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 pa.s.sed through _two sets of capillaries_. The first forms the cl.u.s.ters 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 pa.s.sed from the capillary cl.u.s.ters into a system of small veins (Fig. 90).

From the last set of capillaries the blood is pa.s.sed 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 pa.s.s 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 const.i.tuents of the urine pa.s.s 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 cl.u.s.ters 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 pa.s.sing 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 const.i.tuent 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 quant.i.ties; 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 pa.s.s 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), a.s.sists 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 cla.s.sed 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 pa.s.sed into the food ca.n.a.l, the liver is also cla.s.sed as a _digestive organ_. These different functions make of the liver an organ of the first importance.

*Excretory Work of the Food Ca.n.a.l.*-The glands connected with the food ca.n.a.l, other than the liver, while secreting liquids that aid in digestion, also separate waste materials from the blood. These are pa.s.sed into the ca.n.a.l, whence they leave the body with the undigested portions of the food and the waste from the liver. Though the nature and quant.i.ty 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 pa.s.sing 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 pa.s.sing 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 (Chapter XVI).

*Excretory Work of the Lungs.*-While the lungs cannot be regarded as glands, they do a work in the removal of waste from the body which must be considered in the general process of excretion. They are especially adapted to the removal of gaseous substances from the blood, and it is through them that most of the carbon dioxide leaves the body. The lungs remove also a considerable quant.i.ty of water. This is of course in the gaseous form, being known as water vapor.

*Ductless Glands and Internal Secretion.*-Midway in function between the glands that secrete useful liquids and those that remove waste materials from the blood is a cla.s.s of bodies, found at various places, known as the _ductless glands._ They are so named from their having the general form of glands and from the fact that they have no external openings or ducts.

They prepare special materials which are pa.s.sed into the blood and which are supposed to exert some beneficial effect either upon the blood or upon the tissues through which the blood circulates. The most important of the ductless glands are the thyroid gland, located in the neck; the suprarenal bodies, situated one just over each kidney; and the thymus gland, a temporary gland in the upper part of the chest. The spleen and the lymphatic glands (page 68) are also cla.s.sed with the ductless glands. The liver, the pancreas, and (according to some authorities) the kidneys, in addition to their external secretions, produce materials that pa.s.s into the blood. They perform in this way a function like that of the ductless glands. The work of glands in preparing substances that enter the blood is known as _internal secretion._

*Quant.i.ty of Excretory Products.*-If the weight of the normal body be taken at intervals, after growth has been attained, there will be found to be practically no gain or loss from time to time. This shows that materials are leaving the body as fast as they enter and that the tissues are being torn down as fast as they are built up. It also shows that substances do not remain in the body _permanently_, but only so long perhaps as is necessary for them to give up their energy, or serve some additional purpose in the ever changing protoplasm. The excretory organs then remove from the body a quant.i.ty of material that is equal in weight to the materials absorbed by the organs of digestion and respiration. This is estimated for the average individual to be about five pounds daily. The pa.s.sage of waste from the body is summarized in Table III.

TABLE III. THE Pa.s.sAGE OF WASTE MATERIALS FROM THE BODY Materials State How Formed Condition in How Removed in the Body the Blood from the Blood Carbon Gas By the Dissolved in Separated dioxide oxidation of the plasma from the the carbon and in loose blood at the of proteids, combination alveoli of carbohydrates, with salts the lungs and fats. in the and then blood. forced through the air pa.s.sages into the atmosphere.

Urea Solid By the Dissolved in Removed by oxidation in the plasma. the the liver of uriniferous nitrogenous tubules of compounds. the kidneys and to a small extent by the perspiratory glands.

Water Liquid By the As water. Removed by oxidation of all the the hydrogen organs of of proteids, excretion, carbohydrates, but in the and fats. largest Amount formed quant.i.ties in the body is by the small. kidneys and the skin.

Salts Solid Dissolved in By the the plasma. kidneys, liver, and skin.

HYGIENE

The separation of wastes from the body has such a close relation to the health that all conditions affecting it should receive the most careful attention. Their retention beyond the time when they should be discharged undoubtedly does harm and is the cause of many bodily disorders.

*Value of Water.*-As a rule the work of excretion is aided by drinking _freely_ of pure water. As water is the natural dissolver and transporter of materials in the body, it is generally conceded by hygienists and physicians that the taking of plenty of water is a healthful practice.

People do not as a rule drink a sufficient amount of water, about three pints per day being required by the average adult, in addition to that contained in the food. Most of the water should, of course, be taken between meals, although the sipping of a small amount during meals does not interfere with digestion. As stated elsewhere, the taking of a cup of water on retiring at night and again on rising in the morning is very generally recommended.

