What's the mechanism for pain-induced swoon?
Swooning, fainting otherwise termed Syncope or Primary shock (Triggered by pain or any unpleasant stimuli through the vaso vagal effect), is due to generalised vasodilation. The capillaries loose their tone and dilate resulting in an increase in volume capacity of the vascular system. This results in draining of blood to the dependent areas due to gravity thus hindering blood supply to the brain which shuts down and results in collapse (fainting) of the individual. This is a safety mechanism. Once the patient is in the lying down position the blood supply to the brain returns and the patient recovers. An individual who has fainted should not be propped up as this may result in seizures or worse.