Yes, a decrease in blood pressure when standing from a seated position or sitting up from a lying postion is a common problem known as postural hypotension, a common side effect of which is syncope, or 'blacking out' as you describe above. So why does this happen? Normally, when you stand up after being seated for a while, the pull of gravity causes about 15-25% of the blood in your body to pool in your lower extremities. This pooled blood does not return to the heart, therefore amount of blood your heart pumps out decreases and your blood pressure drops. This decrease in blood pressure is normally transient and goes unnoticed because the body quickly responds by increasing tone in the vasculature to augment blood pressure through direct arterial constriction as well as increased venous return to the heart (this is a reflex known as the carotid body reflex). However, sometimes this reflexive increase in blood pressure fails and the blood pressure stays low for a prolonged period of time resulting in (among other things) decreased perfusion to the brain and an accompanying feeling of lightheadedness or blacking out. So what causes the reflex to fail? The main causes are dysfuction of the nervous systems itself or a decrease in the amount of circulating volume in the veins and arteries. Nervous system dysfunction normally occurs in older patients or patients with other nervous system diseases, however some medications can also disrupt the appropriate neural reflex to augment blood pressure upon standing, most notably tricyclic anti-depressants and blood pressure lower medications such as beta blockers. The other main reason mentioned above (ie a decrease in circulating volume) is commonly due to dehydration and may be ameliorated with increased fluid intake. This is commonly seen with patients who experience these feeling of lightheadedness or blacking out upon standing in the context of GI illness (volume loss through vomiting
) or after a great deal of sweating during exercise or fevers. try to increase your fluid intake and see if that helps with your symptoms. if not, you should go to see your doctor
for further evaluation. hope this helps!