[Sidenote: Meaning of metabolism]
Metabolism is a word used to describe all processes that take place
within the body from the time food is absorbed from the digestive
organs until it is passed out of the body through some of the excretory
channels. To be more accurate, it means the sum of both the anabolic,
or constructive, and the catabolic, or destructive, processes that
continually go on in the animal body.
[Sidenote: Distinction between anabolism and catabolism]
The process of metabolism is chiefly one of tearing apart, or of
breaking down, complex chemical substances into simpler forms of matter.
Formerly, all processes in animal life were considered to be those
of tearing down, or of simplifying, chemical compounds; while plant
life was considered to be chiefly the process of building up complex
substances from simpler forms of matter. This distinction, however, is
rather general with many exceptions. The two terms, "anabolism" and
"catabolism" are sometimes used to distinguish between the processes of
building up complex chemical compounds, and the oxidizing or tearing
down of such compounds by effort or activity. Thus, the formation of
muscular tissue from the digested proteid materials would be a process
of anabolism, or construction, while the conversion of glucose in the
muscle-cells, into carbon dioxid and water would be an example of
catabolism, or destruction.
[Sidenote: Catabolism a process of oxidation]
The process of catabolism is, in general, one of oxidation; that
is, oxygen is added to the chemical compounds taken from the food
we eat, forming simpler substances which are excreted from the body
as waste-products. Oxidized carbon in the body forms carbon dioxid;
hydrogen is oxidized into the form of water, while nitrogen leaves the
body in the more complex and incompletely oxidized substance known
as urea, the chemical formula of which is COH₄N₂. A small portion of
nitrogen leaves the body in the form of uric acid, C₅H₄N₄O₃.
[Sidenote: Why muscular work produces warmth]
The process of anabolism usually absorbs energy or heat from the
surrounding material, while catabolism produces heat as a result of
oxidation, as do ordinary fuels. This explains why muscular work warms
the body.
We may study metabolism best by considering the two purposes food serves
in the animal body, as follows: