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Nitrogen excretion
Nitrogen is a component of many biological molecules in all living things—
amino acids, proteins, nucleic acids, etc. Proteins and other nitrogen-containing
molecules are regular and essential dietary components, and digestion and metabolism
of these compounds result in excess nitrogen that must be excreted. The
primary end product of the metabolic degradation of nitrogen-containing compounds
is ammonia, which in fish is normally excreted directly across the gills.
However, urea and perhaps other nitrogen-containing compounds, formed as direct
end products of nitrogen degradation or from ammonia, are also excreted by
fish under some circumstances. A series of hallmark studies by Homer Smith in
the early 1930s established the fundamental aspects of nitrogen excretion in teleost
and elasmobranch fishes. These studies have served as the foundation for the
steady progress in our understanding of nitrogen metabolism and excretion in fish
over the past 70 years.
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