Why does someone pass out when they stand up?

Someone may pass out when they stand up due to a condition called orthostatic hypotension. This occurs when blood pressure drops suddenly upon standing, leading to reduced blood flow to the brain. Factors such as dehydration, prolonged sitting, or certain medications can contribute to this drop in blood pressure. When the body fails to quickly adjust to the change in posture, it can result in fainting or lightheadedness.