CHAPTER XIII - GLANDS AND THE WORK OF EXCRETION
In our study so far we have been concerned mainly with the introduction of materials into the body. We are now to consider the removal of materials from the body. The structures most directly concerned in this work are known as
Glands.—As generally understood, glands are organs that prepare special liquids in the body and pour them out upon free surfaces. These liquids, known as secretions, are used for protecting exposed parts, lubricating surfaces that rub against each other, digesting food, and for other purposes. They differ widely in properties as well as in function, but are all alike in being composed chiefly of water. The water, in addition to being necessary to the work of particular fluids, serves in all cases as a carrier of solid substances which are dissolved in it.
General Structure of Glands.—While the various glands differ greatly in size, form, and purpose, they present striking similarities in structure. All glands contain the following parts:
1. Gland, or secreting, cells. These are specialized cells for the work of secretion and are the active agents in the work of the gland. They are usually cubical in shape.
2. A basement membrane. This is a thin, connective tissue support upon which the secreting cells rest.
3. A network of capillary and lymph vessels. These[pg 198] penetrate the tissues immediately beneath the secreting cells.
4. A system of nerve fibers which terminate in the secreting cells and in the walls of the blood vessels passing to the glands.
These structures—secreting cells, basement membrane, capillary and lymph vessels, and nerve fibers—form the essential parts of all glands. The capillaries and the lymph vessels supply the secreting cells with fluid, and the nerves control their activities.
Kinds of Glands.—Glands differ from one another chiefly in the arrangement of their essential parts.[73] The most common plan is that of arranging the parts around a central cavity formed by the folding or pitting of an exposed surface. Many such glands are found in the mucous membrane, especially that lining the alimentary canal, and are most numerous in the stomach, where they supply the gastric juice. If these glands have the general form of tubes, they are called tubular glands; if sac-like in shape, they are called saccular glands. Both the tubular and the saccular glands may, by branching, form a great number of similar divisions which are connected with one another, and which communicate by a common opening with the place where the secretion is used. This forms a compound gland which, depending on the structure of the minute parts, may be either a compound tubular or a compound saccular gland. The larger of the compound saccular glands are also called racemose glands, on account of their having the general form of a cluster, or raceme, similar[pg 199] to that of a bunch of grapes. The general structure of the different kinds of glands is shown in Fig. 85.
Fig. 85—Diagram illustrating evolution of glands. A. Simple secreting surface. 1. Gland cells. 2. Basement membrane. 3. Blood vessel. 4. Nerve. B. Simple tubular gland. C. Simple saccular gland. D. Compound tubular gland. E. Compound saccular gland. F. A compound racemose gland with duct passing to a free surface. G. Relation of food canal to different forms of glands. The serous coat has a secreting surface.
Nature of the Secretory Process.—At one time the gland was regarded merely as a kind of filter which separated from the blood the ingredients found in its secretions. Recent study, however, of several facts relating to secretion has led to important modifications of this view. The secretions of many glands are known to contain substances that are not found in the blood, or, if present, are there in exceedingly small amounts. Then again the cells of certain glands have been found to undergo marked changes during the process of secretion. If, for example, the[pg 200] cells of the pancreas be examined after a period of rest, they are found to contain small granular bodies. On the other hand, if they are examined after a period of activity, the granules have disappeared and the cells themselves have become smaller (Fig. 86). The granules have no doubt been used up in forming the secretion. These and other facts have led to the conclusion that secretion is, in part, the separation of materials without change from the blood, and, in part, a process by which special substances are prepared and added to the secretion. According to this view the gland plays the double rôle of a filtering apparatus and of a manufacturing organ.
Fig. 86—Secreting cells from the pancreas (after Langley). A. After a period of rest. B. After a short period of activity. C. After a period of prolonged activity. In A and B the nuclei are concealed by the granules that accumulate during the resting period.
