Gasses are able to move in and out of the lungs through muscular energy exerted on the thorax and changes between intrathoracic and atmospheric pressures. The pressure within the lungs and thorax must be less than atmospheric pressure for inspiration to occur. Air then flows from an area of higher pressure to one of lower pressure. As the diaphragm and intercostal muscles work to increase the size of the thorax, intrathoracic pressure decreases below atmospheric pressure and air moves into the lungs. During exhalation, the inspiratory muscles relax, and the elastic recoil of the lung tissues, combined with a rise in intrathoracic pressure, causes air to move out of the lungs
The diaphragm, a dome shaped structure that separates the thoracic and abdominal cavities, is the major muscle of respiration. The phrenic nerve innervates the diaphragm. The external and internal intercostal muscles are located between the ribs, increasing the anterior-posterior diameter of the thoracic cavity. Breathing may need to be assisted by other muscles, known as secondary or accessory muscles of respiration located in the neck and upper chest These muscles may include the parasternal, scalene, sternocleidomastoid, trapezius, and pectoralis muscles. Accessory respiratory muscles do not function during normal ventilation, but may be needed in some respiratory disorders.
The illustration shows the movement of the diaphragm during the respiratory cycle.
The diaphragm is the major muscle of respiration and is innervated by the phrenic nerve.