Page 1 - LN- EXCRETION

Excretion is the removal of nitrogenous waste products and other metabolites from the
animal body which is normally associated with the process of maintenance of osmibtic
concentrations, i.e., osmoregulation within the body.
Both excretion and osmoregulation are important for the maintenance of homeostasis, i.e.,
for keeping the internal environment of the body constant that is necessary for normal life
processes.
Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by
animals. These substances get accumulated in the animal body either by metabolic activities
or by other means like excess ingestion.

Topic 1 Excretion: Major Products, Human System and Uropoiesis

Types of Nitrogenous Excretion
Depending upon the nature of excretory product, animals exhibit different processes of
nitrogenous excretion.
These are described as follows
(i) Ammonotelism Ammonia is the most toxic form of nitrogenous waste, it requires large
amount of water for its elimination. The organism that excrete ammonia are called
ammonotelic and this , process to eliminate ammonia is known as ammonotelism.
Examples of ammonotelic animals are Many bony fishes, aquatic amphibians and aquatic
insects. Ammonia, as it is readily soluble, is generally excreted by diffusion across body
surfaces or through gill surfaces (in fish) as ammonium ions.
Kidneys does not play any significant role in its removal.
(ii) Ureotelism The process of excreting urea is called ureotelism. Animals, which does not
live in high abundance of ‘water convert ammonia produced in the body into urea (in the
liver) and release into the blood, which is filtered and excreted out by the kidneys.
Examples of ureotelic animals are Mammals, many terrestrial amphibians and marine fishes.
(iii) Uricotelism The process of excreting uric acid is called uricotelism. Uric acid, being the
least toxic nitrogenous waste can be removed with a minimum loss of water from the
animal body.
Thus, it is excreted in the form of pellet or paste (i.e., semi-solid form). Normally, the
animals which live in desert exhibit uricotelism.
Examples of uricotelic animals are Reptiles, birds, land snails and insects.
Note:
Some animals perform dual excretion, i.e., two modes of excretion. For example,
Earthworms excrete ammonia when sufficient water is available, while it excretes urea in
drier surroundings.
Other examples are lung fishes, Xenopus, crocodiles, etc.
Excretory Organs
Different animal groups have a variety of excretory structures (organs) to perform the
process of excretion. In most of the invertebrates, these structures are simple tubular form,
whereas, vertebrates have complex tubular organs called kidneys.
Some of these structures are mentioned below in the given table
Excretory Organs and Main Nitrogenous Wastes of Different Animal Croups