air flow through bird lungs bird respiratory anatomical dead space

The process of air flow rectification which results in unidirectional ventilation of the parabronchi is mainly dependent on aerodynamic forces acting at the junctions between the mesobronchus and secondary bronchi although active constriction of the airways by bronchial smooth muscle cannot be ruled out.

The roles of the cranial and caudal air sacs are reflected in their gas composition, gas in the cranial sacs being similar to end-expired gas, that in the caudal sacs much closer to atmospheric. The caudal sacs draw a mixture of inspired and dead space air from the mesobronchus and store it until expiration when it is used to aerate the parabronchi for a second time. During inspiration the cranial sacs draw end-expiratory air out of the parabronchi and replace it with a stream of relatively fresh air from the mesobronchus. The air sacs can be divided into two groups, a cranial group connected to the ventrobronchi and a caudal group connected to the caudal end of the mesobronchus. The ventrobronchi and dorsobronchi are connected by numerous parabronchi which occupy the bulk of the lung and are responsible for gas exchange. Within each lung a primary bronchus or mesobronchus gives rise to two sets of secondary bronchi: the ventrobronchi, arising from the cranial part of the mesobronchus and the dorsobronchi, arising from the caudal part of the mesobronchus. The respiratory system of birds consists of a pair of inexpansible lungs that are ventilated by a series of air sacs.

Air flow through bird lungs bird respiratory anatomical dead space