TTTK RT:SPTRATf)RV SYSTEM 



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alveoli is undoubtedly of importance in determining the contraction 

 and expansion of the alveoli under varying conditions of pressure. It 

 has been estimated that on forced inspiration an alveolus can expand 

 to three times its resting capacity. Each alveolus communicates 

 not only with its alveolar passage, or alveolar bronchus, by means of a 

 broad opening, but alveoli arc connected with one another by 

 minute openings in their walls. 



The interaheolar connective tissue, while extremely small in amount, 

 serves to separate the alveoli from one another. Somewhat thicker 

 connective tissue separates the alveoli of one alveolar passage from 



Fig. 217. — Section Through Three AlveoU of Human Lung. _X235. Weigert's 

 elastic-tissue stain (technic 3, p. 28) to show arrangement of elastic tissue, a, Alveolus 

 cut through side walls only; b, alveolus cut through side walls and portion of laottom or 

 top; c, alveolus in which either the bottom or top is included in section. 



those of another. Still stronger connective-tissue bands as already 

 noted separate adjacent lobules. 



Blood-vessels. — Two systems of vessels distribute blood to the 

 lungs. One, the bronchial system, carries blood for the nutrition of 

 the lung tissue. The other, the much larger pulmonary system^ 

 carries blood for the respiratory function (Fig. 213). 



The bronchial artery and the pulmonary artery enter the lung at its 

 hilum. Within the lung the vessels branch, following the branchings 

 of the bronchi, which they accompany (Fig. 213). The pulmonary 

 vessels are much the larger and run in the connective tissue outside 

 the . bronchial walls. The bronchial vessels lie within the fibrous 

 coat of the bronchus. A section of a bronchus thus usually shows 



