Whenever food or fuels are burned, oxidized for the energy which they contain, products of oxidation accumulate and must be eliminated. This is as true in the animal body as in the engine. In the latter accumulating smoke gases, acids and clinkers would put out the fires if not promptly disposed of. In a similar way in the animal body accumulated carbon-dioxid gas and nitrogenous and other waste materials would soon extinguish the "spark of life." As in the engine, so in the animal body, accumulating gases are much more rapid in their action than accumulating solids. If the draft smokestack of the engine were stopped, so that the carbon dioxid could not rapidly pass away, it would require only a few minutes to put out the fire; and if the respiratory passages of the animal were closed, death would follow in a few minutes. If clinkers were permitted to accumulate on the surface of the grate the engine's fire would probably continue for a number of hours, perhaps a whole day, but not indefinitely. Similarly, if the nitrogenous and like excreta are allowed to accumulate in the blood through failure of the kidneys to expel them, an animal would die in convulsions within a day or two. This should illustrate once for all the absolute necessity for getting rid of the ever accumulating waste material. This process is called excretion and is participated in by lungs, kidneys, skin, and intestines.

A. The Work Of The Lungs

The lungs are the respiratory organs and perform a double function. First, to take up oxygen from the air, which is absorbed through the moist thin membrane of the air sacs into the blood of the capillaries. Second, to exhale carbon dioxid into the air. This is carried from the active tissues of the body in the venous blood to the lungs, and diffuses through the capillary walls into the air contained in the air cells. Thus the air which is exhaled out of the lungs contains much less oxygen and much more carbon dioxid than the air which is inhaled.

Incidentally, the lungs give up a certain amount of water and minute quantities of organic material; both of these exhalations, however, are incidental. The excretion of importance in the lungs is the carbon-dioxid gas.

B. The Work Of The Kidneys

The sole work of the kidneys is excretion. In this respect the kidneys differ from many organs of the body; the lungs, for example, perform the double function of absorbing oxygen and excreting carbon-dioxid gas. The liver performs several unrelated functions, as detailed above. The kidneys, however, devote their activity to excretion exclusively; they are the excretory organs par excellence. The blood passes through them from a short transverse branch on the abdominal aorta in far greater quantities than would be necessary to supply the kidneys with nourishment and oxygen. This blood is sent to the kidneys, not for the kidneys' sake, but for the blood's sake. It is sent to the kidneys to be purified. We have here, therefore, the unusual occurrence of a greater purity in the venous blood coming away from an organ than the arterial blood going through the organ. The kidney is composed of a very great number of tubes called the uriniferous tubules. Each tubule starts in a little tuft of capillaries inclosed in a sac. The tuft is called a glomerulus. The sac is called Bowman's capsule. Starting thus in a glomerulus, the tubule, lined with secreting epithelium, undergoes a great many changes in direction; in fact, it is almost as tortuous as the small intestine, and empties finally into the pelvis of the kidneys. Each tubule may be subdivided into two portions: the glomerulus and the convoluted portion. These two portions have two very distinct functions.

Glomerular Excretion

When the blood filters through the tuft of capillaries within the glomerulus, water and salts of the blood pass out in a sort of filtrate, are caught by the Bowman's capsule and pass into the lumen of the tubule. The amount of water and salts thus filtering out of the blood will depend, of course, upon the excess of these substances in the blood. If a large amount of water has passed away from the surface of the body as perspiration, there will be less water to be excreted by way of the kidneys. On the other hand, if one has been drinking quite freely of water during a warm spell, and there comes a sudden change in the weather, accompanied by a fall of temperature and much moisture in the air, the perspiration will suddenly be much decreased in quantity. This will leave the blood with a considerable excess of water above the average or normal. Under these conditions there will be a copious filtration of water and salts through the glomeruli into the tubules. This naturally results in a large volume of light-colored, diluted urine, while the urine in dry, hot weather is likely to be dark-colored and more concentrated, due to the small amount of water.

Tubular Excretion

Surrounding each tubule is a rich mesh-work of capillary loops, through which the blood slowly oozes, after it has passed through the tuft or capillary glomerulus. The active secreting cells that make up the wall of the tubules thus have blood on one side and the open lumen of the tubule on the other side, filled with water just filtered off in the glomerulus and making its way along the tubule to the pelvis of the kidneys. The kidney cells take up nitrogenous and other waste materials from the blood of the capillaries and throw it out on the other side into the water.

