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Just to some basics:
Gas exchange of carbon dioxide and oxygen occurs in the alveoli, small sac shaped branches of the lungs, grouped together similarly to grapes. When you breathe in, oxygen from the inhaled air diffuses through the walls of the alveoli into next door capillaries. Cells in the body produce carbon dioxide as they respire, which returns to the lungs to leave the body.
Back to the question:
To maximise this gas exchange - alveoli have a large surface area to volume ratio, to maximise rate of diffusion. They have walls that are extremely thin (one cell thick) so that the gases can diffuse quickly through the alveoli wall to the capillary or visa versa. Alveoli are lined in fluid to allow the gases to dissolve faster. Alveoli are surrounded by capillaries to maximise the surface area in which the gases can diffuse.