Maintaining a Balance
Topic 19: Excretion Systems in Fish and Mammals
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT POINT
 Identify the role of the kidney in the excretory system of fish
and mammals
Introduction
Water and solutes are continually exchanged between the
environment and a living organism’s body fluids. Water
accumulates within the body of an animal when it eats or
drinks, or as a bi-product of cellular metabolism. Nitrogenous
wastes accumulate in animals as a result of metabolism.
flickr.com
Introduction
The water potential of living cells is similar to that of sea
water, but lower than that of fresh water or the surrounding
air. Water potential is the tendency of a solution to lose water
by osmosis, typical of solutions that have a high water
concentration.
ocean.nationalgeographic.com
Introduction
The concentration of water in the immediate environment of
an organism determines its need to conserve (retain) water or
lose it. In aquatic animals such as fish, the concentration of
solutes in the surrounding aquatic environment has a direct
influence on the direction of movement of water – whether it
will move into or out of the body of the fish.
australia.com
Freshwater Fish
Freshwater fish live in rivers and lakes, where the water
potential is high – these habitats contain very few dissolved
salts and water is therefore freely available.
legacy.owensboro.kctcs.edu
Freshwater Fish
Freshwater fish urinate frequently, as water tends to
accumulate in their tissues as a result of passive movement by
osmosis from a higher concentration in the surroundings to a
lower concentration in the animal.
bio-diversity-nevis.org
Freshwater Fish
These fish are faced with the
problem of too much water
being present in their bodies.
The kidneys in freshwater fish
therefore excrete excess water
(gained from their
surroundings), as well as
nitrogenous wastes
(ammonia). Their kidneys are
structurally suited to this role
by having large glomeruli for
the filtration of blood in large
volumes.
biologicalexceptions.blogspot.com
Freshwater Fish
Their kidneys are not involved
in salt balance, since these fish
do not face the problem of salt
accumulation from their
freshwater environment. Any
excess salts that they consume
in their diets are excreted via
the gills.
todayifoundout.com
Marine Fish
Marine fish urinate less. They tend to lose body water by
osmosis, across the body surface and gills, into their salty
surroundings. Excess salt tends to accumulate in their bodies,
moving in by diffusion from the surrounding sea water.
scuba-equipment-usa.com
Marine Fish
The main function of the
kidneys in these fish is
therefore to remove excess
salt. Marine fish tend to drink
sea water, extract the salt from
it and then use the water for
metabolism. They excrete the
extracted salt to keep salt
levels in the body to a
minimum.
legacy.owensboro.kctcs.edu
Marine Fish
Their kidneys tend to play a role in conserving water rather
than excreting it. To meet this need, their kidneys tend to have
small glomeruli as well as a mechanism for removing excess
salt taken in with sea water. The kidney is also responsible for
excreting nitrogenous wastes (usually in the form of urea) in
marine fish.
tutorvista.com
Terrestrial Mammals
Terrestrial mammals lose water and solutes from the body as a
result of evaporation from the lung surface during respiration
and as a result of excretion. For example, the production of
sweat and urine.
blog.kimblechartingsolutions.com
Terrestrial Mammals
The kidneys of mammals excrete
urine that is composed mainly of
water and nitrogenous wastes
(urea), as well as some excess
salts. The mammalian kidney
can adjust the reabsorption of
nitrogenous wastes, water and
salts, varying the concentration
of urine produced.
beastlair.com
Terrestrial Mammals
Mammals have a complex
control mechanism to ensure
that a balance is maintained
between the amounts of sweat
and urine excreted. In hot
weather, more water is
excreted as sweat and as a
result less urine is produced.
In cold weather, more water is
lost in urine and very little in
sweat.
en.wikipedia.org
Terrestrial Mammals
A relatively large quantity of
salts is also lost during
sweating and needs to be
replaced to maintain a stable
osmotic pressure within body
fluids and cells in an
organism. Any adjustment to
water and salt levels in urine
is brought about by the action
of the hormones ADH and
aldosterone on the kidney
tubules.
winergy.com
Activity
-Students to complete activity 3.11 Urine concentrations of
terrestrial mammals, marine fish and freshwater fish.