RESPIRATORY SYSTEM, DISEASES OF. The great extent of the respiratory system reaching from the lips and nostrils to the ultimate air cells of the lungs, the numerous muscles, in trinsically and extrinsically concerned with respiratory movements, the object of respiration, to wit, aeration of the blood, the mani fest interrelationships between heart, kidneys, skin, nervous system and lungs, are sufficient to indicate that diseases of the respiratory system cover a very wide range. Many of the subjects belonging thereto are dealt with in special articles, notably ASTHMA ; BRON CHITIS; BRONCHIECTASIS ; DIPHTHERIA; EMPHYSEMA; IN FLUENZA; LARYNGITIS; MINERS' PHTHISIS ; PHTHISIS ; PLEU RISY; PNEUMONIA ; PNEUMOTHORAX ; TUBERCULOSIS ; WHOOPING COUGH ; HEART AND LUNG, SURGERY OF ; EAR, NOSE AND THROAT, DISEASES OF ; PLEURO-PNEUMONIA ; RADIOTHERAPY.
But viewed from another standpoint these diseases are rela tively simple. The pathological processes underlying them are the familiar ones of inflammation and new growths and the symptoms produced depend upon the particular part of the tract involved. For example, in diphtheria the disease may affect the tonsils and soft palate ; in this case the local symptoms are relatively small and the constitutional effects produced by the absorbed diphtheria toxin are great. Or it may affect the larynx when the local ob struction dominates the picture. Or again, from the larynx the disease may extend to the bronchioles by way of the trachea, bronchi and their branches and lead to a bronchopneumonia sim ilar, except for its ultimate aetiology, to that met with in measles, whooping cough or influenza. This bronchopneumonia, in its turn, resembles that constituting pulmonary tuberculosis with such modifications as depend upon the acuteness or chronicity of the tuberculous inflammatory process and the extent to which it is simple or complicated by secondary infections. The same is true with regard to new growths ; the type of growth may or may not be histologically the same in different parts of the tract but the symptoms produced depend upon the situation of the growth and its size and behaviour relative to the region it affects.
Wide Inter-relationships.—The far-reaching effects of respi ratory diseases may only become obvious on special occasions. The levator ani muscle seems far removed from the possible effects of morbid conditions in the throat or lungs, and yet if an abscess be forming in the neighbourhood of the anus, the act of coughing causes a sharp stab of pain locally because the levator is an extraordinary muscle of respiration and is called into play in coughing. This example serves to show how nerve conditions such
as paralysis of the phrenic nerve which supplies the diaphragm (q.v.) or of intercostal nerves which supply the intercostal mus cles, or morbid processes in other regions such as peritonitis, may interfere with the normal expansion and contraction of the lung tissue and induce pulmonary changes by disturbing the muscular movements that control them.
The pleura, lying between the lung and the thoracic wall, is in the main liable to disease by extension from one side or the other and rarely escapes involvement. Pulmonary tuberculosis is always associated with tubercle of the pleura and in cancer of the breast the disease often extends to the pleura. In both instances nodules in the normally smooth membrane lead to inflammatory changes with exudation of fluid into the serous cavity and adhesions be tween its walls.
If attention be directed more particularly to the lungs and pleura, the concentration of pulmonary and systemic vascular systems in the heart, with participation of each side of the heart in both circulations, results in a peculiar relationship of diseases of the respiratory system with those of the heart. Regurgitation at the mitral valve is immediately felt throughout the lungs and as far back as the pulmonary valves, as is evidenced by the accentuated pulmonary second sound. (See HEART : AUSCULTA TION.) The rise of blood pressure within the lungs is borne ulti mately by the right ventricle, and failure here, with the resulting incompetence of the tricuspid valve, leads to pulmonary conges tion and exudation of oedematous fluid into the air spaces and into the pleural cavities. Similarly, some diseases of the kidney, indirectly by reason of the changes they induce in the heart, and, perhaps, directly by the altered composition of the blood they imply, lead to pulmonary and pleural congestion and exudation. Blood conditions, too, whether dependent upon anaemia or circu lation of toxic materials, bacterial or other, act upon the heart muscle and secondarily upon the lungs. It is in this way that so-called hypostatic congestion and pneumonia are produced in the later stages of acute febrile disorders.