EXCRETORY SYSTEM

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Term Paper Title	EXCRETORY SYSTEM
# of Words	945
# of Pages (250 words per page double spaced)	3.78

EXCRETORY SYSTEM

     Metabolism can be divided into catabolism and anabolism.  During
catabolism, energy-containing compounds are degraded to produce energy
for the chemical, mechanical and electrical processes that occur in the
body.  The compounds degraded by catabolism contain only carbon,
hydrogen and nitrogen. Before being catabolized, more complicated
molecules are reduced in size and other atoms, such as sulfur,
phosphorus and nitrogen, are removed. Only part of the energy contained
in these compounds, usually less than 50%, is converted into useful
energy. The rest is lost as heat. During anabolism new molecules are
synthesized, such as proteins, carbohydrates, nucleic acids and lipids.
In a growing animal anabolism can be intense, but in an adult, anabolism
just replaces compounds that are degraded or used.  Catabolism must be
more intensive than anabolism because of the additional energy required
for mechanical and electrical processes, as well as the energy lost as
heat.  Metabolism produces a number of byproducts, some of which must be
excreted.  However, what is waste to one animal or organ, may be a
treasure to another.  Carbon dioxide, for example, is a byproduct of
metabolism that is produced during oxidative catabolism, and is excreted
in large amounts via the lungs and skin. However, it is also very
important component of the carbonate buffering system of blood and is
used in the synthesis of many compounds. Water is produced as the last
step in oxidative catabolism and is excreted in large amounts by most
animals, but in desert animals it is highly conserved. The water
produced by oxidative catabolism is called metabolic water, and for some
desert animals the only water they have available is metabolic water.
Nitrogen is usually excreted, and in s0ome forms (i.e. ammonia) is
toxic. But elasmobranch fish (i.e. sharks, rays) use nitrogen in the
form of urea to maintain the osmotic pressure of the blood isosmotic
with seawater. In cows, and other ruminents, urea is secreted into the
rumen (via the salivary glands) where it is used as a source of nitrogen
by the symbiotic bacteria and protozoa.  It is evident, therefore, that
what is excreted varies between animals, although the most common
excretory products are carbon dioxide and nitrogen. The topic of
excretion usually covers how animals deal with excess nitrogen. The body
is usually faced with an excess of amino acids derived from the diet and
turnover of cellular proteins. In carnivores, ...

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