Chapter 44 Osmoregulation and Excretion
Overview: A Balancing Act
• The physiological systems of animals operate within a fluid environment.
The relative concentrations of water and solutes must be maintained
within narrow limits, despite variations in the animal’s external
• Metabolism also poses the problem of disposal of wastes.
The breakdown of proteins and nucleic acids is problematic because
ammonia, the primary metabolic waste from breakdown of these
molecules, is very toxic.
• Aosmoregulation, regulating solute balance and the gain and loss of water
and excretion, the removal of nitrogen-containing waste products of
Concept 44.1 Osmoregulation balances the uptake and loss of water and
• All animals face the same central problem of osmoregulation.
Over time, the rates of water uptake and loss must balance.
Animal cells—which lack cell walls—swell and burst if there is a
substantial net loss of water., or shrivel and die if there is a
• Water enters and leaves cells by osmosis, the movement of water across a
selectively permeable membrane.
Osmosis occurs whenever two solutions separated by a membrane
solution).osmotic pressure, or osmolarity (moles of solute per liter of
Th(mosm/L). measurement of osmolarity is milliosmoles per liter
▪ 1 mosm/L is equivalent to a total solute concentration of 10−3 M.
▪ Tseawater has an osmolarity of about 1,000 mosm/L. while
• If two solutions separated by a selectively permeable membrane have the
same osmolarity, they are said to be isoosmotic.
• There is no net movement of water by osmosis between isoosmotic
solutions, although water molecules do cross at equal rates in both
When two solutions differ in osmolarity, the one with the greater
concentration of solutes is referred to as hyperosmotic, and the more
dilute solution is hypoosmotic.
Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Educati44-1Inc. Water flows by osmosis from a hypoosmotic solution to a
Osmoregulators expend energy to control their internal osmolarity;
osmoconformers are isoosmotic with their surroundings.
• Twater loss.o basic solutions to the problem of balancing water gain with
Onsurroundings as an osmoconformer.s—is to be isoosmotic to the
▪ Aosmolarity, osmoconformers often live in water that has a very
stable composition and, hence, they have a very constant internal
• Iosmolarity because its body fluids are not isoosmotic with the outsideal
An osmoregulator must discharge excess water if it lives in a
hypoosmotic environment or take in water to offset osmotic loss if it
inhabits a hyperosmotic environment.
Osmoregulation enables animals to live in environments that are
uninhabitable to osmoconformers, such as freshwater and terrestrial
It also enables many marine animals to maintain internal osmolarities
different from that of seawater.
• Whenever animals maintain an osmolarity difference between the body
and the external environment, osmoregulation has an energy cost.
Because diffusion tends to equalize concentrations in a system,
osmoregulators must expend energy to maintain the osmotic gradients
via active transport.
The energy costs depend mainly on how different an animal’s
osmolarity is from its surroundings, how easily water and solutes can
move across the animal’s surface, and how much membrane-transport
work is required to pump solutes.
Osmoregulation accounts for nearly 5% of the resting metabolic rate
of many marine and freshwater bony fishes.
• Most animals, whether osmoconformers or osmoregulators, cannot
tolerate substantial changes in external osmolarity and are said to be
In contrast, euryhaline animals—which include both some
osmoregulators and osmoconformers—can survive large fluctuations
in external osmolarity.
For example, various species of salmon migrate back and forth
between freshwater and marine environments.
The food fish, tilapia, is an extreme example, capable of adjusting to
that of seawater.ation between freshwater and 2,000 mosm/L, twice
• Most marine invertebrates are osmoconformers.
Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Educatio44-2nc. Their osmolarity is the same as seawater.
However, they differ considerably from seawater in their
concentrations of most specific solutes.
Thus, even an animal that conforms to the osmolarity of its
surroundings does regulate its internal composition.
• Marine vertebrates and some marine invertebrates are osmoregulators.
For most of these animals, the ocean is a strongly dehydrating
environment because it is much saltier than internal fluids, and water
is lost from their bodies by osmosis.
Marine bony fishes, such as cod, are hypoosmotic to seawater and
constantly lose water by osmosis and gain salt by diffusion and from
the food they eat.
The fishes balance water loss by drinking seawater and actively
transporting chloride ions out through their skin and gills.
▪ Sodium ions follow passively.
They produce very little urine.