Among the substances thus removed from the blood and secreted into the water are: urea, uric acid, urates, sulphuric acid, sulphates, sodium phosphate, xanthin bodies, conjugated sulphates. Note that all of these substances contain either nitrogen or sulphur or phosphates. They are substances which cannot be excreted by the lungs, nor by other excretory organs in more than slight traces. They result from the oxidation and catabolism of the protein. Some of the proteins thus catabolized have been a part of the living, active tissues, while others are catabolized direet. In any case, whether tissue protein or fuel protein, its oxidation yields these substances above enumerated, and they can be excreted only through the kidneys. Interference in the action of the kidneys, then, results in the retention of these substances in the blood. Such a condition is called uremia, and rapidly produces a condition of intoxication called uremic poisoning. Such a condition acknowledges itself by profound influence on the neuromuscular system as evidenced by appearance of convulsions. If the condition is not quickly relieved the patient will die in convulsions.

Any substance which causes increased activity of the kidneys is called a diuretic. We have already, in discussing the citrous fruits, mentioned the diuretic effect of juices of these fruits, the active agent in the juice being the citrate of the alkalies.

Water, when taken copiously, also has a diuretic effect. Some diuretics exert their influence indirectly through increasing blood pressure. Digitalis exerts a diuretic effect in this way.

C. The Work Of The Skin

While the skin is usually named among the excretory organs, excretion is more or less incidental, the principal function of the skin being protection; as a part of its protective function, the skin secretes oil from its sebaceous glands. Another part of its protective function is regulating body temperature. Incident to this regulation of body temperature water may be poured out in large quantities; this water is naturally drawn off from the blood and represents an excess above the minimum water content of the blood. If this amount is exceeded one experiences thirst and takes more water to make good the shortage. Any excess above normal water content of the blood, after the skin has made the necessary drafts in its temperature regulation, will be excreted by the kidneys as described above. Thus we have a perfect reciprocal relation between skin and kidneys in excretion of excess water. Incident to the excretion of water from the sweat glands of the skin, certain salts are also excreted, and these salts are practically the same as those excreted by the kidneys, even including urates in traces.

D. The Work Of The Intestines

A considerable bulk of waste matter passes away from the intestines daily as feces. Much the greater part of this fecal matter represents the indigestible and undigested food materials or food accompaniments that have passed throughout the whole length of the alimentary canal. The most important part of this undigested material, so far as bulk is concerned, is the cellulose of vegetable tissues, while a small part of the cellulose, particularly such tender cellulose as that in crisp celery, cabbage, and other vegetables, is digested in the human digestive canal. All the tougher cellulose structure, such as that found in cereals and legumes, and a large part of that in vegetables and fruits, passes through the human alimentary canal undigested. This is not to be looked upon as a disadvantage; on the other hand, it is considered by physiologists to be distinctly advantageous to have a certain amount of undigested material in the alimentary canal, because this bulk of undigested material serves as a carrier or vehicle for the excretory and other materials to be enumerated later. The presence of this cellulose stimulates the peristaltic action of the intestinal walls, and thus favors regularity of the bowels. Besides the cellulose, there are bile pigments, bile salts, mucus, amino acids, and other products of decomposition of proteins, together with a slight amount of unabsorbed fats, and products of bacterial fermentation and putrefaction within the intestines.

Of all this mass of material that makes up the feces only a very small amount can be classified as real excretion, because an excretion is a substance which has been within the tissues. Only those substances, then, in the feces which have been poured out with the bile, or from the walls of the intestinal cells, in order to rid the system of them, should be counted as excretion. All the other substances are simply ejecta. Even the mucus poured out by the wall of the large intestine to facilitate the movement of its contents would not be called an excretion, though it is a part of the feces. It was not poured out into the intestine in order to get rid of it, but was poured out to serve a particular purpose. The bile pigments, on the other hand, are taken out of the blood and poured into the intestine in order to get rid of them. They serve no purpose in the intestine; they represent purely excretory material. Retention of this material, through a derangement of the liver function, makes itself manifest in the yellow, jaundiced color, due to the tingeing of the tissues by the bile pigments.