Page 68 - The Origin of Birds and Flight

66 The Origin of Birds and Flight

The lungs of vertebrate terrestrials have a two-directional structure.
During inhalation, air proceeds down ever-branching passageways and
ends up at tiny air sacs known as alveoli, where oxygen and carbon diox-
ide are exchanged. The CO2-laden air later returns by the same route,
being expelled via the main bronchial tube.
But in a bird’s lung, air always follows a one-way route. The lung's
entrance and exit channels are different from one another, and the air
always flows in one direction. The bird thus has uninterrupted access to
the oxygen in the air.
Evolutionist author Henry Gee, editor of the international scientific
journal Nature, says, “Birds have a remarkable breathing arrangement, in
which the lungs form just one part of a one-way air handling system that
incorporates large air spaces elsewhere in the body and even within the
hollowed bones.” 46
When the bird breathes, air flows from the windpipe both to the lung
and to the rear air sacs. The air already present in the lung moves to the
front air sac. When the bird exhales, clean air in the rear air sac flows to
the lung and leaves the body via the front air sac. Both cycles, in inhala-
tion and exhalation, must occur for every breath the bird takes. Instead
of the alveoli in mammals’ lungs, millions of tiny tubes extend all the
way along the bird lung. This complex system of air sacs ensures that the
air in the bird lung constantly flows in one direction, from back to front.
Thus the direction of air flow is different from that in lizards or mam-
mals, where air retraces its route during exhalation. The fact that air
always flows in a single direction in birds allows them to use the air’s
oxygen more efficiently.
This efficient respiratory system unique to birds reduces the amount
of energy they require to take off and remain aloft. Well-developed
breast muscles, attached to the furcula bone—one of the structures
essential to flight—add power to each wingbeat. The long wing feathers
generate the necessary lift.
Birds have no diaphragm, and therefore make use of pressure differ-
ences in the air sacs within their bones to move the air in their lungs.