Читать книгу Physiology and Hygiene for Secondary Schools - Francis M. Walters - Страница 14

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Through the movements of the blood and the lymph, materials entering the body are transported to the cells, and wastes formed at the cells are carried to the organs which remove them from the body. We are now to consider the passage of materials from outside the body to the cells and vice versa. One substance which the body constantly needs is oxygen, and one which it is constantly throwing off is carbon dioxide. Both of these are constituents of

The Atmosphere.—The atmosphere, or air, completely surrounds the earth as a kind of envelope, and comes in contact with everything upon its surface. It is composed chiefly of oxygen and nitrogen,29 but it also contains a small per cent of other substances, such as water-vapor, carbon dioxide, and argon. All of the regular constituents of the atmosphere are gases, and these, as compared with liquids and solids, are very light. Nevertheless the atmosphere has weight and, on this account, exerts pressure upon everything on the earth. At the sea level, its pressure is nearly fifteen pounds to the square inch. The atmosphere forms an essential part of one's physical environment and serves various purposes. The process[pg 077] by which gaseous materials are made to pass between the body and the atmosphere is known as

Respiration.—As usually defined, respiration, or breathing, consists of two simple processes—that of taking air into special contrivances in the body, called the lungs, and that of expelling air from the lungs. The first process is known as inspiration; the second as expiration. We must, however, distinguish between respiration by the lungs, called external respiration, and respiration by the cells, called internal respiration.

The purpose of respiration is indicated by the changes that take place in the air while it is in the lungs. Air entering the lungs in ordinary breathing parts with about five per cent of itself in the form of oxygen and receives about four and one half per cent of carbon dioxide, considerable water-vapor, and a small amount of other impurities. These changes suggest a twofold purpose for respiration:

1. To obtain from the atmosphere the supply of oxygen needed by the body.

2. To transfer to the atmosphere certain materials (wastes) which must be removed from the body.

The chief organs concerned in the work of respiration are

The Lungs.—The lungs consist of two sac-like bodies suspended in the thoracic cavity, and occupying all the space not taken up by the heart. They are not simple sacs, however, but are separated into numerous divisions, as follows:

1. The lung on the right side of the thorax, called the right lung, is made up of three divisions, or lobes, and the left lung is made up of two lobes.

2. The lobes on either side are separated into smaller[pg 078] divisions, called lobules (Fig. 33). Each lobule receives a distinct division of an air tube and has in itself the structure of a miniature lung.

Fig. 33—Lungs and air passages seen from the front. The right lung shows the lobes and their divisions, the lobules. The tissue of the left lung has been dissected away to show the air tubes.

3. In the lobule the air tube divides into a number of smaller tubes, each ending in a thin-walled sac, called an infundibulum. The interior of the infundibulum is separated into many small spaces, known as the alveoli, or air cells.

The lungs are remarkable for their lightness and delicacy of structure.30 They consist chiefly of the tissues that form their sacs, air tubes, and blood vessels; the membranes that line their inner and outer surfaces; and the connective tissue that binds these parts together. All these tissues are more or less elastic. The relation of the different parts of the lungs to[pg 079] each other and to the outside atmosphere will be seen through a study of the

Air Passages.—The air passages consist of a system of tubes which form a continuous passageway between the outside atmosphere and the different divisions of the lungs. The air passes through them as it enters and leaves the lungs, a fact which accounts for the name.

Fig. 34—Model of section through the head, showing upper air passages and other parts. 1. Left nostril. 2. Pharynx. 3. Tongue and cavity of mouth. 4. Larynx. 5. Trachea. 6. Esophagus.

The incoming air first enters the nostrils. These consist of two narrow passages lying side by side in the nose, and connecting with the pharynx behind. The lining of the nostrils, called mucous membrane is quite thick, and has its surface much extended by reason of being spread over some thin, scroll-shaped bones that project into the passage. This membrane is well supplied with blood vessels and secretes a considerable quantity of liquid. Because of the nature and arrangement of the membrane, the nostrils are able to warm and moisten the incoming air, and to free it from dust particles, preparing it, in this way, for entrance into the lungs (Fig. 34).

