Respiration : Part 3

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 9 of 28)

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. 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 of the thorax. In bringing about these movements the following muscles are employed:

1. The scalene muscles, three in number on each side, which connect at one end with the vertebra 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 intercostals muscles, which form two thin layers between the ribs, known as the internal and the external intercostals 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 intercostals muscles raises the ribs, and the contraction of the internal intercostals tends to lower them.

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. Except in forced expirations the ribs are lowered and converged by their own weight and by the elastic reaction of the surrounding parts.

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.

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 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. 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.

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. At the same time the carbon dioxide which is being formed at the cells passes into the blood. An exchange of gases is therefore taking place between the cells and the blood, similar to 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.

Hygiene of Respiratory Organs

The liability of the lungs to attacks from such dread diseases as consumption and pneumonia makes questions touching their hygiene of first importance. Consumption does not as a rule attack sound lung tissue, but usually has its beginning in some weak or enfeebled spot in the lungs which has lost its "power of resistance." Though consumption is not inherited, as some suppose, lung weaknesses may be transmitted from parents to children. This, together with the fact, now generally recognized, that consumption is contagious, accounts for the frequent appearance of this disease in the same family. Consumption as well as other respiratory affections can in the majority of cases be prevented, and in many cases cured, by an intelligent observation of well-known laws of health.

D.C. Heath and Co. - Publishers
Original copyright 1909