Physiological Action
Smelling salts release ammonia (NH3) gas, which irritates the mucous membranes of the nose and lungs, and thereby triggers an inhalation reflex (that is, it causes the muscles that control breathing to work faster). Additionally, the irritant properties activate the sympathetic system as a response leading to elevated heart rate, blood pressure and brain activity. Specifically, many of the benign fainting events are caused by excessive parasympathetic and vagal activity leading to dilation of blood vessels, slowing of the heart and decreased perfusion of the brain thus leading to decreased muscle tone in general and postural muscles in particular, allowing the person to collapse. The sympathetic irritant effect is exploited to counteract these vagal parasympathetic effects leading to reversal of the faint. Similar action can be produced by cheek slapping which is often administered to fainting persons.
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