LESSON IV.

RESPIRATION.

1. THE blood, the general nature and properties of which have been described in the preceding Lesson, is the highly complex product, not of any one organ or constituent of the body, but of all. Many of its features are doubtless given to it by its intrinsic and proper structural elements, the corpuscles; but the general character of the blood is also profoundly affected by the circumstance that every other part of the body takes something from the blood and pours something into it. The blood may be compared to a river, the nature of the contents of which is largely determined by that of the head waters, and by that of the animals which swim in it; but which is also very much affected by the soil over which it flows, by the water-weeds which cover its banks, and by affluents from distant regions; by irrigation works which are supplied from it, and by drain-pipes which flow into it.

2. One of the most remarkable and important of the changes effected in the blood is that which results, in most parts of the body, from its simply passing through capillaries, or, in other words, through vessels the walls of which are thin enough to permit a free exchange between the blood and the fluids which permeate the adjacent tissues (Lesson II. § 1).

Thus, if blood be taken from the artery which supplies a limb, it will be found to have a bright scarlet colour; while blood drawn, at the same time, from the vein of the limb, will be of a purplish hue, so dark that it is com

monly called "black blood." And as this contrast is met with in the contents of the arteries and veins in general (except the pulmonary artery and veins), the scarlet blood is commonly known as arterial, and the black blood as

venous.

This conversion of arterial into venous blood takes place in most parts of the body, while life persists. Thus, if a limb be cut off and scarlet blood be forced into its arteries by a syringe, it will issue from the veins as black blood.

3. When specimens of venous and of arterial blood are subjected to chemical examination, the differences presented by their solid and fluid constituents are found to be very small and inconstant. As a rule, there is rather more water in arterial blood, and rather more fatty matter. But the gaseous contents of the two kinds of blood differ widely in the proportion which the carbonic acid gas bears to the oxygen; there being a smaller quantity of oxygen and a greater quantity of carbonic acid, in venous than in arterial blood.

And it may be experimentally demonstrated that this difference in their gaseous contents is the only essential difference between venous and arterial blood. For if venous blood be shaken up with oxygen, or even with air, it gains oxygen, loses carbonic acid, and takes on the colour and properties of arterial blood. Similarly, if arterial blood be treated with carbonic acid so as to be thoroughly saturated with that gas, it gains carbonic acid, loses oxygen, and acquires the true properties of venous blood; though, for a reason to be mentioned below, the change is not so complete in this case as in the former. The same result is attained, though more slowly, if the blood, in either case, be received into a bladder, and then placed in the carbonic acid, or oxygen gas; the thin moist animal membrane allowing the change to be effected with perfect ease, and offering no serious impediment to the passage of either gas.

4. The physico-chemical processes involved in the exchange of carbonic acid for oxygen when venous is converted into arterial blood, or the reverse, in the cases mentioned above, are not thoroughly understood, and are probably somewhat complex.

It is known (a) that gases, mechanically held by a fluid in a given proportion, tend to diffuse into any atmosphere to which they are exposed, until they occupy that atmosphere in corresponding proportions; and (6) that gases separated by a dry porous partition, or simply in contact, diffuse into one another with a rapidity which is inversely proportioned to the square roots of their densities. Á knowledge of these physical principles does, in a rough way, lead us to see how the gases contained in the blood may effect an exchange with those in the air, whether the blood be freely exposed, or enclosed in a membrane.

But the application of these principles gives no more than this sort of general insight. For, in the first place, when arterialization takes place through the walls of a bladder, or any other thin animal membrane, the matter is complicated by the circumstance that moisture dissolves carbonic acid far more freely than it will oxygen; hence a wet bladder has a very different action upon carbonic acid from that which it has upon oxygen. A moist bladder, partially filled with oxygen, and suspended in carbonic acid gas, becomes rapidly distended, in consequence of the carbonic acid gas passing into it with much greater rapidity than the oxygen passes out. Secondly, the gases of the blood are not held in a merely mechanical way in it; the oxygen seems to be loosely combined with the red corpuscles (Lesson III. § 16), and there is reason to think that a great part, at least, of the carbonic acid, is chemically connected, in a similarly loose way, with certain saline constituents of the serum. Hence the arterialization of blood in the lungs seems to be a very mixed process, partly physical, and yet, to a certain extent chemical, and consequently very difficult to analyse.

