Summary
Let us sum up the processes which the food undergoes in its conversion into condition to be absorbed by the body; in its absorption through the walls of the intestines and stomach; and the metabolic processes which it undergoes in being converted into heat and energy and again broken down and eliminated as waste.
The Saliva begins the digestion of starches and sugars in the mouth. This digestion is continued by the saliva in the stomach.
The Stomach, when in normal condition, thoroughly digests the proteins. If any proteins fail of digestion in the stomach the process is completed in the intestines.
The Intestines, aside from their work of digestion and absorption, excrete bile pigment, bile salts, animal acids, mucus and other decomposition of proteins, with bacterial fermentation and putrefactions; also such food materials as are not digested.
The small intestine digests and absorbs the fats and continues the digestion of starches, sugars and fats when this digestion is not completed in the stomach.
The large part of the food is absorbed through the small intestine, though a small part is absorbed through the walls of the stomach and through the large intestine.
Fats are almost entirely absorbed in the small intestine. They are absorbed through the lacteals and are carried into the blood stream.
The Liver. The proteins and the starches (converted into maltose) and sugars pass into the liver. The sugar (including the sugar in vegetables, milk, fruits and that used for sweetening, as well as the carbohydrates which have been changed into maltose), is converted into glycogen in the liver, stored here for a time and again broken down into sugar that it may be in condition to be absorbed into the blood.
The proteins pass through the liver but are not acted upon by this organ until they again return to the liver through the blood stream, after they have been partly oxidized in the tissues. The liver further oxidizes them putting them into condition to be excreted by the kidneys and intestines.
The liver also breaks up the worn out red corpuscles, putting them into condition to be eliminated in the bile.
It oxidizes and renders harmless poisonous substances absorbed in the food, such as fermented food products and alcohol.
The Muscles oxidize the fats and sugars liberating the latent heat and energy.
They partly oxidize proteins which are further broken up in the liver.
The Nerves oxidize food materials stored in the nerve cells, providing nervous energy.
The Lungs absorb oxygen and throw off carbon dioxid, watery vapor and some organic substances.
The Kidneys and The Skin purify the blood by excreting water, carbon dioxid and nitrogenous waste.