CHAPTER VII

- RESPIRATION

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 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 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]

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 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]

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 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 (