The mesenchyme of the lung becomes highly vascular, and becomes especially rich in lymph vessels. Epithelial branching will occur with lung mesenchyme, but not with other nearby mesenchymes. However, some more distant mesenchymes will provoke lung formation. There are about 2,000 segments of capillary around each alveolus. The lung of a newborn is, of course, smaller than an adult's lung, but size is so important to function in the lung that lungs of newborns are disproportionately large.
Smaller airways are only about one half the size of adult's.
Such small airways and alveoli would offer fatal resistance to air passage. Even though the smaller air passages are half adult size, their resistance to air flow is 5 to 6 times that of the adult. Up to the early 1970's, about 25,000 newborns died each year from respiratory difficulties. Most of these infants were premature. Immature lungs cannot retain air.