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Your diaphragm, the large muscle that forms the base of your chest cavity, 243

does most of the work during normal breathing. It receives messages from your Lungs brain to contract, which expands the chest cavity and creates a negative pressure that results in inward air flow. Stretch receptors in the lungs inform your brain that enough air has been taken in, and the diaphragm is instructed to relax, allowing the air to move out. Your brain alters the rate and depth of respiration based on the levels of oxygen and carbon dioxide in your blood. Visual, auditory, or other stimuli, such as fear, also can alter the breathing pattern.

Your lungs are very spongelike. The right lung has three sections, or lobes, and is slightly larger than the left lung, which has only two lobes. Air enters the lungs through the trachea (windpipe) and is drawn through the two large bronchi (the main airways that lead into each lung) into smaller airways called bronchioles. The bronchi and the bronchioles are lined with tiny hairs called cilia that catch and sweep out large dust particles and mucus that is secreted by glands in the bronchioles and coats the airways.

Gases are exchanged (carbon dioxide is removed from the bloodstream, and oxygen enters the bloodstream) in the highly folded and moist surfaces of air sacs (also called alveolar sacs) at the end of each bronchiole. Each alveolar sac is made up of up to 30 tiny pouches called alveoli. Alveoli have walls that are only one cell thick and are richly supplied with capillaries (the smallest blood vessels in the body) to facilitate gas exchange. Healthy lungs provide approximately 850 square feet of surface area for this purpose. Only 10 percent of the lungs is solid tissue; the rest is air and blood.

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