The carbonic acid exhaled contains all the carbon of the digestible food, with the exception of a comparatively small quantity given off in urea. It amounts to about 900 grammes per diem.
How are we to determine the quantity of air which an individual requires? We can but make the general statement that it must be sufficient to dilute the carbonic acid exhaled to an extent which precludes poisoning. It is impossible to fix a limit. Breathing becomes embarrassed, and frontal headache and other symptoms make themselves felt when 10 per cent. of pure carbonic acid is mixed with air. Even in so large a proportion as this, carbonic acid is not fatal to life. Yet an atmosphere in which there is present a hundredth part of this amount of carbonic acid, produced by respiration, is extremely injurious to health under the ordinary conditions in which people live. It may be asserted, therefore, that under ordinary conditions 0·1 per cent. is the extreme limit for wholesome living. But again we are obliged to add that air contaminated to this extent is not under all circumstances injurious to health. The explorers on the recent Antarctic Expedition were obliged at times to sleep three men in one sleeping-bag, with the aperture of the bag tightly closed. The atmosphere must have been heavily laden with carbonic acid. Dr. Wilson assures us that it was impossible to keep a pipe alight inside the bag. Not that any man so placed would desire, one would imagine, to add the combustion-products of tobacco to those given off from the lungs! The survival of the explorers proves that it is impossible to fix a limit of safety even for the carbonic acid in air vitiated by respiration. It is, however, a matter of common observation that air which is moist and warm, owing to respiration, and tainted with the odours of humanity, is extremely prejudicial to those who live in it. Such an atmosphere is a favourable medium for the conveyance of germs, whether of the common cold or of a more virulent type. At one time it was supposed that the volatile emanations which can be condensed, along with water, by hanging a vessel of ice to the ceiling of a crowded room, were actively poisonous; but this statement has not been confirmed by recent research. It is unnecessary to call any such evidence in support of the thesis that human beings thrive better in fresh air than in foul. The admirable results achieved by the “fresh air cure” show that there is no degree of vitiation which can be pronounced innocuous. Nevertheless, public opinion demands that sanitarians should give some figure as a guide. Commonly they fix the maximum of carbonic acid compatible with health at 0·06 per cent., the quantity of carbonic acid being taken as the measure of all impurities present. An adult exhales about 0·6 cubic foot of CO₂ per hour. Fresh air already contains about 0·04 per cent. If, therefore, the percentage is not to rise higher than 0·06 per cent., each adult must be supplied with 3,000 cubic feet of air per hour. With good ventilation air may be changed four times an hour, and therefore 800 cubic feet is regarded as sufficient space for each occupant of a room. The figure may pass. It is a reasonable basis from which to calculate the packing capacity of a dormitory. So long as a man has 800 cubic feet of air to himself, he may safely feel that he has room to stretch his lungs. Dwelling on this figure may make him feel uncomfortable when he finds himself in a railway carriage, seated five on a side, with the windows closed. In the theatre or in church he may doubt whether he has all the fresh air to which his humanity entitles him. But, as a philosopher rather than as a physiologist, he reflects that, whether on the Antarctic icecap in a sleeping-bag or standing on a summit in the Alps, he takes all that he can get, for fresh air is one of the few good things of which one can never have enough.
Tissue Respiration.—A frog will live for seventeen hours in an atmosphere of nitrogen. Under these circumstances it is clearly impossible for it to take up oxygen, yet for several hours it gives off as much carbonic acid as it would do if it were living in air. Such an observation as this proves that oxidation does not occur in the lungs, but deeper in the body. At one time the blood was regarded as the seat of oxidation; the products formed by the splitting up of proteins in the tissues were supposed to be passed into the blood, where they came in contact with the oxygen carried by hæmoglobin. A certain amount of oxidation does take place in the blood, as in all other tissues, for blood is a living tissue and needs to respire. But the oxidation which occurs in the blood is small in amount as compared with that in the organs which the vessels traverse. Muscle and other tissues detached from the body and free from blood give off carbonic acid. It is possible to wash the blood out of the vessels of a frog and to replace it with a solution of salt. In an atmosphere of oxygen such a “saline frog” lives for a day or two, taking in the same quantity of oxygen and giving off the same quantity of carbonic acid as a normal frog. The oxygen is chiefly absorbed through the skin, the carbonic acid discharged from the lung. This experiment shows that blood is not essential for oxidation. Oxidations do not occur in the salt solution with which blood is replaced. Taking all the evidence together, it seems to be safe to conclude that the tissues absorb the oxygen which the oxyhæmoglobin brings into their neighbourhood, and that they have some capacity of storing it. A piece of detached muscle which gives off carbonic acid in an atmosphere of nitrogen would appear to be holding a store of oxygen, much as hæmoglobin holds it. The proof is not quite so definite as might be desired; but we are probably justified in holding the belief that the main part of the respiratory exchange occurs in the tissues. Lymph dissolves oxygen which it obtains from the blood. The tissues take it from lymph. Tissues set free carbonic acid which lymph dissolves. Its tension being higher than in blood, carbonic acid diffuses from lymph, through the walls of the capillary vessels, into blood, from which it passes into the air in the lungs.
