Therefore oxygen leaves the alveolus and enters the red blood cells. When a mammal breathes out the opposite happens. The concentration of carbon dioxide is lower in the alveolus than in the red blood cells so carbon dioxide leaves the red blood cells enters the alveolus and is exhaled. In an average human lung there are 480 million alveoli .
A pulmonary alveolus (plural: alveoli from Latin alveolus “little cavity”) is a hollow cup-shaped cavity found in the lung parenchyma where gas exchange takes place. Lung alveoli are found in the acini at the beginning of the respiratory zone.They are located sparsely in the respiratory bronchioles line the walls of the alveolar ducts and are more numerous in the blind-ended alveolar sacs.
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 w…