Syncope is a temporary loss of consciousness due to transient loss of blood supply to the brain.
Symptoms are sensation of warmth, nausea, lightheadedness, sweating, blurring of vision, inability to stand etc.
In syncope blood pressure drops, the vessels of the leg becomes dilated, pulling of blood in the periphery leads to poor circulation in the brain.
When there is low blood supply to the brain, there triggers a compensatory mechanism of the body to reestablish the circulation.
The blood vessels of the other part of the body constricts to give enough blood to the brain, so instantaneously the person revive consciousness.
You can read about it for detailed information in the Link 1 Link 2
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