The same may also be said of the change from arterial to venous blood in the tissues. Owing to the peculiar relation of oxygen to the red blood-corpuscles, the process which takes place in the tissues is not a simple interchange by diffusion of the oxygen of the blood for the carbonic acid of the tissues; on the contrary, the oxygen is given up for purposes of oxidation, the demand being determined by the supply of oxidizable materials in the tissue, while the blood, poor in carbonic acid, takes up,

apparently by an independent action, a quantity of that gas from the tissues rich in it.

Hence venous blood is characterized not only by the large amount of carbonic acid present, but also by the fact that the red corpuscles have given up a good deal of their oxygen for the purposes of oxidation, or, as the chemists would say, have become reduced. This is the reason why arterial blood is not so easily converted into venous blood by exposure to carbonic acid as venous blood into arterial by exposure to oxygen. There is, in the former case, a want of some oxidizable substance to carry off the oxygen from and so to reduce the red corpuscles. When such an oxidizable substance is added (as, for instance, a salt of iron), the blood at once and immediately becomes completely venous.

Practically we may say that the most important difference between venous and arterial blood is not so much the relative quantities of carbonic acid as that the red corpuscles of venous blood have lost a good deal of oxygen, are reduced, and ready at once to take up any oxygen offered to them.

5. The cause of the change of colour of the blood-of its darkening when exposed to carbonic acid, and its brightening when under the influence of oxygen-is not thoroughly understood. There is reason to think, however, that the red corpuscles are rendered somewhat flatter by oxygen gas, while they are distended by the Under the action of carbonic acid (Lesson III. § 4). former circumstances they may, not improbably, reflect the light more strongly, so as to give a more distinct coloration to the blood; while, under the latter, they may reflect less light, and, in that way, allow the blood to appear darker and duller.

This, however, is not the whole of the matter; for solutions of hæmoglobin or of blood-crystals (Lesson III § 9), even when perfectly free from actual blood-cor puscles, change in colour from scarlet to purple, accord ing as they gain or lose oxygen. It has already bee stated (Lesson III. § 16) that oxygen most probably exists in the blood in loose combination with hæmo globin. But, further, there is evidence to show that solution of hæmoglobin, when thus loosely combined wit!

ayet he a scarlet colour, while a solution of hæmopoda, ag rod of oxygen, has a purplish hue. Hence Amma dood in which the hemoglobin is richly proword #sì argen, would naturally be scarlet, while res dont which not only contains an excess of cardone, and but whose hemoglobin also has lost a great ect de 10 angen, would be purple.

& Whever may be their explanation, however, the Bots are certain (1), that arterial blood, separated by erly a thin membrane by carbonic acid, or from a fluid containing a greater amount of carbonic acid than itself, and also carrying ce tain oxidizable materials, becomes wwws; and (2) that venous blood, separated by only a the membrare from exger, or a fluid containing a greater proportion of free oxygen than itself, becomes

In these facts lies the explanation of the conversion of scarlet blood into dark blood as it passes through the capillaries of the body, for the latter are bathed by the juices of the tissues, which contain carbonic acid, the product of their waste and combustion, in excess, together with highly exidizable matters. On the other hand, if we seek for the explanation of the conversion of the dark blood in the veins into the scarlet blood of the arteries, we find, 1st, that the blood remains dark in the right auricle, the right ventricle, and the pulmonary artery; and, that is scarlet not only in the aorta, but in the left ventricle, the left auricle, and the pulmonary veins.

Obviously, then, the change from venous to arterial takes place in the pulmonary capillaries, for these are the sole channels of communication between the pulmonary arteries and the pulmonary veins.

7. But what are the physical conditions to which the blood is exposed in the pulmonary capillaries?

These yesse's are very wide, thin walled, and closely set, so as to form a network with very small meshes, which is contained in the substance of an extremely thin membrane This membrane is in contact with the air, so that the blood in each capillary of the lung is separated from the air by only a delicate pellicle formed by its own wall and the lung membrane. Hence an exchange very readily takes place between the blood and the air; the latter