The blood has been found, occasionally, in cases of Diabetes Mellitus, to contain traces of sugar. The great discordance of the results obtained, may result, perhaps, from the sugar contained in the blood only for a short time after meals, and then being rapidly evacuated by the kidneys. In the jaundice the green coloring matter of the bile has been found mixed with the blood.
Other observations of morbid constituents of the blood are too indefinite to justify me in occupying space with them.
Color of the Blood.—In the living body, the blood in the veins and arteries is well known to differ essentially in color; in the former being of a dark purple-red, in the latter of a bright vermilion color. The change from the venal to the arterial state occurs during the passage of the blood through the capillary vessels of the lungs, where it is exposed to the action of an extensive surface of atmospheric air; while the arterial blood, in traveling the general capillary system of the body, assumes the red, dark condition in which it is carried back to the heart by the veins. Yet, although the vital properties of the blood depend essentially upon this change of color, we are not able to connect it with any alteration in the composition of the constituents of the blood, or even in their relative proportions.
Arterial and venous blood contains sensibly the same quantity of water, fibrine, globules, albumen and salts; and, by analysis, the composition of those bodies is found to be identical, no matter what kind of blood they may be derived from. To trace the difference of nature between arterial and venous blood, it is therefore necessary to study it under different points of view than its approximate or elementary composition. So far as we have yet explained it, the air which has been employed in respiration, is found to have undergone an important change of constitution; its volume is but slightly, if at all, altered; but a quantity of oxygen has disappeared, and is replaced by carbonic acid, in generally equal volume. Air which has been once respired is found to contain from three to four per cent. of carbonic acid, and if the same quantity of air be continually breathed, the animal dies with all the symptoms of narcotic poisoning. When the carbonic acid has accumulated to from eight to ten per cent., the action of the air in expiration is therefore to remove carbon from the blood. The quantity so taken from the system in twenty-four hours is very large, and makes up the principal portion of that element which we take in with our food; yet, such is the activity with which its assimilation provides, that no perceptible change in the solid elements of the blood can be perceived.
It was, at one time, a much disputed point, whether the carbon so separated from the system was directly excreted from the lungs, and carried off as it were, by contact with the oxygen of the air, or whether the oxygen was first absorbed by the blood and carried by the circulation to every portion of the body, where it combined with the carbon, which was there present in excess, and the carbonic acid so produced, being dissolved by the venous blood, was thrown off on arriving at the surface of the atmosphere, in the lungs. The progress of science has, however, finally decreed in favor of the latter view, to which the fullest confirmation has been given by the careful and elaborate researches of Magnus.
Gases in the Blood.—It was found that both arterial and venous blood contain dissolved quantities of gases, oxygen, nitrogen and carbonic acid, which amount to from one-tenth to one-twentieth the volume of the blood; the proportions of these two gases to each other are different in arterial and venous blood; the oxygen in arterial blood being about one-half of the carbonic acid, while in the venous blood it seldom amounts to more than one-fifth; the difference is greatest in young persons, and probably is proportional to their activity of nutrition.
The quantity of nitrogen appears to be the same in both kinds of blood, making from one-fifth to one-tenth of the gaseous mixture.
The physico-chemical conditions of respiration are simply explicable upon these results, by the principle of gaseous diffusion, the fine lining pulmonary membrane being permeable to gases. When the venous blood arrives at the surface of the lungs, a portion of the carbonic acid which it contains is evolved, and a quantity of oxygen gas absorbed in place of it; these two quantities are not necessarily equal at each moment, though ultimately they become so, and hence the volume of oxygen absorbed is generally, though not universally, equal to that of the carbonic acid given out. There appears, from the presence of nitrogen in equal quantity in both kinds of blood, to be an absorption and evolution of that gas, simply from physical laws, and independent of any application of it to the nutrition of the animal; hence the volume of nitrogen in air is sometimes increased, and at others diminished, by respiration, and a man evolves much nitrogen when respiring an atmosphere of oxygen and hydrogen, while it has been shown that the rate of nutrition of a man is proportionate to the quantity of nitrogen it receives as food, and that none of that principle is really assimilated from the air.
It is still by no means easy to decide upon the changes of color which occur in the blood during respiration; for this should appear connected, not merely with the presence of certain gases in the blood, but upon a true change in the composition of hematosine, which analysis cannot direct.
Stevens first attracted the attention upon the influence which saline bodies have upon the color of the blood. If dark, venous blood is put in contact with a solution of common salt, glauber salt, nitre, or carbonate of soda, it becomes as vermilion colored as if it had been truly arterialized; on the contrary, the presence of carbonic acid impedes this action, and gives to blood, so reddened by a salt, not in excess, the dark tint of venous blood.