Editing Gas exchange (section)

===Birds===
{{Main|Bird anatomy#Respiratory system}}
[[File:BirdRespiration.svg|thumb|right|'''Fig. 10.''' Inhalation-exhalation cycle in birds.]]
[[File:Cross-current exchanger.jpg|thumb|300 px|left|'''Fig. 9.'''  A diagrammatic representation of the cross-current respiratory gas exchanger in the lungs of birds. Air is forced from the air sacs unidirectionally (from right to left in the diagram) through the parabronchi. The pulmonary capillaries surround the parabronchi in the manner shown (blood flowing from below the parabronchus to above it in the diagram).<ref name= graham /> Blood or air with a high oxygen content is shown in red; oxygen-poor air or blood is shown in various shades of purple-blue.]]
Birds have [[Bird anatomy#Respiratory system|lungs but no diaphragm]]. They rely mostly on [[air sacs]] for [[Ventilation (physiology)|ventilation]]. These air sacs do not play a direct role in gas exchange, but help to move air unidirectionally across the gas exchange surfaces in the lungs. During inhalation, fresh air is taken from the trachea down into the posterior air sacs and into the [[parabronchi]] which lead from the posterior air sacs into the lung. The air that enters the lungs joins the  air which is already in the lungs, and is drawn forward across the gas exchanger into anterior air sacs. During exhalation, the posterior air sacs force air into the same [[parabronchi]] of the lungs, flowing in the same direction as during inhalation, allowing continuous gas exchange irrespective of the breathing cycle. Air exiting the lungs during exhalation joins the air being expelled from the anterior air sacs (both consisting of "spent air" that has passed through the gas exchanger) entering the trachea to be  exhaled  (Fig. 10).<ref name=AvResp /> Selective [[bronchoconstriction]] at the various bronchial branch points ensures that the air does not ebb and flow through the bronchi during inhalation and exhalation, as it does in mammals, but follows the paths described above.

The unidirectional airflow through the parabronchi exchanges respiratory gases with a ''crosscurrent'' blood flow (Fig. 9).<ref name="graham"/><ref name="AvResp"/> The partial pressure of O<sub>2</sub> (<math>P_{{\mathrm{O}}_2}</math>) in the parabronchioles declines along their length as O<sub>2</sub> diffuses into the blood. The capillaries leaving the exchanger near the entrance of airflow take up more O<sub>2</sub> than capillaries leaving near the exit end of the parabronchi.  When the contents of all capillaries mix, the final <math>P_{{\mathrm{O}}_2}</math> of the mixed pulmonary venous blood is higher than that of the exhaled air, but lower than that of the inhaled air.<ref name= graham /><ref name=AvResp />