Kinds of Secretion.—In a general way all the liquids produced by glands may be considered as belonging to one or the other of two classes, known as the useful and the useless secretions. To the first class belong all the secretions that serve some purpose in the body, while the second includes all those liquids that are separated as waste from the blood. The first are usually called true secretions, or secretions proper, while the second are called excretions. The most important glands producing liquids of the first class are those of digestion (Chapter X).
[pg 201]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[pg 202] 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—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[pg 203] 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—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—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.
[pg 204]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—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.
[pg 205]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[pg 206] 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[pg 207] 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—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 (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[pg 208] remove also a considerable quantity 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 class 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 passed 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 classed with the ductless glands. The liver, the pancreas, and (according to some authorities) the kidneys, in addition to their external secretions, produce materials that pass 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.
Quantity 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 quantity 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 passage of waste from the body is summarized in Table III.
| TABLE III. THE PASSAGE OF WASTE MATERIALS FROM THE BODY | ||||
|---|---|---|---|---|
| Materials | State | How Formed in the Body | Condition in the Blood | How Removed from the Blood |
| Carbon dioxide | Gas | By the oxidation of the carbon of proteids,carbohydrates, and fats. | Dissolved in the plasma and in loosecombination with salts in the blood. | Separated from the blood at thealveoli of the lungs and then forced through the air passages into theatmosphere. |
| Urea | Solid | By the oxidation in the liver of nitrogenous compounds. | Dissolved in the plasma. | Removed by the uriniferous tubules of thekidneys and to a small extent by the perspiratory glands. |
| Water | Liquid | By the oxidation of the hydrogen of proteids,carbohydrates, and fats. Amount formed in the body is small. | As water. | Removed by all the organs of excretion, but in the largest quantities bythe kidneys and the skin. |
| Salts | Solid | Dissolved in the plasma. | By the 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[pg 211] 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 passed 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[pg 212] 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 passes through the lymph in the blood. From the blood it is separated by the excretory organs and passed to the exterior of the body.
Exercises.—1. What general purposes are served by the glands in the body?
[pg 213]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 pass 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 pass.
12. Trace carbon dioxide from the cells to the outside atmosphere.
13. How does the quantity 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
To suggest the Double Work of Glands.—Prepare a simple filter by fitting a piece of porous paper into a glass funnel. Through this pass pure water and also water having salt dissolved in it and containing some sediment, as sand. The water and the dissolved salt pass through, while the sediment remains on the filter. Now substitute a fresh piece of paper in the funnel and drop on its surface a little solid coloring matter, such as cochineal. Again pass the liquid through the funnel. This time it comes through colored, the color being added by the filter. Compare the filter and materials filtered to the gland and the materials concerned in secretion (blood, the liquid secreted, substances added by the gland, etc.).
Fig. 92—The physiological scheme. Diagram suggesting the essential relation of the bodily activities. See Summary of Part I, page 215, and Summary of Part II, page 413.
SUMMARY OF PART I
The body is an organization of different kinds of cells; it grows through the growth and reproduction of these cells; and its life as a whole is maintained by providing such conditions as will enable the cells to keep alive. Of chief importance in the work of the body is a nutrient fluid which supplies the cells with food and oxygen and relieves them of waste. A moving portion of this fluid, called the blood, serves as a transporting agent, while another portion, called the lymph, passes the materials between the blood and the cells. Through their effects upon the blood and the lymph, the organs of circulation, respiration, digestion, and excretion minister in different ways to the cells, and aid in the maintenance of life. By their combined action two distinct movements are kept up in the body, as follows:
1. An inward movement which carries materials from the outside of the body toward the cells.
2. An outward movement which carries materials from the cells to the outside of the body.
Passing inward are the oxygen and food materials in a condition to unite with each other and thereby change their potential into kinetic energy. Passing outward are the oxygen and the elements that formed the food materials after having united at the cells and liberated their energy.
As a final and all-important result, there is kept up a continuous series of chemical changes in the cells. These liberate the energy, provide special substances needed by the cells, and preserve the life of the body (Fig. 92).
In the chapters which follow, we are to consider the problem of adjusting the body to and of bringing it into proper relations with its surroundings.