CHAPTER VIII
EXCRETION
Many things enter into the alimentary canal. If an analysis were made of a day’s food and drink, from the cup of tea on waking to the cocoa or other potion which is regarded as a necessary preliminary to settling for the night, it would be found that a great variety of substances were included in the food or taken as adjuvants to food. All these things, differing widely in chemical constitution, must leave the body. Some are not digested. They do not, properly speaking, enter into the diet. Such are the cellulose of vegetables, especially skins, husks, woody fibres; elastic fibres of meat; horny substances, etc. The quantity varies greatly, according to the nature of the diet. About 2 ounces (weighed dry) is the average. With this indigestible refuse is included undigested food, if the diet be excessive, and a variety of substances secreted by the liver, such as cholesterin and bile-pigment, some residues of the secretions of the alimentary canal, and products of bacteric fermentations. All food which is digested and absorbed is oxidized. It leaves the body by the lungs, the kidneys, or the skin. Foods, as already stated, are classified as proteins, carbohydrates, and fats. The chief excreta are carbonic acid, water, and urea. Carbonic acid makes its exit from the lungs; water from the lungs, the kidneys, and the skin; urea from the kidneys. The three great groups of foods and the three great groups of excreta overshadow in amount all the other substances which pass through the system. A balance-sheet in which proteins, carbohydrates, and fats appear on one side, carbonic acid, water, and urea on the other, is substantially correct. The energy which is set free by burning in a calorimeter the items entered on the debit side, after deducting that yielded by burning the urea (carbonic acid and water are incapable of further oxidation), gives a day’s income. Other constituents of the diet are so small in quantity as to be negligible in making up the body’s accounts. The chemical changes which they undergo add practically nothing to its capacity for work. Yet some of them are essential to the maintenance of health. Of such are common salt (sodic chloride), alkaline and earthy carbonates, sulphur, phosphorus, etc. These things, together with some products of action of the bacteria in the alimentary canal, the final stage of hæmoglobin, imperfectly oxidized nitrogenous substances, and other soluble substances which enter with, or are formed from the food, are removed by the kidneys. We speak of the elimination of waste products, as excretion. Not that there is any physiological distinction between excretion and secretion. Both terms refer to the selection or production and the discharge of materials by cells. If the product discharged has a useful function to perform—if it be a digestive ferment, for example—it is said to be secreted. If it is of no further use to the economy, we say that it is excreted—got rid of. In some cases either term is equally appropriate. The sebum prepared by the sebaceous glands is useful as a lubricant of the skin. It is thrown off. We may speak of the glands as either secreting or as excreting this fatty substance.
The Kidney.—From worms upwards, all animals possess organs for the removal of waste products in solution. This statement might, indeed, be widened so as to include animals even lower than worms. All animals which have a cœlomic cavity—a space between the alimentary canal and the body-wall—have organs for the removal of soluble waste. The segmental organs of worms are obviously the same organs as the kidneys of mammals; the latter are distinguished from their prototypes by greater concentration of structure and specialization of function. The kidney is the oldest of organs, if its antiquity be estimated as the length of time during which it has had a form practically identical with that which it now presents. The lungs are of late appearance in the animal scale. Alimentary canal, heart, brain, have passed through many transformations. The kidney assumed its permanent form very far back in the history of the animal kingdom. The most primitive animal which has a digestive cavity, and vessels in which the products of digestion circulate, needs an organ which provides for the overflow from the body-fluids of all substances which are injurious or effete.
The kidney is an aggregation of long urinary tubules. The head of each tubule is dilated into a globular capsule, into which a tuft of bloodvessels depends. This is the sink into which the waste-water of the blood drips. The long urinary tubules are lined with cells well qualified by form and constitution to search the blood in the capillaries which border them, for substances which, not being easily diffusible, have to be forcibly dragged from it and added to the water trickling down the pipe which connects the rain-water head with the sewer. The hydrostatic conditions of this apparatus—the provision for greater or less flow of blood through the tufts (glomeruli) which hang in the capsules, and for longer or shorter exposure of the blood to the purifying activity of the epithelium of the renal tubules—will be described after a very brief account has been given of the structure of the organ.
The outer border of the kidney is convex, its inner border concave. The concavity is termed the “hilus.” The central depression of the hilus is embraced by the expanded end of the ureter—the tube which carries the secretion of the kidney to the bladder. The renal artery and the renal nerves enter, and the renal vein leaves, the kidney at the hilus.