ABOUT THE LUNGS: The lungs are located in the chest cavity, and are protected by the rib cage. The lungs are responsible for the exchange of oxygen and carbon dioxide between the body and its surrounding environment. They are made of a spongy, elastic type of tissue filled with tiny holes or bubbles, each surrounded by a fine network of tiny blood vessels. This tissue stretches and contracts as you breathe. The total surface area of the lungs is about the size of a football field. When you breathe in, the diaphragm and intercostals or chest wall muscles contract, causing the air to travel from your nose and mouth through the windpipe (trachea), then through large and small tubes in the lungs called bronchial tubes. At the end of these tubes are groups of tiny air sacs called alveoli. They have very thin walls filled with small blood vessels called capillaries. Oxygen passes from the air sacs into the blood vessels, and carbon dioxide -- the waste byproduct from the body's metabolism -- passes from the blood into the air sacs. The carbon dioxide is then expelled into the atmosphere when you exhale.
SLEEP STAGES: Stage 1: Drowsiness. Stage 2: Light sleep. Stages 3 and 4: Deep sleep. Stage 5: Rapid-eye movement (REM) sleep. REM is when people dream, perhaps because the brain is more active and the muscles are relaxed. These five stages occur cyclically; a person may complete five cycles in a typical night's sleep.