4. That not only the oxygen that disap pears from the inspired air is united chemi cally in the arterial blood, but also the carbonic acid formed during its circulation through the systemic capillaries enters into chemical combination with some one of the constituent parts of the venous blood ; that the combination thus formed is decomposed in the pulmonic capillaries by the agency of the absorbed oxygen, and the carbonic acid thus set free is evolved and escapes in the expired air.
The first view, viz. that the carbonic acid that appears in the expired air is formed in the lungs by the combination of part of the. oxy gen of the inspired air with the carbon of the venous blood, must now be regarded as unte nable. The existence of free gases in the blood, the evolution of carbonic acid from the blood at the lungs in animals made to breathe gases devoid of oxygen, the small increase of temperature the blood acquires in its change from the venous to the arterial condition*, and the result of observations made upon the blood out of the body, when subjected to alternate applications of oxy-gen and carbonic acid gas, are all opposed to the supposition that the formation of carbonic acid gas takes place to any great extent in the lungs. The existence of a quantity of free carbonic acid in the venous blood, more than sufficient to furnish the whole of this gas thrown off at the lungs, and the avowedly conjectural explana tion of the manner in which the carbonic acid is combined and the agency by which its com binations are decomposed in the lungs, given hy those who advocate this view, justify the adoption of the opinion that the carbonic acid gas evolved at the lungs exists in a free state in the venous blood before it reaches the lungs.
An interchange, therefore, takes place be tween the air in the cells of the lungs and the blood in the pulmonic capillaries, the latter receiving oxygen and giving up part of the free carbonic acid held by it in solution. These gases, from their solubility, readily per meate the thin moist membranes interposed between the blood and the atmospheric air contained in the cells of the lungs. We have already mentioned that Valentin and Brunner have concluded from their experiments that this interchange of oxygen and carbonic acid gas is regulated by the law of the diffusion of gases established by Graham ; but besides the objections that may be urged against this view, drawn from the considerable diversity in the relative proportions of these gases inter changed during respiration as ascertained by different experimenters, the conditions under which the two gases are placed in respiration are very different from those in the experi ments instituted by Graham.t In respiration the gases are separated by moist animal mem branes, and one of these, viz. the carbonic acid, is held in solution in a fluid subjected to an increased pressure caused by the action of the heart.* We are not, in the present state of our knowledge, in a condition to form any thing like an accurate estimate of the various cir cumstances which regulate this interchange between the oxygen of the air and the car bonic acid gas of the blood, but it is obvious that it will be affected in a most important manner by the relative proportion of these gases in the air contained in the air-cells of the lungs and in the blood, and by the quan tities of atmospheric air and blood trans mitted through the respiratory apparatus.
We have seen, from the experiments of Vierordt, that when the air is rapidly renewed in the lungs, though the percentage of car bonic acid in the expired air is diminished, yet the total amount of this gas thrown off' from the lungs within a given time is pro portionally increased ; while, on the other hand, when the respirations are diminished below the natural standard, though the per centage of carbonic acid in the expired air is increased, yet the total quantity thrown off from the lungs in a given time is propor tionally diminished. When the atmospheric air in the lungs is rapidly renewed by an in creased frequency of the respiratory move ment, the diffusion of the oxygen in the higher, and of the carbonic acid in the deeper, parts of the air tubes will proceed more rapidly, and the air in the deeper parts or in the air-cells will contain a less percentage of carbonic acid, and a greater percentage of oxygen, than when the respirations are carried on with the usual frequency and force. This diminution of the usual quantity of carbonic acid gas and increase of oxygen in the deeper parts of the lungs will accelerate the inter change between the oxygen of the air and the carbonic acid of the blood, provided the blood holds its normal amount of free gases in solu tion, and a larger quantity than usual of car bonic acid will be separated from the blood at the lungs, and carried out in the expired air. If, then, we add an increased flow of blood through the capillaries of the lungs to an in creased frequency of the respiratory move ments, as occurs in exercise, the interchange between the oxygen of the air and the free carbonic acid of the blood will be carried on with greater activity. When, on the other hand, the air is renewed in the lungs less fre quently than usual, as happens when the respiratory movements are diminished in number and in extent, the air in the deeper parts of the lungs will contain less oxygen and more carbonic acid than usual, and the interchange between the oxygen of the atmo spheric air and the free carbonic acid of the blood will proceed more slowly. When the respirations are reduced to about one half of their nortnal frequency, as occurs in the course of some diseases, and after division of the vagi nerves, the carbonic acid gas gradually accumulates in the blood, less oxygen is ab sorbed, and the individual generally sooner or later dies of asphyxia. When the quantity of carbonic acid gas in the air-cells reaches a certain amount, the evolution of this gas from the blood will cease; and when this is carried still farther, there will be an absorption of a part of the carbonic acid gas by the blood.