Chapter 44 Osmoregulation and Excretion
Overview: A Balancing Act
• The physiological systems of animals operate within a fluid
• The relative concentrations of water and solutes must be
maintained within narrow limits, despite variations in the
animal’s external environment.
• 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.
• An organism maintains a physiological favorable environment by
osmoregulation, regulating solute balance and the gain and loss of
water and excretion, the removal of nitrogen-containing waste
products of metabolism.
Concept 44.1 Osmoregulation balances the uptake and loss of water
• 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
continuous net uptake of water, or shrivel and die if there is a
substantial net loss of water.
• 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 differ in osmotic pressure, or osmolarity (moles of
solute per liter of solution).
• The unit of measurement of osmolarity is milliosmoles per liter
(mosm/L). • 1 mosm/L is equivalent to a total solute concentration of
• The osmolarity of human blood is about 300 mosm/L,
while seawater has an osmolarity of about 1,000 mosm/L.
• 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.
• Water flows by osmosis from a hypoosmotic solution to a
Osmoregulators expend energy to control their internal osmolarity;
osmoconformers are isoosmotic with their surroundings.
• There are two basic solutions to the problem of balancing water gain
with water loss.
• One—available only to marine animals—is to be isoosmotic to
the surroundings as an osmoconformer.
• Although they do not compensate for changes in external
osmolarity, osmoconformers often live in water that has a
very stable composition and, hence, they have a very
constant internal osmolarity.
• In contrast, an osmoregulator is an animal that must control its
internal osmolarity because its body fluids are not isoosmotic with the
• 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
• 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
• 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 any salt concentration between freshwater and
2,000 mosm/L, twice that of seawater.
• Most marine invertebrates are osmoconformers.
• 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
• 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 than seawater.
• 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 surroundings.
• Salmon and other euryhaline fishes that migrate between seawater
and freshwater undergo dramatic and rapid changes in
• While in the ocean, salmon osmoregulate as other marine
fishes do, by drinking seawater and excreting excess salt from
• 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 dilute environment—the same as fishes that spend
their entire lives in fresh water.
• 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 water.
• 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 membrane protectant.
• 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 terrestrial vertebrates.
• Being nocturnal also reduces evaporative water loss.
• Despite these adaptations, most terrestrial animals lose considerable
water 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
• Some animals are so well adapted for minimizing water loss that they
can survive in deserts without drinking.
• For example, kangaroo rats lose so little water that they can
recover 90% of the loss from metabolic water and gain the
remaining 10% in 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
• In animals with an open circulatory system, this fluid is
• 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 body surface.
• 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 ocean.
• The counter-current system in these glands removes salt from
the blood, allowing these organisms to drink seawater during
their months at sea.
• 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
Concept 44.2 An animal’s nitrogenous wastes reflect its phylogeny
• 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
• 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 to produce.
• Animals that excrete nitrogenous wastes as ammonia need access to
lots of water.
• This is because ammonia is very soluble but can be tolerated
only at very low concentrations.
• Therefore, ammonia excretion is most common in aquatic
• Many invertebrates release ammonia across the whole body
• In fishes, most of the ammonia is lost as ammonium ions
(NH4+) at the gill epithelium. • Freshwater fishes are able to exchange NH4+ for Na+
from the environment, which helps maintain Na+
concentrations in body fluids.
• 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 sufficient water.
• 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
• This reduces the amount of water needed for nitrogen excretion
when releasing a concentrated solution of urea rather than a
dilute solution of ammonia.
• 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
• They switch largely to urea when they are land-dwelling
• 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 loss.
• 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 these alternatives.
• Soluble wastes can diffuse out of a shell-less amphibian egg
(ammonia) or be carried away by the mother’s blood in a
mammalian embryo (urea).
• 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
• 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
• 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
• 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 available.
• 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
• Because they use energy at high rates, endotherms eat more
food—and thus produce more nitrogenous wastes—per unit
volume than ectotherms.
• 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
• Although the problems of water balance on land or in salt water or
fresh water are very different, the solutions all depend on the
regulation of solute movements between internal fluids and the
• 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
Most excretory systems produce urine by refining a filtrate derived
from body fluids.
• 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
• The initial fluid collection usually involves filtration through
selectively permeable membranes consisting of a single
layer of transport epithelium. • Hydrostatic pressure forces water and small solutes into
the excretory system.
• 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
• 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.
• Apparently, the tubules reabsorb most solutes before the urine
exits the body.
• In these freshwater flatworms, the major function of the flame-
bulb system is osmoregulation, while most metabolic wastes
diffuse across the body surface or are excreted into the
• However, in some parasitic flatworms, protonephridia do
dispose of nitrogenous wastes. • Protonephridia are also found in rotifers, some annelids, larval
molluscs, and lancelets.
• Metanephridia, another tubular excretory system, consist of internal
openings that collect body fluids from the coelom through a ciliated
funnel, the nephrostome, and release the fluid to the outside through
• Each segment of an annelid worm has a pair of metanephridia.
• An earthworm’s metanephridia have both excretory and
• As urine moves along the tubule, the transport epithelium
bordering the lumen reabsorbs most solutes and returns them
to the blood in the capillaries.
• Nitrogenous wastes remain in the tubule and are dumped
• 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
• 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
• 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
• Like the excretory organs of most animal phyla, kidneys are
built of tubules.
• 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 were segmented.
• However, the kidneys of most vertebrates are compact,
nonsegmented organs containing numerous tubules arranged
in a highly organized manner.
• The vertebrate excretory system includes a dense network of
capillaries intimately associated with the tubules, along with
ducts and other structures that carry urine out of the tubules
and kidney 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.
• 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 o