The nostrils are separated from the mouth by a thin layer of bone, and back of both the mouth and the nostrils is the pharynx. The pharynx and the mouth serve as parts of the food canal, as well as air passages, and are[pg 080] described in connection with the organs of digestion (Chapter X). Air entering the pharynx, either by the nostrils or by the mouth, passes through it into the larynx. The larynx, being the special organ for the production of the voice, is described later (Chapter XXI). The entrance into the larynx is guarded by a movable lid of cartilage, called the epiglottis, which prevents food particles and liquids, on being swallowed, from passing into the lower air tubes. The relations of the nostrils, mouth, pharynx, and larynx are shown in Fig. 34.

From the larynx the air enters the trachea, or windpipe. This is a straight and nearly round tube, slightly less than an inch in diameter and about four and one half inches in length. Its walls contain from sixteen to twenty C-shaped, cartilaginous rings, one above the other and encircling the tube. These incomplete rings, with their openings directed backward, are held in place by thin layers of connective and muscular tissue. At the lower end the trachea divides into two branches, called the bronchi, each of which closely resembles it in structure. Each bronchus separates into a number of smaller divisions, called the bronchial tubes, and these in turn divide into still smaller branches, known as the lesser bronchial tubes (Fig. 33). The lesser bronchial tubes, and the branches into which they separate, are the smallest of the air tubes. One of these joins, or expands into, each of the minute lung sacs, or infundibula. Mucous membrane lines all of the air passages.

General Condition of the Air Passages.—One necessary condition for the movement of the air into and from the lungs is an unobstructed passageway.31 The air passages[pg 081] must be kept open and free from obstructions. They are kept open by special contrivances found in their walls, which, by supplying a degree of stiffness, cause the tubes to keep their form. In the trachea, bronchi, and larger bronchial tubes, the stiffness is supplied by rings of cartilage, while in the smaller tubes this is replaced by connective and muscular tissue. The walls of the larynx contain strips and plates of cartilage; while the nostrils and the pharynx are kept open by their bony surroundings.

Fig. 35—Ciliated epithelial cells. A. Two cells highly magnified. c. Cilia, n. Nucleus. B. Diagram of a small air tube showing the lining of cilia.

The air passages are kept clean by cells especially adapted to this purpose, known as the ciliated epithelial cells. These are slender, wedge-shaped cells which have projecting from a free end many small, hair-like bodies, called cilia (Fig. 35). They line the mucous membrane in most of the air passages, and are so placed that the cilia project into the tubes. Here they keep up an inward and outward wave-like movement, which is quicker and has greater force in the outward direction. By this means the cilia are able to move small pieces of foreign matter, such as dust particles and bits of partly dried mucus, called phlegm, to places where they can be easily expelled from the lungs.32

Fig. 36—Terminal air sacs. The two large sacs are infundibula; the small divisions are alveoli. (Enlarged.)

[pg 082]The Alveoli.—The alveoli, or air cells, are the small divisions of the infundibula (Fig. 36). They are each about one one-hundredth of an inch (¼ mm.) in diameter, being formed by the infolding of the infundibular wall. This wall, which has for its framework a thin layer of elastic connective tissue, supports a dense network of capillaries (Fig. 37), and is lined by a single layer of cells placed edge to edge. By this arrangement the air within the alveoli is brought very near a large surface of blood, and the exchange of gases between the air and the blood is made possible. It is at the alveoli that the oxygen passes from the air into the blood, and the carbon dioxide passes from the blood into the air. At no place in the lungs, however, do the air and the blood come in direct contact. Their exchanges must in all cases take place through the capillary walls and the layer of cells lining the alveoli.

Fig. 37—Inner lung surface (magnified), the blood vessels injected with coloring matter. The small pits are alveoli, and the vessels in their walls are chiefly capillaries.

Fig. 38.—Diagram to show the double movement of air and blood through the lungs. The blood leaves the heart by the pulmonary artery and returns by the pulmonary veins. The air enters and leaves the lungs by the same system of tubes.

Fig. 39—Diagram to show air and blood movements in a terminal air sac. While the air moves into and from the space within the sac, the blood circulates through the sac walls.

Blood Supply to the Lungs.—To accomplish the purposes of respiration, not only the air, but the blood also, must be passed into and from the lungs. The chief[pg 084] artery conveying blood to the lungs is the pulmonary artery. This starts at the right ventricle and by its branches conveys blood to the capillaries surrounding the alveoli in all parts of the lungs. The branches of the pulmonary artery lie alongside of, and divide similarly to, the bronchial tubes. At the places where the finest divisions of the air tubes enter the infundibula, the little arteries branch into the capillaries that penetrate the infundibular walls (Figs. 38 and 39). From these capillaries the blood is conveyed by the pulmonary veins to the left auricle.

