Excretion Systems in Fish

& Mammals

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Excretion Systems in Fish and

Mammals

DOT POINT

Identify the role of the kidney in the excretory system of fish

and mammals

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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.

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