Due to its system of nine air sacs connected to the lungs, the respiratory system of birds is arguably the most effective respiratory system of all air-breathers. Upon inhalation, air first flows into posterior air sacs, then into the lungs, and then into anterior air sacs on the way to being exhaled. Thus, there is one-way flow of air through the lungs, along thousands of tubules called parabronchi. If Archaeopteryx had air sacs, then which of its features would have had the opposite effect on enabling Archaeopteryx to fly long distances from that provided by air sacs?