The Body at Work: A Treatise on the Principles of Physiology

by Alex Hill

The walls of the air-chambers, or alveoli, are formed of a membrane upon which is spread a network of capillary bloodvessels. The air-chambers are so closely packed together that a common wall separates one chamber from the next adjoining. Minute bloodvessels pierce the partitions which separate the chambers, appearing now on one side of the wall, now on the other. The air-chambers are lined by thin epithelial scales or tiles. The blood in the capillary vessels is separated from the air in the air-chambers by the wall of the capillary; by a lymph-space, probably rather potential than actual; and by the epithelial tiles. This covering suffices to prevent the escape both of red corpuscles and of plasma, yet offers very little resistance to the passage of gases from the blood into the air, and from the air into the blood.

Leucocytes make their way between the tiles, and creep over their internal surfaces, searching for cell débris or foreign matter. Anything that they find they carry to the clumps of lymphoid tissue which occur in the outer wall of the bronchi. In a town-dweller, leucocytes are found in these lymph-thickets, charged with particles of soot. They show droplets of fat and other evidences of degeneration. At other spots are to be seen little collections of soot which have been left behind after the dissolution of the leucocytes which brought them there.

It is not possible to make anything like an accurate estimate of the number of alveoli in the lungs; 725,000,000 is a figure arrived at by measuring the average cubic capacity of an alveolus, and comparing it with the total cubic capacity of the lungs. Each alveolus supports some forty or fifty capillary vessels. The superficial area of vascular membrane exposed is placed at 90 square metres, or about 100 times the area of the skin. Figures such as these convey very little meaning, but they help one to realize the magnitude of the provision made for the aeration of the blood.

Pneumonia is a condition in which the lining of the air-chambers is inflamed, usually, possibly always, owing to the entrance of bacteria. Lymph exudes through the walls of the alveoli. Epithelial scales flake off. Pus cells (dead leucocytes) accumulate in the air-chambers. Respiration is curtailed, and dyspnœa results. After a time, if the case progresses favourably, “resolution,” as it is technically termed, begins to occur. The exuded substances are either expectorated or absorbed, and the lung-tissue returns to a normal condition.

Here a few words may be devoted to respiratory sounds. Spirare means to sigh. Breathing received the name by which it is known in physiology from the sound which accompanies the exit of air from the nostrils. Since the introduction of auscultation as a means of ascertaining the condition of the lungs, other sounds, not heard until the ear or a stethoscope is placed against the chest, have acquired great importance. These sounds, termed “murmurs,” may be divided into two classes. (a) When the ear is placed against the windpipe, or in the middle of the back between the shoulder-blades, a murmur is audible, due to the movement of air through the larynx. If the larynx, the trachea, or the bronchi contain mucus, it is a harsh, rough, bubbling, or crackling sound. It accompanies both inspiration and expiration. (b) A softer, more delicate murmur is heard when the ear is placed against the front or the side of the chest. This is the vesicular or pulmonary murmur. It is heard during inspiration, and is due to the passage of air out of the smallest bronchi into the more spacious infundibula in which they end. These two kinds of murmur must be rigidly distinguished—the laryngeal murmur, heard in situations in which no lung-tissue intervenes between the ear and the great tracheal or bronchial tubes; and the pulmonary murmur, heard over all regions where the bronchi are buried in lung. Healthy lung is as bad a conductor of sound as a sponge or a wad of cotton-wool. The laryngeal murmur is inaudible in regions in which lung lies beneath the chest-wall. It would be far beyond the scope of this book to attempt to describe the very varied alterations in the chest-sounds which may be produced by disease. The student would do well to familiarize himself with the nature of the sounds which are heard in health, and the situations in which they are heard, in order that he may be able, in abnormal conditions, to recognize that something is wrong.

The chief departures from the normal may be grouped under the following heads: (1) The pulmonary murmur may lose its soft, smooth, sighing character owing to inflammation of the alveoli and infundibula. It may be as loud in expiration as in inspiration. Only a practised ear can estimate the significance of these changes. (2) The laryngeal murmur may be reinforced by “râles”—a convenient term for supplementary sounds. The source of such râles may be a cold in the chest, laryngitis, or bronchitis of various degrees. (3) The laryngeal murmur may be heard in situations in which lung intervenes between the ear and the larger bronchial tubes. This can be due only to the lung being in an abnormal condition as a conductor of sound. Instead of being as spongy as well-made Vienna bread, its air-spaces are filled with solid or fluid deposit. It is as firm as dough. To such a condition it attains at the height of pneumonia—a stage termed “hepatization” because in section it looks like liver rather than lung.

Breathing is the enlargement and diminution of the chest, which causes air to be drawn into and expressed from the lungs. The windpipe being open, the air inside the lungs is, of course, at the same pressure as the atmosphere. Expansion of the chest results in the equal expansion of the lungs. Since there is no air-space between the outer surface of the lungs and the inner surface of the chest-wall, the lungs cannot separate from the chest-wall when it expands. But the lungs contain elastic tissue always slightly on the stretch. If the chest be punctured, and air admitted between the chest-wall and the lungs, the lungs collapse. The expiratory movement, the contraction of the chest, is due to the elasticity of the lungs. This tendency on the part of the lungs to contract is sufficient in quiet respiration to restore the chest to its usual size after inspiration, and thus to expel air. The lungs are held open owing to the negative pressure in the space which separates them from the chest-wall. This negative pressure has a most important relation to their permeability by air. Imagine the condition reversed. Picture a lung into which air is forced by a muscular pump. After each stroke of the pump the lung would collapse. Its finest tubes and their dilated terminations could be maintained as open spaces, between the strokes of the pump, only by giving a considerable thickness and firmness to their walls. Such a substantial structure would be unfavourable to an interchange of gases between the blood and the air. The reverse of this condition is found in Nature. The lung is stretched from without. Its tissue, delicate as crêpe, cannot collapse even at the end of the deepest expiration.

The ribs are united by intercostal muscles, disposed in two sheets. The fibres of the external intercostals are directed downwards and forwards, those of the internal intercostals downwards and backwards. In tranquil respiration the chest is enlarged by the external intercostal muscles, which raise the ribs, and the diaphragmatic muscle, which renders peripheral portions of the diaphragm flat. The rôle of the internal intercostal muscles is a subject still under discussion. For the most part, physiologists regard them as accessory to expiration, but some hold that they combine with the external intercostals in raising the ribs and twisting them outwards during inspiration. The diaphragm is a partition which separates the thoracic from the abdominal cavity. It is in the form of a vault. The central portion of the dome is membranous, its margins muscular. Its membranous centre is in contact with the pericardium, which encloses the heart. The level of this part is therefore fixed, except in forced inspiration, when it descends slightly. It constitutes a fixed plane for the muscles of the diaphragm, which are attached below to the vertebral column and the ribs. When the muscles contract in inspiration, the curvature of the marginal portions of the diaphragm is diminished, and the chest-cavity consequently enlarged. During expiration the space between the muscle of the diaphragm and the chest-wall closes up, and the lower border of the lung slips out of it.

There is a marked difference in the relative extent of the costal and diaphragmatic movements in men and women. In women respiration is chiefly costal; in men it is chiefly diaphragmatic. In men the abdomen moves forwards, as the diaphragm descends in tranquil breathing; in women the chest rises. Men who wish, for the purposes of athletics, or singing, or public speaking, to retain the power of making the most of their chest-capacity are wise in not allowing themselves to fall into the habit of lazy, abdominal breathing.