The lungs have two primary functions: to acquire oxygen from the
air, which is required for life, and to remove carbon dioxide
from the body. Carbon dioxide is a byproduct of many of the
chemical reactions that sustain life. During breathing, air
enters and exits the lungs. It flows in through increasingly
smaller airways, finally filling tiny sacs called alveoli. Blood
circulates around the alveoli through capillaries (tiny blood
vessels). Where the capillaries and alveoli meet, oxygen crosses
into the bloodstream. At the same time, carbon dioxide crosses
from the bloodstream into the alveoli to be exhaled. The lungs
are continuously being exposed to particles in the air,
including smoke, pollen, dust, and microorganisms. Some of these
inhaled substances can cause lung disease if enough is inhaled
or if the body is particularly sensitive to them. People
normally make new alveoli until about age 20. After that, the
lungs begin to lose some of their tissue. The number of alveoli
decreases, and there is a corre