The Bezold-Jarisch reflex involves a variety of cardiovascular and neurological processes which cause hypopnea (overly shallow breathing or an abnormally low respiratory rate) and bradycardia (abnormally low resting heart rate).
Prolonged upright posture results in some degree of pooling of blood in the lower extremities that can lead to diminished intracardiac volume.
This phenomenon is accentuated if the individual is dehydrated.
They may respond by sending signals that trigger paradoxical bradycardia and decreased contractility, resulting in additional and relatively sudden arterial hypotension. It usually occurs in the nitrate therapy and in the use of serotonin agonists. 
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