are osmoconformers
Most marine vertebrates and some invertebrates are osmoregulators

18. Marine bony fishes are hypoosmotic to sea water
They are in a hypertonic environment
Constantly lose water (as if they were in a desert)
They lose water by osmosis and gain salt by diffusion and from food
They balance water loss by drinking seawater
Gain of water and
salt ions from food
and by drinking
seawater
Osmotic water loss
through gills and other parts
of body surface
[hypotonic]
[hypertonic]
Excretion of salt ions
and small amounts
of water in scanty
urine from kidneys
Excretion of
salt ions
from gills
Osmoregulation in a saltwater fish

19. Transport Epithelia
Transport epithelia are specialized cells that regulate solute movement
They are essential components of osmotic regulation and metabolic waste disposal
They are arranged in complex tubular networks
An example is in salt glands of marine birds, which remove excess sodium chloride from the blood
Nasal salt gland
(acts as a filter)
Nostril
with salt
secretions

20. Freshwater Animals
Freshwater animals constantly take in water from their hypoosmotic environment
Need: get rid of extra water, keep salts
They lose salts by diffusion and maintain water balance by excreting large amounts of dilute urine
Salts lost by diffusion are replaced by foods and uptake across the gills
Osmotic water gain
through gills and other parts
of body surface
Uptake of
water and some
ions in food
[hypotonic]
[hypertonic]
Uptake of
salt ions
by gills
Excretion of
large amounts of
water in dilute
urine from kidneys
Osmoregulation in a freshwater fish

21. Animals That Live in Temporary Waters
Some aquatic invertebrates in temporary ponds lose almost all their body water and survive in a dormant state
This adaptation is called anhydrobiosis (“life without water”)
Hydrated tardigrade
Dehydrated
tardigrade
100 µm

22. Land Animals
Water
balance in a
kangaroo rat
(2 mL/day)
balance in
a human
(2,500 mL/day)
gain
loss
Derived from
metabolism (1.8 mL)
Ingested
in food (0.2 mL)
metabolism (250 mL)
in food
(750 mL)
in liquid
(1,500 mL)
Evaporation (900 mL)
Feces (100 mL)
Urine
Evaporation (1.46 mL)
Feces (0.09 mL)
(0.45 mL)
Land animals manage water budgets by drinking and eating moist foods and using metabolic water

23. Desert animals get major water savings from simple anatomical features4 3
(L/100 kg body mass)
Water lost per day 2 1
Control group
(Unclipped fur)
Experimental group
(Clipped fur)