The lungs also receive blood from two (in some individuals three) small arteries branching from the aorta, known as the bronchial arteries. These convey to the lungs blood that has already been supplied with oxygen, passing it into the capillaries in the walls of the bronchi, bronchial tubes, and large blood vessels, as well as the connective tissue between the lobes of the lungs. This blood leaves the lungs partly by the bronchial veins and partly by the pulmonary veins. No part of the body is so well supplied with blood as the lungs.

Fig. 40—The pleuræ. Diagram showing the general form of the pleural sacs as they surround the lungs and line the inner surfaces of the chest (other parts removed). A, A'. Places occupied by the lungs. B, B'. Slight space within the pleural sacs containing the pleural secretion, a, a'. Outer layer of pleura and lining of chest walls and upper surface of diaphragm. b, b'. Inner layer of pleura and outer lining of lungs. C. Space occupied by the heart. D. Diaphragm.

The Pleura.—The pleura is a thin, smooth, elastic, and tough membrane which covers the outside of the lungs and lines the inside of the chest walls. The covering of each lung is continuous with the lining of the chest wall on its respective side and forms with it a closed sac by[pg 085] which the lung is surrounded, the arrangement being similar to that of the pericardium. Properly speaking, there are two pleuræ, one for each lung, and these, besides inclosing the lungs, partition off a middle space which is occupied by the heart (Fig. 40). They also cover the upper surface of the diaphragm, from which they deflect upward, blending with the pericardium. A small amount of liquid is secreted by the pleura, which prevents friction as the surfaces glide over each other in breathing.

The Thorax.—The force required for breathing is supplied by the box-like portion of the body in which the lungs are placed. This is known as the thorax, or chest, and includes that part of the trunk between the neck and the abdomen. The space which it incloses, known as the thoracic cavity, is a variable space and the walls surrounding this space are air-tight. A framework for the thorax is supplied by the ribs which connect with the spinal column behind and with the sternum, or breast-bone, in front. They form joints with the spinal column, but connect with the sternum by strips of cartilage. The ribs do not encircle the cavity in a horizontal direction, but slope downward from the spinal column both toward the front and toward the sides, this being necessary to the service which they render in breathing.

How Air is Brought into and Expelled from the Lungs.—The principle involved in breathing is that air flows from a place of greater to a place of less pressure. The construction of the thorax and the arrangement of the lungs within it provide for the application of this principle in a most practical manner. The lungs are suspended from the upper portion of the thoracic cavity, and the trachea and the upper air passages provide the only opening to the outside atmosphere. Air entering the thorax must on[pg 086] this account pass into the lungs. As the thorax is enlarged the air in the lungs expands, and there is produced within them a place of slightly less air pressure than that of the atmosphere on the outside of the body. This difference causes the air to flow into the lungs.

Fig. 41—Diagram illustrating the bellows principle in breathing. A. The human bellows. B. The hand bellows. Compare part for part.

When the thorax is diminished in size, the air within the lungs is slightly compressed. This causes it to become denser and to exert on this account a pressure slightly greater than that of the atmosphere on the outside. The air now flows out until the equality of the pressure is again restored. Thus the thorax, by making the pressure within the lungs first slightly less and then slightly greater than the atmospheric pressure, causes the air to move into and out of the lungs.

Breathing is well illustrated by means of the common hand bellows, its action being similar to that of the thorax. It will be observed that when the sides are spread apart air flows into the bellows. When they are pressed together the air flows out. If an air-tight sack were hung in the bellows with its mouth attached to the projecting tube, the arrangement would resemble closely the general plan of the breathing organs (Fig. 41). One respect, however, in which the bellows differs from the thorax should be noted. The thorax is never sufficiently compressed to drive out all the air. Air is always present in the lungs. This keeps them more or less distended and pressed against the thoracic walls.

How the Thoracic Space is Varied.—One means of varying the size of the thoracic cavity is through the movements of the ribs and their resultant effect upon the walls[pg 087] of the thorax. In bringing about these movements the following muscles are employed:

1. The scaleni muscles, three in number on each side, which connect at one end with the vertebræ of the neck and at the other with the first and second ribs. Their contraction slightly raises the upper portion of the thorax.

