Respiratory system
Breathing is the first step in respiration. For respiration to happen, the body needs a constant supply of oxygen, which is done by breathing. Inhalation is the breathing in of air. To inhale, the lungs expand, decreasing the air pressure in the lungs. This is caused by two actions. The diaphragm (a sheet of muscular tissue that separates the lungs from the abdomen) is pulled downward. Also the muscles between the ribs contract to expand the chest. Both of these actions expand the lungs. To fill the enlarged lungs, air from outside at higher pressure comes rushing into the area of low pressure in the lungs. Air first passes through the nose and mouth, then through the larynx (voice box), then down the trachea (windpipe), and into the lungs.