EXCRETION IN
ANIMALS:
Keeping in view the definition
of excretion, that it is the elimination of waste
metabolites several
products may be included in the list of excretory products.
Water:
Water may be considered
as waste due to its removal in hypo osmotic (Internal) environment. 
Salts:
Salts removed by animals
in hypertonic (External) environment are the excretory products for these
animals, e.g. Marine cartilaginous fishes.
Nitrogenous Wastes:
Otherwise,
overwhelmingly, nitrogenous waste metabolites constitute the excretory
products.
Formation of Nitrogenous Waste:
Primarily, in the
catabolism of animo-acids the amino group (---NH2) is released (deamination) or
transferred to another molecule for removal or reuse.
Amino group not reused
for recycling of amino acids is essentially dissolved in water and excreted to
avoid toxic rise in the plasma. Elevated levels of these wastes can cause
convulsions, coma and eventually death.
Major Nitrogenous Wates:
Mostly excess nitrogen is
excreted by animals as
1.
Ammonia
2.
Urea
3.
Uric acid
Minor Excretory products:
Lower quantities of
nitrogen are excreted in the form of other compounds such as creatinine,
creatine or trimethylamine oxide
Trace Waste Products:
Some nitrogenous wastes
are excreted in very small quantities such as
amino acids, purines and
pyrimidines.
Metabolism of purine and
pyrimidine bases produces significant amount of nitrogenous wastes of
1.
uric acid
2.
urea
3.
ammonia
4.
Hypoxanthine
5.
Xanthine
6.
Allantoin
Metabolic Wastes:
Generation of wastes is primarily done
at metabolic level and these are called
metabolic wastes.
Examples:
1.
Urea,
produced from the metabolism of amino acids;
2.
Creatinine,
produced from muscle creatine;
3.
Uric
acid, from nucleic acids;
4.
Bilirubin,
end products of hemoglobin breakdown
5.
Metabolites
of various hormones.
6.
Toxins
produced within the body
7.
Toxins
ingested into the body such as pesticides, drugs and food additives
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