Why do we get dizzy when we stand up too fast?

Standing up can cause a sudden loss of blood pressure in your brain. This is especially true if you have been sitting in such a way as to partially cut off the blood supply to your legs. The blood rushing back to the deprived area causes the drop in blood pressure.

If this happens often, you may be suffering from hypotension (low blood pressure) and might want to consult your doctor. Hypotension can lead to fainting episodes, among other problems.