This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
A man takes into his body daily several pounds of foods of various kinds, as meats, bread, vegetables, and water, yet he grows no heavier; it is, therefore, clear that his body must in every twenty-four hours return, on the average, to the outside world about as great a weight of matter as it receives from it. Even in childhood, while growth is taking place and the body becoming heavier, the gain is never nearly equal to the weight of the foods swallowed. The materials given off daily from the body to the external universe, and compensating more or less accurately for the receipts from the outside world, are its wastes, and are chiefly things which cannot be burned. Much of the food taken in can be, and is, oxidized to enable us to move and keep warm. When burned it is of no further use to us, and would only clog up the various organs, as the ashes and smoke of an engine would soon put its fire out if they were allowed to accumulate in the furnace. The chief wastes of the body are carbon dioxide gas, water, and a kind of ammonia called urea.