*Protection of Kidneys and Liver.*-The kidneys and liver are closely related in their work and in many instances are injured or benefited by the same causes. Both, as already stated (page 124), are liable to injury from an _excess of proteid food_, especially meats, and also by a condition of inactivity of the bowels (page 166). The free use of alcohol also has an injurious effect on both of these organs.(75) On the other hand, increasing the activity of the skin has a beneficial effect upon them, especially the kidneys. Exercise and bathing, which tend to make the skin more active, are valuable aids both in ridding the body of impurities and in lessening the work of the other excretory organs. One having a disease of the kidneys, however, needs to exercise great care in bathing on account of the bad results which follow getting chilled.

*Special Care after Certain Diseases.*-Certain diseases, as measles, diphtheria, scarlet fever, and typhoid fever, sometimes have the effect of weakening the kidneys (and other vital organs) and of starting disease in them. When this occurs it is usually the result of exposure or of over-exertion while the body is in a weakened condition. Severe chilling at such a time, by driving blood from the surface to the parts within, often causes inflammation of the kidneys. On recovering from any wasting disease one should exercise great caution both in resuming his regular work and in exposing his body to wet or cold.

*Misunderstood Symptoms.*-Pains in the small of the back, an increase in the secretions of the kidneys, and a sediment in the urine very naturally suggest some disorder of the kidneys. It is a fact, however, that these symptoms have little or no relation to the state of the kidneys and may occur when the kidneys are in a perfectly healthy condition. The kidneys are not located in the small of the back, but above this place, so that pains in this region are evidently not from the kidneys, while the increase in the flow of the urine may arise from a number of causes, one of which is an increase of certain waste products pa.s.sed into the blood.

The symptoms referred to are frequently the results of nervous exhaustion, resulting from overstudy, worry, eye strain, or some other condition that overtaxes the nervous system. When this is the case, relief is obtained through resting the nerves. Actual disease of the kidneys can only be determined through a chemical and microscopic examination of the urine. To resort to some patent medicine for kidney trouble without knowing that such trouble exists, as is sometimes done, is both foolish and unhygienic.

*Alcoholic Beverages and the Elimination of Waste.*-Causing as it does such serious diseases as cirrhosis of the liver and Bright's disease of the kidneys (footnote, page 210), alcohol will greatly interfere in this way with the elimination of waste. There is also evidence to the effect that it interferes with waste elimination before the stage is reached of causing disease of these organs. Researches have shown that alcohol increases the amount of uric acid in the body and decreases the amount of urea found in the urine. The conclusion to be drawn is that alcohol interferes in some way with the change of the harmful uric acid into the comparatively harmless urea-an interference which in some instances results in great harm. It has also been shown that malted liquors, such as beer and ale, contain substances which, like the caffein of tea and coffee (page 167), are readily converted into uric acid.(76) Wines contain acids which may also act injuriously. The harm which such substances do is, of course, additional to that caused by the alcohol.

*Summary.*-As a result of the oxidations and other changes at the cells, substances are produced that can no longer serve a purpose in the body.

They are of the nature of waste, and their continuous removal from the body is as necessary to the maintenance of life as the introduction of food and oxygen. The organs whose work it is to remove the waste, excepting the lungs, are glands; and the material which they remove are of the nature of secretions. From the cells, the waste pa.s.ses through the lymph in the blood. From the blood it is separated by the excretory organs and pa.s.sed to the exterior of the body.

*Exercises.*-1. What general purposes are served by the glands in the body?

2. What are the parts common to all glands? What purpose is served by each of these parts?

3. How do tubular glands differ in structure from saccular glands? What is a racemose gland? Why so called?

4. Describe the nature of the secretory process.

5. What conditions render necessary the formation of waste materials in the body? Why must these be removed?

6. How do the waste materials get from the cells to the organs of excretion?

7. Show by a drawing the connections of the kidneys with the large blood vessels and the bladder. Name parts of drawing.

8. In what do the uriniferous tubes have their beginning? In what do they terminate? With what are they lined?

9. Why should the blood pa.s.s through two sets of capillaries in the kidneys?

10. Bright's disease of the kidneys affects the uriniferous tubes and interferes with their work. What impurity is then left in the blood?

11. Trace water and salts from the Malpighian capsules to the bladder, naming parts through which they pa.s.s.

12. Trace carbon dioxide from the cells to the outside atmosphere.

13. How does the quant.i.ty of material introduced into the body compare with that which is removed by the organs of excretion?

14. Name two ways of lessening the work of the kidneys.

15. Why is the drinking of plenty of pure water a healthful practice?

PRACTICAL WORK