Physiology and Hygiene for Secondary Schools. Francis M. Walters
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