|MadSci Network: Anatomy|
For an adult of average size, the lungs can hold a total of about 5 L (liters). During normal breathing, the tidal volume of air entering and leaving the lungs is about one-tenth of the total volume, or 500 ml. Through conscious, deep breathing, a person can inhale a full 3 L, the inspiratory reserve, and exhale an extra 1 L, the expiratory reserve. That adds up to 4 L of air being moved during deep breathing, but the total volume of air in the lungs is 5 L. The remaining 1 L of air in the lungs is called the residual volume, and is the result of the architecture of the lungs preventing them from collapsing completely like a balloon, which would allow them to expel all of their contents.
Breathing is controlled both consciously and unconsciously by the brain through a complex system of sensory and motor nerves. The motor nerves run to the diaphragm and intercostal ("between the ribs") muscles to expand and contract the chest for breathing, and the sensory nerves run to various receptors that tell the brain: how much oxygen is in the blood, how much carbon dioxide is in the blood, how much air pressure is in the lungs, and how much tension is on the chest muscles during breathing. These last two give important feedback to the brain to allow it to coordinate the muscles so that breathing runs smoothly.
When someone or something impacts against your chest with enough force, the air forced out of the lungs can exceed the normal expiratory reserve, such that the residual volume is below the normal limits of the lungs. When this happens, miscues from the sensory nerves confuse the brain, so that it cannot "feel" the lungs to take a breath, until the natural elasticity of the lungs recoils them from the blow and returns the volume to above normal residual levels. This recoil takes at most a couple seconds, during which the individual is unable to breath.
So the term, "having your breath knocked out," is actually a very accurate description of the physiological cause of this loss of the ability to breath.
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