The bronchial |arterioles| and |venules| supply ~blood~ to the alveolar sacs for regeneration and carry the regenerated ~blood~ back to the |heart|, respectively. The |arterioles| branch off of the |pulmonary artery|, which originates at the |heart|. These |arterioles| lead to smaller vessels called metarterioles which, in turn, lead to tiny |capillaries| in the alveolar tissue. The semipermeable membrane of the capillary wall allows oxygen to transport from the ~air~ to the |blood cells| (binding to the ~hemoglobin~ in ~blood~), while allowing excess carbon dioxide and other waste gases to transport from the ~blood~ to the ~air~ to be exhaled. The |capillaries| then carry the |blood cells| to larger vessels, called metavenules, which lead to |venules| and then to the |pulmonary vein|. The |pulmonary vein| returns this regenerated ~blood~ to the |heart| to be pumped throughout the body. It is worthwhile to note that, in most graphic representations, as in the body itself, oxygen-poor ~blood~ is blue or dark purple, while oxygen-rich ~blood~ is bright red. For most parts of the body, therefore, the arteries and |arterioles| are depiced as red and the |veins| and |venules| as blue. In the |lungs|, however, the reverse is true. ~Blood~ passing through the |pulmonary artery| and |arterioles| is oxygen-poor (and is shown here as blue) while the ~blood~ passing back to the |heart| through the |pulmonary vein| and |venules| is oxygen-rich (and so is depicted as the red ~blood~ vessel in the diagram).
