Although folk wisdom views coughing as a grave portent of illness — What did one casket say to the other? "I hear you coffin" — the cough is actually a vital player in the body's defense against disease. Coughing expels mucus, microbes, and foreign particles from the respiratory tract, protecting the lungs from infection and inflammation.
The cough begins with an initial gasp that draws air deep into the lungs. Next, the glottis snaps shut, putting a lid over the trachea, or windpipe. The third step is the forceful contraction of the muscles of the chest cage, abdomen, and diaphragm (see figure). In normal breathing, these muscles push air gently from the lungs up through the nose and mouth. But when the glottis is closed, the air can't move out, so tremendous pressure builds up in the air passages. Finally, the glottis swings open and the air rushes out. And it is quite a rush; in a vigorous cough, the air travels out at nearly the speed of sound, creating the barking or whooping noise that we call a cough.