The alveoli are where the crucial gas exchange takes place. There are enough alveoli in just one lung to cover an area roughly the size of a tennis court. The average adult's lungs contain about 600 million of these spongy, air-filled structures.

Within the lungs, the mucus-lined bronchi split like the branches of a tree into tens of thousands of ever smaller tubes (bronchioles), which connect to tiny sacs called alveoli. The air is warmed up before passing down the windpipe, where it's divided at the bottom between two airways called bronchi that lead to either lung. Air is drawn in via the mouth and the nose, the latter acting as an air filter by trapping dust particles on its hairs. Our two lungs are made up of a complex latticework of tubes, which are suspended, on either side of the heart, inside the chest cavity on a framework of elastic fibers.

Central to the human respiratory system, they breathe in between 2,100 and 2,400 gallons (8,000 and 9,000 liters) of air each day-the amount needed to oxygenate the 2,400 gallons (9,000 liters) or so of blood that is pumped through the heart daily. Since the lungs process air, they are the only internal organs that are constantly exposed to the external environment. Without this vital exchange, our cells would quickly die and leave the body to suffocate. When we exhale, we produce carbon dioxide as a byproduct. Oxygen drives the process of respiration, which provides our cells with energy. They breathe in air, then extract the oxygen and pass it into the bloodstream, where it's rushed off to the tissues and organs that require it to function. Our lungs fuel us with oxygen, the body's life-sustaining gas.

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