Alveolar size, number, and surface area: developmentally dependent response to 13% O2.

Nonpregnant female rats were kept in 13% O2 for greater than 3 wk before being bred, throughout pregnancy, and, with their pups, after birth; control rats were only in air. The average volume (v), number (N) and surface area (Sa) of gas-exchange structures (saccules or alveoli) were estimated by stereological means. Saccule conversion to alveoli by septation between age 2 and 14 days was impaired in 13% O2 rats; there was less decrease in v (signifying less septation) and less increase in N (indicating the formation of fewer alveoli) in 13% O2 pups than in air pups. Between age 14 and 40 days, v rose 2-fold in air pups and 1.3-fold in 13% O2 pups; N increased 1.7-fold in air rats and 2.8-fold in 13% O2 rats. In other experiments, 23-day-old rats, previously only in air, were continued in air or were placed in 13% O2 until 44-days-old. At age 44 days, Sa was 25% and v 27% greater in 13% O2 rats than in air rats, but N was the same in both groups. We conclude there are multiple mechanisms for forming alveoli and increasing Sa and these mechanisms exhibit a developmentally dependent response to 13% O2.