Explain How do insects, reptiles, and birds
eliminate ammonia and how do mammals
eliminate ammonia
Apply Concepts How do kidneys help maintain
homeostasis while processing nitrogenous
Review In what form do (a) annelids and
mollusks, (b) insects and arachnids, (c) mammals
and land amphibians, and (d) reptiles and birds
excrete nitrogenous wastes
Relate Cause and Effect Explain how differing
water balance needs relate to an animals
conversion of ammonia to wither urea or uric
27.4 Excretion
The Ammonia Problem
Breakdown of proteins by cells releases a nitrogencontaining waste: ammonia
Ammonia is poisonous
Moderate concentrations of ammonia can kill most
 Elimination
of metabolic wastes, such as ammonia
Small animals in wet environments get rid of
ammonia by allowing it to diffuse out of their body
fluids across their skin
Most larger animals have excretory systems that
process ammonia and eliminate it from the body.
Animals that cannot dispose of ammonia
continuously have evolved ways to store
nitrogenous wastes until they can be eliminated.
Storing Nitrogenous Wastes
Ammonia itself cannot be stored in body fluids
because it is too toxic
Insects, reptiles, and birds convert ammonia into a
sticky white compound called uric acid
 Much
less toxic than ammonia and is less soluble in
Mammals and some amphibians convert ammonia
to urea
 Urea
is less toxic than ammonia
 Is highly soluble in water.
Maintaining Water Balance
Excretory systems are extremely important in
maintaining the proper balance of water in blood
and body tissues
 May
also excrete excess water or have to conserve
Separate wastes and excess water from blood to
form urine
 Pump
ions from salt to create osmotic gradients
 Water then “follows” those ions passively by osmosis
 Usually cannot excrete excess salt.
Freshwater Animals
Freshwater invertebrates lose ammonia to their
environment by simple diffusion across their skin.
Freshwater fishes and amphibians eliminate
ammonia by diffusion across the same gill
membranes they use for respiration.
Bodies of freshwater
animals, such as fishes,
contain a higher
concentration of salt
than the water they live
Water moves into their
bodies by osmosis
Salt diffuses out.
Freshwater fish excrete
lots of watery urine
Don't drink water
Actively pump salt in
across their gills.
Saltwater Animals
Typically release ammonia by diffusion across their
body surfaces or gill membranes.
Many marine invertebrates have body fluids with
water concentrations similar to that of the
seawater around them.
Many saltwater animals,
such as fishes, contain a
lower concentration of
salt than the water they
live in.
Lose water through
osmosis, and salt
diffuses in.
Saltwater fish conserve
water by producing very
little concentrated urine
Drink water
Actively pump salt out
across their gills.
Excretion in Terrestrial Animals
Land animals can lose large amounts of water from
respiratory membranes that must be kept moist
Must eliminate nitrogenous wastes in ways that
require disposing of water.
Terrestrial Invertebrates
Produce urine in nephridia
 Tubelike excretory
structures that filter body fluid.
Terrestrial Invertebrates
Body fluid enters the nephridia through
nephrostomes and becomes more concentrated as
it moves along the tubes
Urine leaves the body through excretory pores.
Terrestrial Invertebrates
Insects and arachnids convert
ammonia into uric acid
Malpighian tubules
 Absorb
uric acid from body
 Concentrate the wastes and
add them to digestive wastes.
Water is absorbed from
Crystals form a thick paste
which leaves the body
through the anus
Paste contains little water
 Minimizes
water loss.
Terrestrial Vertebrates
Mammals and land amphibians
convert ammonia into urea
Excreted in urine by the
Reptiles and birds convert ammonia into uric acid
Passed through ducts into a cavity that also
receives digestive wastes from
the gut
Walls of cavity absorb water
 Uric
acid separates as thick, milky-white paste
recognized as “bird droppings.”
Most vertebrate kidneys cannot excrete
concentrated salt
Most vertebrates cannot survive by drinking
All that extra salt would overwhelm the kidneys,
and the animal would die of dehydration.