Describe the process of breathing.

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Breathing begins with inhalation. This starts with the contraction of the diaphragm, which flattens on contraction pulling the lungs down. The intercostal muscles contract to move the lungs up and out. The contraction of these muscles increases the volume of the lungs, this reduces the air pressure in the lung to below that of the external air. This pressure difference forces air to move in to the lungs where gas exchange occurs. The relaxation of the diaphragm and intercostals reduces the volume of the lungs which therefore increases the air pressure inside the lung to that above the external air. This forces air out of the lungs. The cycle is then repeated. 

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