The term “respiration” has been extended until it is synonymous with “oxidation.” At one time it was supposed that the combination of oxygen with oxidizable substances occurred in the lungs. The lungs were the hearth of the body, to which the blood brought fuel which burned in the air drawn into them. When it was understood that the actual combination of combustible material with oxygen occurs, not in the lungs, but in the tissues, a somewhat illogical distinction was made between “external respiration”—the combination of oxygen and blood in the lungs—and “internal respiration”—the combination of oxygen and tissue-substances. The terms are not comparable. The taking up of oxygen by the hæmoglobin of blood is a different process to the union of oxygen, after the hæmoglobin has parted with it, with the carbon, hydrogen, and nitrogen of the tissue-substances.
The blood-stream carries both fuel and oxygen to the tissues, but the fuel while in the blood is not in an oxidizable condition. The foods are taken up by the tissues. They enter into combination with their protoplasm. Oxygen also combines with tissue-substances. In proportion as the tissues are active oxidized compounds are split off. They fall into the lymph, whence they are absorbed by the blood. If they are nitrogenous compounds, they are carried to the liver, formed into urea, and passed to the kidneys for elimination. If carbonic acid, it is carried to the lungs for exhalation. The water formed by combination of hydrogen and oxygen may escape from the lungs, the kidneys, or the skin.
Two or three pounds of mixed foods are consumed every day. By the blood they are carried to the tissues, whence an equivalent quantity of waste—that is to say, oxidized—material is removed. About 1½ pounds of oxygen is required to burn the day’s fuel.
The problems of respiration are twofold. In the first place we have to consider the physics and chemistry of the combination of hæmoglobin with oxygen, and of the elimination of carbonic acid from the blood in the lungs; secondly we have to explain the transference of oxygen from hæmoglobin to the tissues, and the reception in the blood of carbonic acid produced by the tissues.
The apparatus by which air is brought into relation with the blood consists of lungs and windpipe. At its upper end, where it joins the portion of the alimentary tract common to deglutition and respiration, the special respiratory tube is protected by the larynx. The nasal chambers belong to the respiratory tract; the gullet, or pharynx, is common to the two functions.