The |capillary network| in the alveolar tissue allows transmission of gases between the ~air~ in the |alveoli| and the |blood cells| within the |capillaries|. The tiny |capillaries| are so small that they only allow |blood cells| to pass through them one at a time. This single-file order, combined with the delicate, semipermeable membrane separating the alveolar sac from the |capillaries|, allows osmosis to occur. Osmosis is the act of a substance (in this case, oxygen and carbon dioxide) passing through a semipermeable membrane from a region of high concentration to one of lower concentration. The |blood cells| passing through the |capillaries| are depleted of oxygen and rich in carbon dioxide and other waste gases. As a result, the carbon dioxide diffuses through the membrane to the ~air~ within the alveoli (which is less rich in carbon dioxide). Similarly, the oxygen in the ~air~ within the |alveoli| osmoses through the membrane to the |blood cells|. In this way, the ~blood~ gets rid of excess carbon dioxide (which is subsequently exhaled) and is regenerated by oxygen. The regenerated |blood cells| continue on through the pulmonary metavenules, |venules|, and |vein| to the |heart|, from which it is pumped to the rest of the body.
