In optimal breathing, the diaphragm plays a major role. It moves downward during inhalation, decreasing pressure in the chest cavity, which causes air to come in and expand the lungs. In exhalation, the diaphragm moves upward as the lungs return to their normal size and air is pushed out. This is the external respiration process, in which gas is exchanged between the external environment and the lungs. The whole valve system coordinates with breath, which each valve pumping one direction on inhalation and the other direction on exhalation.
In the internal respiration process, gas is exchanged between the lungs and the internal systems of the body. This happens in the apnea phases – the pauses after the end of the inhalation as exhalation is initiated, and after the end of exhalation as inhalation is initiated.
In certain stages of fight or flight, the valve system – including the diaphragm – will stop pumping, and in some stages the valves will pump backwards. Compensatory patterns will take over so that breathing still occurs, though these patterns are not ideal for long-term function. Normal respiration supports blood oxygenation and oxygen delivery, circulation, creation of internal pressure, hydration and elasticity of muscle and connective tissues, and support for good posture.