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 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 (Figs. 38 and 39). 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.
Fig. 40—The pleuræ. Diagram showing the general form of the pleural sacs as they surround the lungs and line the inner surfaces of the chest (other parts removed). A, A'. Places occupied by the lungs. B, B'. Slight space within the pleural sacs containing the pleural secretion, a, a'. Outer layer of pleura and lining of chest walls and upper surface of diaphragm. b, b'. Inner layer of pleura and outer lining of lungs. C. Space occupied by the heart. D. Diaphragm.
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[pg 085] which the lung is surrounded, the arrangement being similar to that of the pericardium. Properly speaking, there are two pleuræ, one for each lung, and these, besides inclosing the lungs, partition off a middle space which is occupied by the heart (Fig. 40). 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 incloses, 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[pg 086] 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.
