Airflow the exhaled air. In mammals, the

Airflow in birds is unidirectional through a pair of small
yet rigid lungs and nine thin-walled, expandable air sacs. Unlike in mammals,
the volume of the lungs doesn’t change to effect airflow; the air sacs work
like bellows to create a continuous flow of air through the lungs – the system
is more efficient than a bidirectional air flow and allows for the high
metabolic rate found in birds.

Physiology

1. The cycle begins when inspiratory muscles contract,
moving the sternum outwards, increasing the volume of the air sacs and
decreasing the pressure in the cavity below atmospheric pressure.

2. Oxygenated air enters the mouth or nares. The air passes
the pharynx and into the trachea, the trachea divides into two primary bronchi
at the syrinx.

3. The air sacs exist in two functional groups, the cranial
air sacs and the caudal air sacs, they extend from the main bronchus at cranial
and caudal ends respectively. Air enters the caudal air sacs first, due to
aerodynamic valving,  air previously in
those sacs being drawn through the lungs into the cranial air sacs as they
expand.

4. Primary bronchi enter the lungs and are then known as
intrapulmonary bronchi, these branch into smaller dorsobronchi, which branch
into even smaller parabronchi (Figure 1).

5. The parabronchi contain hundreds of tiny inlets called
air capillaries. They are surrounded by a network of blood capillaries, this is
where gas exchange takes place via diffusion. After passing through the
parabronchi, the now deoxygenated air moves into the ventrobronchi, which then
connects back to the main bronchus.

6. Contraction of the expiratory muscles pulls the sternum
towards to spine, reducing the volume of the system and increasing pressure
within the air sacs. Air from the caudal air sacs is pushed through the lungs
into the cranial air sacs, with the air in the cranial air sacs being expelled
through the trachea and then out of the mouth or nares. (Sjaastad, 2010)

Cross-current Gas Exchange

The parabronchi’s air capillaries and blood capillaries cross
each other perpendicularly (Figure 2), this is to sustain a high concentration
gradient and increase the amount of O2 diffusion into the blood. Capillaries
towards the start of the parabronchi contain air with a higher pO2 than those
at the end. The overall pooled capillary blood returning to the heart has a
higher pO2 than the exhaled air. In mammals, the partial pressures remain
relatively constant, this means they can never achieve blood with a higher pO2
than exhaled air. This means birds can adequately oxygenate their blood at much
higher altitudes than mammals can. (Sjaastad, 2010)

How does it differ from a typical mammal?

• Birds have a higher tidal volume, and therefore lower
respiratory rate, than mammals. This is beneficial as there is a large amount
of dead space within the bird, the increased tidal volume allows more of the
air in the system to be used for gas exchange.

• They also have pneumatic bones which contain some of the
air sacs (Figure 3). They are beneficial for flying as they decrease the
overall weight of the animal, as there is no marrow, but do not compromise
strength, as avian bones are denser than those of mammals. (Dumont, 2010)