Breathing may be likened to the functioning of the old-fashioned blacksmith's bellows. Lift up the handle, opening the bellows, and air is sucked in. Let go, the bellows collapse, and the air rushes out.

The components of the process are volume and pressure. As the volume of the bellows increases, the internal pressure decreases, creating a vacuum effect that draws in the outside air. When the bellows collapses, the volume decreases, and the increasing internal pressure forces the air back out again.

The bellows at work within your body consists of the lung tissue's elasticity as well as a number of fascinating muscles. The most active of these is the diaphragm - a dome-shaped muscle located at the base of the rib cage and above the stomach. It contracts during inhalation and pulls down on the bottom of the chest, increasing the volume inside the chest cavity. At the same time, the chest capacity can be further increased by elevating the ribs slightly and moving them outward and upward. When all is working smoothly, what results is maximum volume and minimal pressure. Air is drawn in - you are inspiring." Relax these muscles and the elastic property of the lungs causes them to contract. The diaphragm is pulled back up, ribs move back in and down, the pressure builds, and the air is exhaled. The illustration [below] will further clarify this.

[image of bellows]

Adult human beings breathe an average of 16,000 quarts of air each day.

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