• Marine sharks and most other cartilaginous fishes (chondrichthyans) use
a different osmoregulatory “strategy.”
Like bony fishes, salts diffuse into the body from seawater, and these
salts are removed by the kidneys, a special organ called the rectal
gland, or in feces.
Unlike bony fishes, marine sharks do not experience a continuous
osmotic loss because high concentrations of urea and trimethylamine
oxide (TMAO) in body fluids leads to an osmolarity slightly higher
▪ TMAO protects proteins from damage by urea.
Consequently, water slowly enters the shark’s body by osmosis and in
food, and is removed in urine.
• In contrast to marine organisms, freshwater animals are constantly
gaining water by osmosis and losing salts by diffusion.
This happens because the osmolarity of their internal fluids is much
higher than that of their surroundings.
However, the body fluids of most freshwater animals have lower
solute concentrations than those of marine animals, an adaptation to
their low-salinity freshwater habitat.
Many freshwater animals, including fish such as perch, maintain
water balance by excreting large amounts of very dilute urine, and
regaining lost salts in food and by active uptake of salts from their
• Salmon and other euryhaline fishes that migrate between seawater and
freshwater undergo dramatic and rapid changes in osmoregulatory status.
While in the ocean, salmon osmoregulate as other marine fishes do,
by drinking seawater and excreting excess salt from the gills.
When they migrate to fresh water, salmon cease drinking, begin to
produce lots of dilute urine, and their gills start taking up salt from the
Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education44-3c. dilute environment—the same as fishes that spend their entire lives in
• Dehydration dooms most animals, but some aquatic invertebrates living
in temporary ponds and films of water around soil particles can lose
almost all their body water and survive in a dormant state, called
anhydrobiosis, when their habitats dry up.
For example, tardigrades, or water bears, contain about 85% of their
weight in water when hydrated but can dehydrate to less than 2%
water and survive in an inactive state for a decade until revived by
• Anhydrobiotic animals must have adaptations that keep their cell
While the mechanism that tardigrades use is still under investigation,
researchers do know that anhydrobiotic nematodes contain large
amounts of sugars, especially the disaccharide trehalose.
Trehalose, a dimer of glucose, seems to protect cells by replacing
water associated with membranes and proteins.
Many insects that survive freezing in the winter also use trehalose as a
• The threat of desiccation is perhaps the largest regulatory problem
confronting terrestrial plants and animals.
Humans die if they lose about 12% of their body water.
Camels can withstand twice that level of dehydration.
• Adaptations that reduce water loss are key to survival on land.
Most terrestrial animals have body coverings that help prevent
These include waxy layers in insect exoskeletons, the shells of land
snails, and the multiple layers of dead, keratinized skin cells of most
Being nocturnal also reduces evaporative water loss.
• Dwater from moist surfaces in their gas exchange organs, in urine and
feces, and across the skin.
Land animals balance their water budgets by drinking and eating
moist foods and by using metabolic water from aerobic respiration.
• Ssurvive in deserts without drinking.minimizing water loss that they can
Fo90% of the loss from metabolic water and gain the remaining 10% inr
their diet of seeds.
These and many other desert animals do not drink.
Water balance and waste disposal depend on transport epithelia.
• The ultimate function of osmoregulation is to maintain the composition
of cellular cytoplasm, but most animals do this indirectly by managing
the composition of an internal body fluid that bathes the cells.
Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education44-4c. In animals with an open circulatory system, this fluid is hemolymph.
In vertebrates and other animals with a closed circulatory system, the
cells are bathed in an interstitial fluid that is controlled through the
composition of the blood.
The maintenance of fluid composition depends on specialized
structures ranging from cells that regulate solute movement to
complex organs such as the vertebrate kidney.
• In most animals, osmotic regulation and metabolic waste disposal depend
on the ability of a layer or layers of transport epithelium to move
specific solutes in controlled amounts in specific directions.
Some transport epithelia directly face the outside environment, while
others line channels connected to the outside by an opening on the
The cells of the epithelium are joined by impermeable tight junctions
that form a barrier at the tissue-environment barrier.
• In most animals, transport epithelia are arranged into complex tubular
networks with extensive surface area.
For example, the salt-secreting glands of some marine birds, such as
the albatross, secrete an excretory fluid that is much more salty than
The counter-current system in these glands removes salt from the
blood, allowing these organisms to drink seawater during their months
• The molecular structure of plasma membranes determines the kinds and
directions of solutes that move across the transport epithelium.
