The lungs are a paired set of organs in your chest that bring oxygen-rich air into your body and allow you to get rid of a cellular waste product called carbon dioxide. Your lungs also perform a supporting role in your body's regulation of the relative acidity of your bloodstream.
Past your nose and mouth, the passageway to your lungs begins with your trachea, or windpipe, according to the Nemours Foundation. At its far end, your trachea branches off into two primary airways called the main stem bronchi, each of which enters a single lung. Inside each lung, the main bronchus further branches into smaller bronchi, then into even smaller pathways called bronchioles, which are about as thick as a human hair. Each of your lungs contains roughly 30,000 bronchioles. At its far end, each bronchiole contains clumps of minuscule air sacs called alveoli, which in turn have a covering of tiny blood vessels called capillaries. Your lungs have a total of roughly 600 million alveoli.
When you inhale, air enters your lungs through the bronchi, then passes down through the bronchioles into your alveoli, the Nemours Foundation explains. As the alveoli fill with air, your lungs expand. With each inward breath, oxygen passes from your alveoli to their surrounding capillaries, where it then attaches to red blood cells in your bloodstream. Next, this oxygen-rich blood passes to your heart, which pumps the blood and its oxygen supply to the rest of your body. Your cells then use this oxygen to perform various processes that keep you alive from day to day.
As your cells perform their various functions, they create carbon dioxide and other forms of waste, the Nemours Foundation notes. These waste products are passed into your bloodstream back to your lungs through the capillaries surrounding your alveoli. When you exhale, these wastes pass up through your alveoli, bronchioles and bronchi until they reach your windpipe and pass out of your body through your mouth or nose. In addition to passing waste material, your lungs also transfer heat from your body's interior, thereby helping you regulate your internal temperature.
The concentration of carbon dioxide and certain other substances in your bloodstream influences your blood's relative acidity, or pH, according to Discovery Health. The concentration of these substances also influences the acidity of your cerebrospinal fluid. In turn, the acidity of your blood and cerebrospinal fluid affects the part of your autonomic--or unconscious--nervous system that controls your breathing rate. As a result, when your blood levels of carbon dioxide rise, your breathing rate automatically increases and your lungs remove the carbon dioxide buildup from your body. Your breathing rate will also increase if your blood oxygen levels fall.
The presence of irritating substances, such as pollen and smoke, in your airways also triggers part of your autonomic nervous system, Discovery Health notes. In response to this trigger, the muscles associated with your lungs contract, forcing you to cough or sneeze. In turn, coughs and sneezes clear irritants from your airways and lungs.