Chapter X

).

*Excretory Work of Glands.*--The process of removing wastes from the body is called _excretion_. While in theory excretion may be regarded as a distinct physiological act, it is, in fact, leaving out the work of the lungs, but a phase of the work of glands. From the cells where they are formed, the waste materials pass into the lymph and from the lymph they find their way into the blood. They are removed from the blood by glands and then passed to the exterior of the body.

*The Necessity for Excretion* is found in the results attending oxidation and other chemical changes at the cells (page 107). Through these changes large quantities of materials are produced that can no longer take any

## part in the vital processes. They correspond to the ashes and gases of

ordinary combustion and form wastes that must be removed. The most important of these substances, as already noted (page 110), are carbon dioxide, water, and urea.(74) A number of mineral salts are also to be included with the waste materials. Some of these are formed in the body, while others, like common salt, enter as a part of the food. They are solids, but, like the urea, leave the body dissolved in water.

Waste products, if left in the body, interfere with its work (some of, them being poisons), and if allowed to accumulate, cause death. Their removal, therefore, is as important as the introduction of food and oxygen into the body. The most important of the excretory glands are

*The Kidneys.*--The kidneys are two bean-shaped glands, situated in the back and upper portion of the abdominal cavity, one on each side of the spinal column. They weigh from four to six ounces each, and lie between the abdominal wall and the peritoneum. Two large arteries from the aorta, called the _renal arteries_, supply them with blood, and they are connected with the inferior vena cava by the _renal veins_. They remove from the blood an exceedingly complex liquid, called the _urine_, the principal constituents of which are water, salts of different kinds, coloring matter, and urea. The kidneys pass their secretion by two slender tubes, the _ureters_, to a reservoir called the _bladder_ (Fig. 87).

[Fig. 87]

Fig. 87--*Relations of the kidneys.* (Back view.) 1. The kidneys. 2. Ureters. 3. Bladder. 4. Aorta. 5. Inferior vena cava. 6. Renal arteries. 7. Renal veins.

*Structure of the Kidneys.*--Each kidney is a compound tubular gland and is composed chiefly of the parts concerned in secretion. The ureter serves as a duct for removing the secretion, while the blood supplies the materials from which the secretion is formed. On making a longitudinal section of the kidney, the upper end of the ureter is found to expand into a basin-like enlargement which is embedded in the concave side of the kidney. The cavity within this enlargement is called the _pelvis of the kidney_, and into it project a number of cone-shaped elevations from the kidney substance, called the _pyramids_ (Fig. 88).

From the summits of the pyramids extend great numbers of very small tubes which, by branching, penetrate to all parts of the kidneys. These are the _uriniferous tubules_, and they have their beginnings at the outer margin of the kidney in many small, rounded bodies called the _Malpighian capsules_ (_A_, Fig. 88). Each capsule incloses a cluster of looped capillaries and connects with a single tubule (Fig. 89). From the capsule the tubule extends toward the concave side of the kidney and, after uniting with similar tubules from other parts, finally terminates at the pyramid. Between its origin and termination, however, are several convolutions and one or more loops or turns. After passing a distance many times greater than from the surface to the center of the kidney, the tubule empties its contents into the expanded portion of the ureter.

[Fig. 88]

Fig. 88--*Sectional view of kidney.* 1. Outer portion or cortex. 2. Medullary portion. 3. Pyramids. 4. Pelvis. 5. Ureter. _A._ Small section enlarged to show the tubules and their connection with the capsules.

[Fig. 89]

Fig. 89--*Malpighian capsule* highly magnified (Landois). _a._ Small artery entering capsule and forming cluster of capillaries within. _e._ Small vein leaving capsule and branching into _c_, a second set of capillaries, _h._ Beginning of uriniferous tubule.

The uriniferous tubules are lined with secreting cells. These differ greatly at different places, but they all rest upon a basement membrane and are well supplied with capillaries. These cells provide one means of separating wastes from the blood (Fig. 90).

[Fig. 90]

Fig. 90--*Diagram illustrating renal circulation.* 1. Branch from renal artery. 2. Branch from renal vein. 3. Small artery branches, one of which enters a Malpighian capsule (5). 6. Small vein leaving the capsule and branching into the capillaries (7) which surround the uriniferous tubules. 4. Small veins which receive blood from the second set of capillaries. 8. Tubule showing lining of secreting cells.

