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 (1/4 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.