478 THE RESPIRATORY MECHANISM. 



of the pulmonary structures which belongs to the domain of 

 the suprarenal glands and to the vasoconstrictor nerves that 

 govern the flow of venous blood which carries their secretion 

 to the pulmonary lobules. 



If we now retrace our steps, a corresponding identity of 

 the two blood-systems, the bronchial and the respiratory, will 

 assert itself. The bronchial arteries follow the subdivisions of 

 the bronchi, but only as far as the lobular bronchiole. The 

 terminal arteriole does not penetrate the lobule, but furnishes 

 branches to the pulmonary connective tissue, to the lymphatic 

 ganglia, and to the pleura. This appears to us as an impor- 

 tant feature in pathogenesis: The bronchial arteries supply 

 the blood upon which the parenchyma depends for its oxidiz- 

 ing substance. Insufficiency of the adrenals, therefore, by re- 

 ducing the oxidation processes correspondingly reduces the nu- 

 trition of the pulmonary tissues: a predominating feature of 

 phthisis. 



The pulmonary artery that containing the adrenal secre- 

 tion in its venous blood also closely follows the bronchi and 

 their subdivisions, and only leaves them at the terminal bron- 

 chiole, where the alveolar ducts begin and subdivide into the 

 capillary net-works which closely enmesh the alveoli. The net- 

 works thus formed are the closest in the organism. The re- 

 turn arterialized blood passes into the venules, which carry it 

 to the pulmonary veins: the channels directly connected with 

 the left auricle. 



The veins of the bronchial system, however, do not return 

 their blood, as do similar vessels in other parts of the organism, 

 along the path of their arteries. The capillaries which are 

 distributed to the connective tissue and lymphatic structures 

 between the lobules, etc., return their blood to the superior 

 vena cava by the bronchial veins of the venas azygos, but those 

 which end at the alveoli anastomose with capillaries derived 

 from the pulmonary venous tree. Their blood is thus mixed 

 with that of the lobular capillaries, and is returned to the heart 

 with the now arterial blood of the pulmonary vein. 



We thus have two distinct vascular systems: the bron- 

 chial, the arteries of which supply blood to the bronchial and 

 peribronchial structures; and the respiratory, the vessels of 



