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I have experienced this phenomena many, many times and I'm sure it's not only me. I guess the sudden rise in arousal opens up the nostrils in a kind of fight or flight response, but I'm interested in the more intricate details of why this happens.

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up vote 12 down vote accepted

Norepinephrine is responsible for, among many other things, causing a constriction the nasal vasculature and slowing capillary leakage that would lead to a runny nose.

Both the startle response (sympathetic) and the orgasm response (sharp transition between high peak of sympathetic response and descent into parasympathetic) result in large releases of epinephrine and norepinephrine into the bloodstream from the adrenal medulla.

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So, you mean that norepinephrine consticts the nasal vasculature? My intuition was that they got broader, thereby leaving more room for air to pass through. – Speldosa Jun 21 '12 at 23:09
@Speldosa Yes, it does constrict the nasal vasculature. Think of one of those leak hoses, if you turn the water on at the other end, you get a lot of leakage into the soil, but if you pinch off the length just before the leak holes, the ground will dry up. It may seem like the nostrils are getting bigger, but likely they are less puffy. – Chuck Sherrington Jun 21 '12 at 23:11
@Speldosa See vasoconstriction:Two common stimuli for eliciting smooth muscle contraction is circulating epinephrine and activation of the sympathetic nervous system (through release of norepinephrine) that directly innervates the muscle. This occurs on the smaller arterioles just before the capillaries. – Chuck Sherrington Jun 21 '12 at 23:15

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