PHYSIOLOGY OF RESPIRATION. 



OBJECTS AND SUBDIVISIONS. 



The purpose of respiration is to convey to the body the oxygen 

 necessary for its oxidation-processes, as well as to remove the carbon 

 dioxid resulting from the combustion processes. The activity required 

 for this purpose is most effectively rendered by the lungs. A distinction 

 is made between external and internal respiration. The first embraces 

 the exchange of gases between the outer air and the gases of the blood 

 contained in the respiratory organs (lungs and skin) ; the second in- 

 cludes the exchange of gases between the capillary blood of the systemic 

 circulation and the body tissues. 



STRUCTURE OF THE AIR-PASSAGES AND THE LUNGS. 



The lungs are compound tubular (grape-like?) glands that secrete carbon 

 dioxid, and each of which sends its excretory duct (bronchus) to the common air- 

 passage, the trachea. 



The trachea has for its foundation a number of C-shaped, superposed, hya- 

 line, cartilaginous arches, held together by a rigid fibrous membrane of closely 

 woven elastic network, intermixed with connective tissue, arranged principally 

 in a longitudinal direction. The cartilages serve the function of keeping the 

 lumen of the tube patulous under the varying pressure-relations. They subserve 

 a similar purpose in the bronchi and their branches. They do not occur in air- 

 passages having a diameter of i mm. or less; and even in bronchioles of greater 

 size they are less numerous and more irregular, occurring especially at the bifurca- 

 tions in the form of irregular platelets. 



An outer layer of connective and elastic tissue covers the air-passages and 

 branches of the bronchial tree. On the side toward the esophagus this layer is 

 reinforced by additional elastic elements and a few bundles of longitudinal un- 

 striated muscle-fibers. The trachea contains unstriated muscle-fibers, especially 

 arranged transversely, connecting the ends of the cartilaginous arches posteriorly 

 and being inserted into the cartilages by means of elastic tendons. This transverse 

 layer is again covered by longitudinal bundles. The mucous membrane, besides 

 containing connective tissue and leukocytes, is especially rich in longitudinal 

 elastic fibers, which attain their greatest size immediately beneath the epithelial 

 basement membrane. The outer, narrow, scarcely separable submucosa is com- 

 posed principally of connective tissue, and attaches the mucous membrane to the 

 cartilages with their connecting fibrous membrane. The epithelium of the trachea 

 is a stratified, ciliated epithelium, with the cilia waving toward the glottis, and 

 with many interspersed goblet-cells. Numerous branched, tubular, mucous 

 glands, with larger, brighter cells and smaller, darker ones (Gianuzzi's crescents) 

 are founxl beneath the muscular layer of the trachea and bronchi. These glands 

 are of a mixed type and have secretory ducts connected with their serous alveoli, 

 but not with the mucous tubules. They secrete the viscid mucus that catches the 

 dust -particles of the inspired air and is then removed from the bronchial tree and 

 larynx by means of the ciliated epithelium. The air-passages are richly supplied 

 with lymph-vessels and lymph-follicles, but are rather poor in nerves and blood- 

 vessels. Ganglia are found on the nerve-trunks. 



The direction in which the branches of the bronchi penetrate into their respec- 

 tive lobes corresponds with the inspiratory movement of the chest-wall covering 

 each lobe; for example, the direction of the bronchi in the upper lobe is upward, 

 forward, and outward. 



The small bronchi are distinguished from the larger ones by a diminution in 



201 



