The most obvious change which the blood undergoes in its passage through the lungs is that of color, the dark venous blood being exchanged for the bright scarlet arterial blood. It gains oxygen, loses carbonic acid, becomes 1° to 2° F. warmer; it coagulates sooner and more firmly, and contains more fibrine.
The venous blood as it issues from the right ventricle is loaded with carbonic acid. The oxygen present is insufficient to the whole of the hæmoglobin of the red corpuscles; much reduced hæmoglobin is present, hence the purple color of venous blood. As the blood-vessels pass through the capillaries of the lungs, this reduced hæmoglobin takes from the pulmonary air its complement of oxygen, all or nearly all the hæmoglobin of the red corpuscles becomes oxy-hæmoglobin, and the purple color forthwith shifts into scarlet. The hæmoglobin of arterial blood is saturated or nearly saturated with oxygen. Passing from the left ventricle to the capillaries, some of the oxy-hæmoglobin gives up its oxygen to the tissues, becomes reduced hæmoglobin, and the blood in consequence becomes once more venous, with a purple hue. Thus the red corpuscles by virtue of their hæmoglobin are emphatically oxygen-carriers. Undergoing no intrinsic change in itself, the hæmoglobin combines in the lungs with oxygen, which it carries to the tissues; these, more greedy of the oxygen than itself, rob it of its charge, and the reduced hæmoglobin hurries back to the lung in venous blood for another portion. Hæmoglobin combines loosely with carbonic oxide just as it does with oxygen, but the affinity with the former is greater than with the latter. While carbonic oxide readily turns out oxygen, oxygen cannot so readily turn out carbonic acid. This property of carbonic oxide explains its poisonous nature.
Respiratory changes in the tissues. Arterial blood passing through the several tissues, becomes once more venous. A considerable quantity of the oxy-hæmoglobin becomes reduced, and a quantity of carbonic acid passes from the tissue into the blood. The blood which comes from a contracting muscle, is not only richer in carbonic acid, but also, though not to a corresponding amount, poorer in oxygen, than the blood which flows from a muscle at rest.
A muscle is always producing carbonic acid, and when it contracts there is a sudden and extensive increase of the normal production. Oxygen is necessary for the life of the muscle. When venous blood instead of arterial blood is sent through the blood-vessel of a muscle, the irritability speedily disappears, and unless fresh oxygen be administered the muscle soon dies.
Our knowledge of the respiratory changes in muscle is more complete than in the case of any other tissue; but we have no reason to suppose the phenomena of muscle are exceptional. On the contrary, all the available evidence goes to show that in all the tissues the oxidation takes place in the tissues and not in the adjoining blood. It is a remarkable fact, that lymph, serous fluid, bile, urine, and the other secretions contain no free or loosely combined oxygen, while the tension of carbonic acid in peritoneal fluid is as high as six per cent, and in bile and urine is still higher, etc.
All these facts point to the conclusion, that it is the tissues, and not the blood, which become primarily loaded with carbonic acid, the latter simply receiving the gas from the former by diffusion; and that the oxygen which passes from the blood into the tissues is at once taken up in the same combinations, so that it is no longer removable by diminished tension.
The production of carbonic acid in the muscle is not directly dependent on the consumption of oxygen. The muscles produce carbonic acid in an atmosphere of hydrogen. What is true of muscle is true also of other tissues and of the body at large.
Oxygen helps to wind up the vital clock; but once wound up, the clock will go on for a period without further winding (Pflüger).
To sum up, then, the result of respiration in its chemical aspect. As the blood passes through the lungs, the low oxygen tension of the venous blood permits the entrance of oxygen from the air of the pulmonary alveolus, through the thin alveolar wall, through the thin capillary sheath, through the thin layer of blood plasma, to the red corpuscles, and the reduced hæmoglobin of the venous blood becomes wholly, or all but wholly, oxy-hæmoglobin. Hurried to the tissues, the oxygen, at a comparatively high tension in the arterial blood, passes largely into the tissues, in which the oxygen tension is always kept at an exceedingly low pitch, by the fact that the tissues, in some way at present unknown to us, pack away, at every moment, into some stable combination each molecule of oxygen which they receive from the blood. With much, but not all, of its oxy-hæmoglobin reduced, the blood passes on as venous blood. How much hæmoglobin is reduced will depend on the activity of the tissue itself. The quantity of hæmoglobin in the blood is the measure of limit of the oxidizing power of the body at large; but within that limit the amount of oxidation is determined by the tissue, and by the tissue alone.
The skin is an excretory tissue, and consists principally of two layers, an external covering of epithelium, termed the cuticle or epidermis, and a layer of vascular tissue, named the corium derma or cutis vera. The integument serves (1) for the protection of deeper tissues, (2) as a sensitive organ in the exercise of touch, (3) as an excretory organ, (4) as an absorbing organ, (5) for regulating the temperature of the body. Within and beneath the corium are imbedded several organs with special functions, namely, sudoriferous or sweat glands, sebaceous or fat glands, and hair follicles; and on its surface are sensitive papillæ. The so-called appendages of the skin, the hair and nails, are modifications of the epidermis.