Complex sentence: Avian lungs do not have alveoli as mammalian lungs do, but instead contain millions of narrow passages known as parabronchi, connecting the dorsobronchi to the ventrobronchi at either ends of the lungs.
Split into simpler sentences: Avian lungs do not have alveoli as mammalian lungs do. Instead they contain millions of narrow passages known as parabronchi, connecting the dorsobronchi to the ventrobronchi at either ends of the lungs.