Diseases of the Respiratory System

The human body cannot survive for more than a very few minutes in an environment that lacks oxygen. Oxygen is required for the normal functioning of all living body cells. This vital gas reaches the body cells via the bloodstream; each red blood cell transports oxygen molecules to the body tissues. The oxygen comes from the atmosphere one breathes, and it enters the bloodstream through the very thin membrane walls of the lung tissue, a fresh supply of oxygen entering the bloodstream each time a person inhales. As the red blood cells circulating through the walls of the lung tissue pick up their fresh supply of oxygen, they release molecules of carbon dioxide given off by the body cells as a waste product of metabolism. When a person exhales, the lungs are squeezed somewhat like a bellows, and the carbon dioxide is expelled from the lungs.

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