Lungs are the only internal organs that are constantly exposed to the external environment. 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. The rate at which we breathe is controlled by the brain, which is quick to sense changes in gas concentrations. This is certainly in the brain's interests—it's the body's biggest user of oxygen and the first organ to suffer if there's a shortage. The actual job of breathing is done mainly by the diaphragm, the sheet of muscles between the chest and abdomen. These muscles contract when we breathe in, expanding the lungs and drawing in air. We breathe out simply by relaxing the diaphragm; the lungs deflate like balloons. __