*Blood Supply to the Kidneys.*--The method by which the kidneys do their work is suggested by the way in which the blood circulates through them. The renal artery entering each kidney divides into four branches and these send smaller divisions to all parts of the kidney. At the outer margin of the kidney, called the _cortex_, the blood is passed through _two sets of capillaries_. The first forms the clusters in the Malpighian capsules and receives the blood directly from the smallest arteries. The second forms a network around the uriniferous tubules and receives the blood which has passed from the capillary clusters into a system of small veins (Fig. 90). From the last set of capillaries the blood is passed into veins which leave the kidneys where the artery branches enter, uniting there to form the main renal veins.

*Work of the Kidneys.*--Why should the blood pass through two systems of capillaries in the kidneys? This is because the separation of waste is done in part by the Malpighian capsules and in part by the uriniferous tubules. Water and salts are removed chiefly at the capsules, while the remaining solid constituents of the urine pass through the secreting cells that line the tubules. It was formerly believed that the kidneys obtained their secretion by a process of filtration from the blood, but this belief has been gradually modified. The prevailing view now is that the processes of filtration and secretion are both carried on by the kidneys,--that the capillary clusters in the Malpighian bodies serve as delicate filters for the separation of water and salts, while the secreting cells of the tubules separate substances by the process of secretion.

On account of the large volume of blood passing through the kidneys this liquid is still a bright red color as it flows into the renal veins (Fig. 90). The kidney cells require oxygen, but the amount which they remove from the blood is not sufficient to affect its color noticeably. The blood in the renal veins, having given up most of its impurities and still retaining its oxygen, is considered the purest blood in the body.

*Urea* is the most abundant solid constituent of the urine and is the chief waste product arising from the oxidation of nitrogenous substances in the body. Although secreted by the cells lining the uriniferous tubules, it is not formed in the kidneys. The secreting cells simply separate it from the blood where it already exists. The muscles also have been suggested as a likely source of urea, for here the proteids are broken down in largest quantities; but the muscles produce little if any urea. Its production has been found to be the _work of the liver_. In the muscular tissue, and in the other tissues as well, the proteids are reduced to a lower order of compounds, such as the compounds of ammonia, which pass into the blood and are then taken up by the liver. By the

## action of the liver cells these are converted into urea and this is turned

back into the blood. From the blood the urea is separated by the secreting cells of the kidneys.

*Work of the Liver.*--The liver, already described as an organ of digestion (page 152), assists in the work of excretion both by changing waste nitrogenous compounds into urea and by removing from the blood the wastes found in the bile. While the chief work of the liver is perhaps not that of excretion, its functions may here be summarized. The liver is, first of all, a _manufacturing organ_, producing, as we have seen, three distinct products--bile, glycogen, and urea. On account of the nature of the urea and the bile, the liver is properly classed as an _excretory organ_; but in the formation of the glycogen it plays the part of a _storage organ_. Then, on account of the use made of the bile after it is passed into the food canal, the liver is also classed as a _digestive organ_. These different functions make of the liver an organ of the first importance.

*Excretory Work of the Food Canal.*--The glands connected with the food canal, other than the liver, while secreting liquids that aid in digestion, also separate waste materials from the blood. These are passed into the canal, whence they leave the body with the undigested portions of the food and the waste from the liver. Though the nature and quantity of the materials removed by these glands have not been fully determined, recent investigations have tended to enhance the importance attached to this mode of excretion.

*The Perspiratory Glands.*--The perspiratory, or sweat, glands are located in the skin. They belong to the type of simple tubular glands and are very numerous over the entire surface of the body. A typical sweat gland consists of a tube which, starting at the surface of the cuticle, penetrates to the under portion of the true skin and there forms a ball-shaped coil. The coiled extremity, which forms the secreting portion, is lined with secreting cells and surrounded by a network of capillaries. The portion of the tube passing from the coil to the surface serves as a duct (Figs. 91 and 121).

[Fig. 91]

Fig. 91--*Diagram of section through a sweat gland.* _a._ Outer layer of skin or cuticle. _b._ Dermis or true skin. _d, e._ Sections of the tube forming the coiled portion of the gland. _c._ Duct passing to the surface. The other structures of the skin not shown.

The sweat glands secrete a thin, colorless fluid, called _perspiration_, or sweat. This consists chiefly of water, but contains a small per cent of salts and of urea. The excretory work of these glands seems not to be so great as was formerly supposed, but they supplement in a practical way the work of the kidneys and, during diseases of these organs, show an increase in excretory function to a marked degree. The perspiration also aids in the regulation of the temperature of the body (