2. The elevators of the ribs, twelve in number on each side, which are so distributed that each single muscle is attached, at one end, to the back portion of a rib and, at the other, to a projection of the vertebra a few inches above. The effect of their contraction is to' elevate the middle portion of the ribs and to turn them outward or spread them apart.

3. The intercostal muscles, which form two thin layers between the ribs, known as the internal and the external intercostal muscles. The external intercostals are attached between the outer lower margin of the rib above and the outer upper margin of the rib below, and extend obliquely downward and forward. The internal intercostals are attached between the inner margins of adjacent ribs, and they extend obliquely downward and backward from the front. The contraction of the external intercostal muscles raises the ribs, and the contraction of the internal intercostals tends to lower them.

Fig. 42—Simple apparatus for illustrating effect of movements of the ribs upon the thoracic space; strips of cardboard held together by pins, the front part being raised or lowered by threads moving through attachments at 1 and 2. As the front is raised the space between the uprights is increased. The front upright corresponds to the breastbone, the back one to the spinal column, the connecting strips to the ribs, and the threads to the intercostal muscles.

By slightly raising and spreading apart the ribs the thoracic space is increased in two directions—from front to back and from side to side. Lowering and converging the ribs has, of course, the opposite effect (Fig. 42). Except in forced expirations the ribs are lowered and converged by their own weight and by the elastic reaction of the surrounding parts.

[pg 088]The Diaphragm.—Another means of varying the thoracic space is found in an organ known as the diaphragm. This is the dome-shaped, movable partition which separates the thoracic cavity from the cavity of the abdomen. The edges of the diaphragm are firmly attached to the walls of the trunk, and the center is supported by the pericardium and the pleura. The outer margin is muscular, but the central portion consists of a strong sheet of connective tissue. By the contraction of its muscles the diaphragm is pulled down, thereby increasing the thoracic cavity. By raising the diaphragm the thoracic cavity is diminished.

The diaphragm, however, is not raised by the contraction of its own muscles, but is pushed up by the organs beneath. By the elastic reaction of the abdominal walls (after their having been pushed out by the lowering of the diaphragm), pressure is exerted on the organs of the abdomen and these in turn press against the diaphragm. This crowds it into the thoracic space. In forced expirations the muscles in the abdominal walls contract to push up the diaphragm.

Interchange of Gases in the Lungs.—During each inspiration the air from the outside fills the entire system of bronchial tubes, but the alveoli are largely filled, at the same time, by the air which the last expiratory effort has left in the passages. By the action of currents and eddies and by the rapid diffusion of gas particles, the air from the outside mixes with that in the alveoli and comes in contact with the membranous walls. Here the oxygen, after being dissolved by the moisture in the membrane, diffuses into the blood. The carbon dioxide, on the other hand, being in excess in the blood, diffuses toward the air in the alveoli. The interchange of gases at the lungs, however, is not fully understood, and it is possible that other forces than osmosis play a part.

Fig. 43—Diagram illustrating lung capacity.

Capacity of the Lungs.—The air which passes into and from the lungs in ordinary breathing, called the tidal air, is but a small part of[pg 089] the whole amount of air which the lungs contain. Even after a forced expiration the lungs are almost half full; the air which remains is called the residual air. The air which is expelled from the lungs by a forced expiration, less the tidal air, is called the reserve, or supplemental, air. These several quantities are easily estimated. (See Practical Work.) In the average individual the total capacity of the lungs (with the chest in repose) is about one gallon. In forced inspirations this capacity may be increased about one third, the excess being known as the complemental air (Fig. 43).

Fig. 44—Diagram illustrating internal respiration and its dependence on external respiration. (Modified from Hall.) (See text.)

Internal, or Cell, Respiration.—The oxygen which enters the blood in the lungs leaves it in the tissues, passing through the lymph into the cells (Fig. 44). At the same time the carbon dioxide which is being formed at the cells passes into the blood. An exchange of gases is thus taking place between the cells and the blood, similar to[pg 090] that taking place between the blood and the air. This exchange is known as internal, or cell, respiration. By internal respiration the oxygen reaches the place where it is to serve its purpose, and the carbon dioxide begins its movement toward the exterior of the body. This "breathing by the cells" is, therefore, the final and essential act of respiration. Breathing by the lungs is simply the means by which the taking up of oxygen and the giving off of carbon dioxide by the cells is made possible.