This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Amount Of Oxygen Taken Into The Blood In Respiration does not bear any constant proportion to the amount of carbonic acid given off; but it is generally somewhat greater, and is always so when the period of observation is extended over twenty-four hours. Carbonic acid contains exactly its own volume of oxygen; and therefore if the oxygen taken in corresponded with the carbonic acid given off in each respiration, there would be just sufficient oxygen to account for the formation of the carbonic acid. But there is an additional amount of oxygen inhaled, rendering the volume of air expired smaller than that which is inspired; and this additional amount must be used for some other purpose than the formation of the carbonic acid escaping by the lungs. A small portion may be used in the formation of the carbonic acid which escapes by the skin, estimated at one-fiftieth of what is exhaled by the lungs; but experiments on the total respiration, both pulmonary and cutaneous, made by placing a man in an air-tight chamber and estimating the carbonic acid evolved, agree with those confined to the pulmonary in showing that the oxygen given off in twenty-four hours, in form of carbonic acid, is less than what is taken up; and we must therefore suppose that the excess of the oxygen is used in other processes of oxidation, converting the hydrogen of organic matters into water, and their sulphur and phosphorus into sulphuric and phosphoric acids. This is in keeping with the observation that the proportion of oxygen absorbed is greater in feeding on animal than on vegetable food; for the carbohydrates, it will be recollected, already contain as much oxygen as would combine with their hydrogen to form water, whereas oils and nitrogenous substances are comparatively deficient in oxygen.
* Eight ounces is the amount generally mentioned in text-books; but that means troy ounces formerly in use in matters medical.