Numerous, small air spaces called air capillaries open into the atria. Larger blood vessels between the parabronchi form capillaries which surround the air capillaries.
Gas exchange occurs at the level of the air capillaries.
The avian esophagus is similar to other species but large mucous glands are present in the submucosa. The avian crop is lined with stratified squamous epithelium which is sloughed to form crop milk in some species. The crop milk is fed to the young. It is characterized by mucosal and submucosal glands. The submucosal glands are very large.