In mechanical terms, our lungs can be described as the site of gas exchange: Oxygen--the fuel all the cells and organs of our body need to function--is extracted there from the air we inhale and infused into the bloodstream, to be distributed to other organs and tissues. With each exhalation, we dispose of the carbon dioxide that is the by-product of our bodily processes. In our lungs, in the course of a single day, an astonishing 8,000 to 9,000 liters of breathed-in air meet 8,000 to 10,000 liters of blood pumped in by the heart through the pulmonary artery. The lungs relieve the blood of its burden of waste and return a refreshed, oxygen-rich stream of blood to the heart through the pulmonary vein.
The lungs are internal organs. Yet they are, uniquely, constantly exposed to our external environment--a direct interface with the world outside. With each breath, a host of alien substances enter our bodies--pollens, dust, viruses, bacteria; the constituents of the air in our homes and offices and factories, ranging from animal dander and tobacco smoke to radon and airborne lead; the toxic chemicals spewed into our atmosphere by smokestacks and tailpipes.
The lungs, with their tiny air sacs called alveoli, have sometimes been simplistically compared to sponges. They are actually far more complex than many other organs. The heart, for example, is a relatively uncomplicated muscular pump designed, with one-way mechanical valves, for one purpose: to keep the bloodstream flowing in one direction. The lungs must play multiple roles--supplier of oxygen, remover of wastes and toxins, defender against hostile intruders. They contain at least three dozen distinct types of cells, each with its special tasks and abilities. Some scavenge foreign matter. Others, equipped with delicate, hairlike cilia, sweep the mucous membranes lining the smallest air passages. Still others act on substances crucial to blood-pressure control, or serve as sentries to spot invading agents of infection. And the roles of many others remain mysteries, posing challenges to researchers.
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