For example, the salt-excreting glands of the albatross remove excess
sodium chloride from the blood.
By contrast, transport epithelia in the gills of freshwater fishes
actively pump salts from the dilute water passing by the gill filaments
into the blood.
Transport epithelia in excretory organs often have the dual functions
of maintaining water balance and disposing of metabolic wastes.
habitat 44.2 An animal’s nitrogenous wastes reflect its phylogeny and
• Because most metabolic wastes must be dissolved in water when they are
removed from the body, the type and quantity of waste products may
have a large impact on water balance.
• Nitrogenous breakdown products of proteins and nucleic acids are among
the most important wastes in terms of their effect on osmoregulation.
During their breakdown, enzymes remove nitrogen in the form of
ammonia, a small and very toxic molecule.
Some animals excrete ammonia directly, but many species first
convert the ammonia to other compounds that are less toxic but costly
Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education44-5c. • Animals that excrete nitrogenous wastes as ammonia need access to lots
This is because ammonia is very soluble but can be tolerated only at
very low concentrations.
Therefore, ammonia excretion is most common in aquatic species.
Many invertebrates release ammonia across the whole body surface.
Inthe gill epithelium. ammonia is lost as ammonium ions (NH ) at 4
▪ Freshwater fishes are able to exchange NH +or N4 from the
fluids.ment, which helps maintain Na concentrations in body
• Ammonia excretion is much less suitable for land animals.
Because ammonia is so toxic, it can be transported and excreted only
in large volumes of very dilute solutions.
Most terrestrial animals and many marine organisms (which tend to
lose water to their environment by osmosis) do not have access to
• Instead, mammals, most adult amphibians, sharks, and some marine bony
fishes and turtles excrete mainly urea.
Urea is synthesized in the liver by combining ammonia with carbon
dioxide and is excreted by the kidneys.
• The main advantage of urea is its low toxicity, about 100,000 times less
than that of ammonia.
Urea can be transported and stored safely at high concentrations.
This reduces the amount of water needed for nitrogen excretion when
of ammonia. concentrated solution of urea rather than a dilute solution
• The main disadvantage of urea is that animals must expend energy to
produce it from ammonia.
In weighing the relative advantages of urea versus ammonia as the
form of nitrogenous waste, it makes sense that many amphibians
excrete mainly ammonia when they are aquatic tadpoles.
▪ They switch largely to urea when they are land-dwelling adults.
• Land snails, insects, birds, and many reptiles excrete uric acid as the
main nitrogenous waste.
Like urea, uric acid is relatively nontoxic.
But unlike either ammonia or urea, uric acid is largely insoluble in
water and can be excreted as a semisolid paste with very little water
While saving even more water than urea, it is even more energetically
expensive to produce.
• Uric acid and urea represent different adaptations for excreting
nitrogenous wastes with minimal water loss.
• Mode of reproduction appears to have been important in choosing among
Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education,44-6. Soluble wastes can diffuse out of a shell-less amphibian egg
(ammonia) or be carried away by the mother’s blood in a mammalian
However, the shelled eggs of birds and reptiles are not permeable to
liquids, which means that soluble nitrogenous wastes trapped within
the egg could accumulate to dangerous levels.
▪ Even urea is toxic at very high concentrations.
Uric acid precipitates out of solution and can be stored within the egg
as a harmless solid left behind when the animal hatches.
• The type of nitrogenous waste also depends on habitat.
For example, terrestrial turtles (which often live in dry areas) excrete
mainly uric acid, while aquatic turtles excrete both urea and ammonia.
In some species, individuals can change their nitrogenous wastes
when environmental conditions change.
▪ For example, certain tortoises that usually produce urea shift to
uric acid when temperature increases and water becomes less
• Excretion of nitrogenous wastes is a good illustration of how response to
the environment occurs on two levels.
Over generations, evolution determines the limits of physiological
responses for a species.
During their lives, individual organisms make adjustments within
these evolutionary constraints.
• The amount of nitrogenous waste produced is coupled to the energy
budget and depends on how much and what kind of food an animal eats.
Because they use energy at high rates, endotherms eat more food—
and thus produce more nitrogenous wastes—per unit volume than
Carnivores (which derive much of their energy from dietary proteins)
excrete more nitrogen than animals that obtain most of their energy
from lipids or carbohydrates.
