Editing Gas exchange (section)

==Mammals==
The gas exchanger in mammals is internalized to form lungs, as it is in most of the larger land animals.{{citation needed|date=July 2017}} Gas exchange occurs in microscopic dead-end  air-filled sacs called [[pulmonary alveolus|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.
[[File:An alveolus, is an anatomical structure that has the form of a hollow cavity. Mainly found in the lung, the pulmonary alveoli are spherical outcroppings of the respiratory bronchioles and are the.png|thumb|300 px|'''Fig. 3.'''  An alveolus (plural: alveoli, from Latin alveus, "little cavity"), is an anatomical structure that has the form of a hollow cavity. They occur in the mammalian lung. They are spherical outcroppings of the respiratory bronchioles and are the primary sites of gas exchange with the blood.]][[File:Alveolar Wall.svg|thumb|300 px|left|'''Fig. 4.'''  A histological cross-section through an alveolar wall showing the layers through which the gases have to move between the blood plasma and the alveolar air. The dark blue objects are the nuclei of the capillary [[endothelial]] and alveolar type&nbsp;I&nbsp;[[epithelial]] cells (or type&nbsp;1&nbsp;[[pneumocyte]]s). The two red objects labeled "RBC" are [[red blood cell]]s in the alveolar capillary blood.]]