Respiration : Part 2

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 8 of 28)

General Condition of the Air Passages. - One necessary condition for the movement of the air into and from the lungs is an unobstructed passageway. The air passages 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.

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. 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.

The Alveoli. - The alveoli, or air cells, are the small divisions of the infundibula. They are each about one one-hundredth of an inch 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, 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.

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 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. 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.

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 which the lung is surrounded, the arrangement being similar to that of the pericardium. Properly speaking, there are two pleura, one for each lung, and these, besides inclosing the lungs, partition off a middle space which is occupied by the heart. 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 encloses, 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 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.

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. Therefore 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.

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