Concept 44.3 Diverse excretory systems are variations on a tubular
• Awater are very different, the solutions all depend on the regulation of
solute movements between internal fluids and the external environment.
Much of this is handled by excretory systems, which are central to
homeostasis because they dispose of metabolic wastes and control
body fluid composition by adjusting the rates of loss of particular
body fluids.ory systems produce urine by refining a filtrate derived from
Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education44-7c. • While excretory systems are diverse, nearly all produce urine in a process
that involves several steps.
First, body fluid (blood, coelomic fluid, or hemolymph) is collected.
▪ The initial fluid collection usually involves filtration through
selectively permeable membranes consisting of a single layer of
▪ Hydrostatic pressure forces water and small solutes into the
This fluid is called the filtrate.
Filtration is largely nonselective.
▪ It is important to recover small molecules from the filtrate and
return them to the body fluids.
▪ Excretory systems use active transport to reabsorb valuable solutes
in a process of selective reabsorption.
▪ Nonessential solutes and wastes are left in the filtrate or added to it
by selective secretion, which also uses active transport.
The pumping of various solutes also adjusts the osmotic movement of
water into or out of the filtrate.
▪ The processed filtrate is excreted as urine.
• Flatworms have an excretory system called protonephridia, consisting
of a branching network of dead-end tubules.
These are capped by a flame bulb with a tuft of cilia that draws water
and solutes from the interstitial fluid, through the flame bulb, and into
the tubule system.
• The urine in the tubules exits through openings called nephridiopores.
Excreted urine is very dilute in freshwater flatworms.
Apthe body. the tubules reabsorb most solutes before the urine exits
Insystem is osmoregulation, while most metabolic wastes diffuse across
the body surface or are excreted into the gastrovascular cavity.
However, in some parasitic flatworms, protonephridia do dispose of
Protonephridia are also found in rotifers, some annelids, larval
molluscs, and lancelets.
• Mopenings that collect body fluids from the coelom through a ciliated
funnel, the nephrostome, and release the fluid to the outside through the
Each segment of an annelid worm has a pair of metanephridia.
• An earthworm’s metanephridia have both excretory and osmoregulatory
Asthe lumen reabsorbs most solutes and returns them to the blood in the
Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education,44-8. Nitrogenous wastes remain in the tubule and are dumped outside.
Because earthworms experience a net uptake of water from damp soil,
their metanephridia balance water influx by producing dilute urine.
• Insects and other terrestrial arthropods have organs called Malpighian
tubules that remove nitrogenous wastes and also function in
These open into the digestive system and dead-end at tips that are
immersed in the hemolymph.
• The transport epithelium lining the tubules secretes certain solutes,
including nitrogenous wastes, from the hemolymph into the lumen of the
Water follows the solutes into the tubule by osmosis, and the fluid
then passes back to the rectum, where most of the solutes are pumped
back into the hemolymph.
Water again follows the solutes, and the nitrogenous wastes, primarily
insoluble uric acid, are eliminated along with the feces.
▪ This system is highly effective in conserving water and is one of
several key adaptations contributing to the tremendous success of
insects on land.
• The kidneys of vertebrates usually function in both osmoregulation and
Like the excretory organs of most animal phyla, kidneys are built of
The osmoconforming hagfishes, which are not vertebrates but are
among the most primitive living chordates, have kidneys with
segmentally arranged excretory tubules.
▪ This suggests that the excretory segments of vertebrate ancestors
However, the kidneys of most vertebrates are compact, nonsegmented
organs containing numerous tubules arranged in a highly organized
The vertebrate excretory system includes a dense network of
other structures that carry urine out of the tubules and kidney ands and
eventually out of the body.
Concept 44.4 Nephrons and associated blood vessels are the functional
units of the mammalian kidney
• Mammals have a pair of bean-shaped kidneys.
Each kidney is supplied with blood by a renal artery and drained by
a renal vein.
In humans, the kidneys account for less than 1% of body weight, but
they receive about 20% of resting cardiac output.
• Urine exits each kidney through a duct called the ureter, and both ureters
drain through a common urinary bladder.
Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Educatio44-9nc. During urination, urine is expelled from the urinary bladder through a
tube called the urethra, which empties to the outside near the vagina
in females or through the penis in males.
Sphincter muscles near the junction of the urethra and the