- Air flow in the mammal lung is bi-directional, due to cyclic volume changes in the lung itself, indirectly caused by movements of the diaphragm and intercostal muscles. The lungs may be divided into several lobes depending on species.
- Air flow in the bird lung is unidirectional and continuous, due to changes in air sac pressures on either end of the lung created by movements of the intercostal muscles. Lung volume is relatively constant.
- In the bird, gas exchange occurs in the parabronchi (3¡ bronchi), where radially projecting atria give rise to air capillaries which contact blood capillaries.
- In the mammal, gas exchange occurs in the sack-like alveoli, which are surrounded by blood capillaries. Alveolar walls are made of squamous type I cells and surfactant producing cuboidal type II cells. Macrophages on the air side of alveoli are actively phagocytic.
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