Originally Posted by myates1980

The dizziness you feel when you stand up quickly is caused by a decrease in blood flow to the brain. Inside your body you have several major arteries—the biggest one is called the aorta. When you are sitting down, the arteries and the blood vessels in your legs are bent in various positions. When you stand up, those major vessels become straight, kind of like a garden hose, and gravity takes a lot of blood towards your toes. Normally the body responds by pushing the blood through the circulatory system with more force (greater pressure) to overcome the effects of gravity. But if all factors that govern blood pressure are not "in sync" (and you’ve been sitting for a long while), the increase in blood pressure will not be fast enough and you will feel dizzy.

Feeling dizzy is the body’s way of telling you to stop for a moment until the flow of blood to the brain can catch up with the sudden change in position. If there is too much delay in getting blood to your brain, you also could experience blurred vision, fuzzy thinking, weakness, and even an upset stomach. These symptoms tell you to sit back down (or lie down) so that more blood can flow to the brain. If you don’t heed these symptoms, your body could force you to faint, making you lie down whether you want to or not! In the future, I would recommend getting up slower—unless you just miss that feeling of being on a carnival ride.