338 Pulmonic Interstitial Emphysema 



wall within and the ring of pulmonic alveoli without. Specifically, the outer 

 boundary of this vascular sheath is made up of the contiguous bases of the 

 perivascular alveoli which, collectively, form a mosaic or pavementlike layer. 

 This extremely thin membrane is all that intervenes between the air of the 

 lung and the fluid-filled connective tissue of the underlying vascular sheath. 



Now the interesting thing, functionally, about this perivascular ring of 

 alveoli (or rather branched tunnel system, for the alveolar layer is continuous 

 around the pulmonary blood vessels from the smaller branches to the larger 

 trunks) is that it ividens with inspiration. This action has the effect of in- 

 creasing the caliber of the pulmonary arteries and veins during inspiration, 

 and thus of lowering the resistance of the pulmonary circulatory system. In 

 other words, the inspiratory enlargement of this distensible perivascular en- 

 velope of alveoli steepens the down-gradient from right ventricle to left 

 atrivun. It is of less importance, at this juncture, to remark that this widening 

 of the arterial and venous streambeds of the pulmonary system is accom- 

 plished by the stroma pull of the lung, which is motivated by the pressure of 

 the incoming air, which, in turn, is permitted to act thus because of the muscu- 

 lar action of the inspiratory mechanism; that, through this functional anatom- 

 ical conception, further light is shed on the fortunate natural arrangement 

 whereby the operation which brings air into the lungs synchronously brings 

 more blood into them; and that this explanation of an inspiratory "swelling 

 action" on the pulmonary blood vessels makes less puzzling, perhaps, such 

 cryptic terms as "negative pressure" and "suction." Rather for us, now, the 

 important thing to note is that this perivascular tunnel system of alveoli does 

 widen during inspiration with, consequentially, a certain amount of stretch- 

 ing of each and every alveolar base which separates the air from the under- 

 lying connective tissue of the vascular sheath; and that this stretching due 

 to widening of the tunnel is augmented by that due to its lengthening, for 

 inspiratory lengthening of the pulmonary blood vessels and their environ- 

 mental structures is just as much a part of inflational adjustment as is the 

 lengthening of the bronchial tree during the same respiratory phase.^ This 

 much is physiology. 



As long as the swelling of the blood vessel keeps pace with the enlargement 

 of the surrounding zone of alveoli there is no disturbance of the pressure rela- 

 tions between the alveolar air and the underlying connective tissue; or, more 

 explicitly, there is no tendency for air to break through the thin alveolar 

 bases and invade the vascular sheath. All that happens in the sheath is a passive 

 enlargement in diameter in accommodation with the similar enlargements 

 of its neighbors. But if the alveolar zone becomes overdistended without there 

 being a simultaneous and equivalent swelling of the blood vessel, then a down- 

 ward pressure gradient is set up between air and connective tissue, and in con- 

 sequence minute ruptures may be formed in the thin alveolar bases, weakened 

 through divarication of their network of fibers, through which air enters the 

 sheath. The same imbalance may occur if the blood vessel shrinks (as it does 



