The |lungs| are the two sponge-like sacs which expand with diaphragmatic |contraction| to admit ~air~, and which house the |alveoli| where oxygen and carbon dioxide diffusion regenerates |blood cells|. The |lungs| are divided into the right and left halves, which have three and two |lobes|, respectively. Each half is anchored by the mediastinum and rests on the |diaphragm| below. The medial surace of each half features an aperture, called a |hilus|, through which the |bronchus|, ~nerves~, and ~blood~ vessels pass. The bronchii lead to narrower subdivisions, called |bronchioles|. These, in turn, branch off to the alveolar ducts which lead to grape-like clusters of |alveoli| in the alveolar sacs. Oxygen poor ~blood~ is pumped to the |lungs| from the |heart| through the |pulmonary artery|. This |artery| splits to go to each |lung|, subdividing into |arterioles| and metarterioles deep within the |lung| tissue. These metarterioles lead to networks of smaller vessels, called |capillaries|, which pass through the alveolar surface. The ~blood~ diffuses waste carbon dioxide through the membranous wall of the |alveoli| and takes up oxygen from the ~air~ within. The regenerated ~blood~ is then sent to metavenules and |venules| which are tributaries to the |pulmonary vein|. This |vein| takes the regenerated ~blood~ back to the |heart| to be pumped throughout the body for nourishment of its cells.
A number of terms are used to refer to |lung| capacity: tidal ~air~, complemental ~air~, supplemental (reserve) ~air~, vital capacity, residual ~air~, minimal ~air~, and total |lung| capacity. Each of these terms refers to a different aspect of the |lung|'s ~air~ capacity. Total |lung| capacity refers to all of the ~air~ which can be exhaled from the |lungs|, plus any residual ~air~ left in the |lung|'s chambers. One can never completely exhale all of the ~air~ from the |lungs|, as this would require the |lungs|, the bronchii, and the |bronchioles| to collapse. Even then, some ~air~ remains in the |alveoli|, and this is called the minimal ~air~. The maximum ~air~ which can be exhaled is called the vital capacity (about a gallon, or just under four liters, on average), with the residual ~air~ being what is left in the |lungs| (a quart, or liter, on average). The total |lung| capacity, therefore, is about five quarts (just under five liters) of ~air~. Tidal ~air~ is that ~air~ inhaled and exhaled in normal respiration (about a pint, or half of a liter). Following a normal expiration, the amount of ~air~ taken in by the deepest inhalation possible is referred to the complemental ~air~, and amounts to about three quarts (just under three liters). Following a normal inspiration, the amount of ~air~ expelled by the greatest exhalation possible is called the supplemental, or reserve, ~air~, and amounts to about a quart (just under a liter) of ~air~. Diseases and disorders of the |lungs|, such as |tobacco|-induced |lung| disease, emphysema, bronchitis, pneumonia, and |asthma|, can drastically diminish the capacity of the |lungs|.
