تبادل الغازات
The gas exchanger in mammals is internalized to form lungs, as it is in most of the larger land animals. Gas exchange occurs in microscopic dead-end air-filled sacs called alveoli, where a very thin membrane (called the blood-air barrier) separates the blood in the alveolar capillaries (in the walls of the alveoli) from the alveolar air in the sacs.