Aerobic activity is defined by Wikipedia as “physical exercise that intends to improve the oxygen system and refers to the use of oxygen in the body’s metabolic or energy-generating process.” The ability to absorb oxygen is what it’s all about. The body of a well-conditioned person absorbs oxygen much more efficiently than the body of an inactive person.

The lungs of a normal-sized adult carry about 300 million tiny, microscopic sacks called alveoli. The walls of these sacks are only one cell thick and are each covered with a mesh of blood vessels. When the lungs fill with air the oxygen flows through the sack walls into the blood vessels and enters the blood stream. These 300 million microscopic alveoli have a combined surface area of about 150 square meters (about the size of a tennis court) through which the oxygen is absorbed.

The problem is that nobody can use all of his 300 million alveoli when he breathes. Everyone has a certain portion of his lungs which can never expel the air that is in them. The portion of the lungs which always retains its air is called “residual volume,” and its purpose is to keep the lungs from collapsing. The portion of the lungs that is useful for breathing is called the “vital capacity.” In an unconditioned body, a much smaller portion of the lungs is vital capacity and the residual volume is much larger than what it needs to be. As a body becomes conditioned with regular aerobic activity, the vital lung capacity increases substantially. In a well conditioned body the vital lung capacity will be about 75%. Read the rest of this entry »