minor cause of the darker color of the blood is the swelling of the corpuscles by absorption of carbonic acid.
The corpuscles are the oxygen-carriers, seizing the oxygen in the lungs and conveying it to the tissues, where it unites with the carbon
Fig. 12.—Bronchial Tube, with its Bronchules and Ultimate Ramifications(natural size).
and hydrogen. The corpuscles also convey carbonic acid to the lungs, but they divide this labor with the serum.
In large quantity, carbonic acid acts as a narcotic poison; for, on account of its superior attraction for the hæmoglobin, it replaces and excludes the oxygen. Other gases, as nitric oxide, have the same effect. Nitrogen, on the other hand, is entirely negative in its effect on the blood, and consequently serves to dilute the oxygen of the atmosphere, without injury to animal life.
More oxygen is inhaled than is exhaled as a component of carbonic acid. The extra amount doubtless unites with hydrogen to form a portion of the exhaled water, and to produce sulphuric and phosphoric acids.
The human lungs exhale, in twenty-four hours, about two pounds of carbonic acid. This is the product of the burning of nine ounces of carbon. As giving some idea of the forces within the body, it is interesting to know that the combustion of nine ounces of carbon liberates over six million foot-pounds of energy. This is equivalent to
