 surfact fetal lamb tracheal fluid lamb deliv cesarean intact fetal circul fluid fill trachea analysi reveal fluid contain materi high surfac activ lamb deliv term surfac activ prematur lamb administr 10 cent oxygen ew 1 hour prior deliveri alter surfact properti fetal tracheal fluid analys fetal tracheal fluid reveal 146 198 lipid 100 30 40 cent phospholipid activ compon surfact investig report offer model intrauterin factor pathogenesi hyalin membran diseas 