| Urea (C N2 H4 O), | 512 | grains. | |
| Chloride of sodium (Na Cl), | 177 | grains.,, | |
| Phosphoric acid (H3 P O4), | 48 | grains.,, | |
| Sulphuric acid (H2 S O4), | 31 | .11 | grains.,, |
| Uric acid (C5 N4 H4 O3), | 8 | .53 | grains.,, |
The substances excreted consist mainly of carbonic acid gas (C O2), which is expired by the lungs, and urea (C N2 H4 O), which is expelled by the urine.
These excretions, or expenditures, or waste products of the human body, present the carbohydrates—starch, sugars, and fats—and the proteids—meats and albumen—taken into the system as food.
The daily average loss by the expenditure or waste products of the body is estimated to be about:
| Carbon, | 4,500 | grains. |
| Nitrogen, | 3 to 500 | grains. |
| Besides salts and water. |
Of all the elements of the income and outcome, the nitrogen, the carbon, and the free oxygen of respiration, are by far the most important. Since water is of use to the body for merely mechanical purposes, and not as food in the strict sense of the word, the hydrogen element becomes a dubious one; the sulphur of the proteids, and phosphorus of the fats, are insignificant in amount; while the saline matters stand on a wholly different footing from the other parts of the food, inasmuch as they are not sources of energy, and pass through the body with comparatively little change.
The correct income will consist of so much nitrogen, carbon, hydrogen, oxygen, sulphur, phosphorus, saline matters, and water, contained in the proteids, fats, carbohydrates, salts, and water of the food, together with the oxygen absorbed by the lungs, skin, and alimentary canal.
The outcome will consist of: 1. The respiratory products of the lungs, skin, and alimentary canal, consisting chiefly of carbonic acid and water, with small quantities of hydrogen and carburetted hydrogen, these two latter coming exclusively from the alimentary canal; 2. Perspiration, consisting chiefly of water and salts, with urea by the skin, and other organic constituents of sweat amounting to very little; 3. The urine, which contains practically all the nitrogen really excreted by the body, as well as a large quantity of saline matter and water.