Which of the following statements about bird respiration is false?
a. air flowing through the air channels in the lungs moves in a tidal fashion.
b. it takes two ventilation cycles for a single breath of air to move through a bird's respiratory system.
c. the unidirectional flow of air through a bird's lungs maintains a larger concentration gradient for diffusion than can be obtained through tidal breathing.
d. bird lungs receive fresh air during both inhalation and exhalation.
e. oxygen is exchanged between the air and the bloodstream in the lungs, but not in the air sacs.