Orthostatic hypotension

Orthostatic hypotension is a condition where the patient's blood pressure falls quickly when going from sitting or lying down to standing, often resulting in lightheadness and even fainting, which restores the blood pressure. 
In a normal person, the blood vessels in the lower body constrict when a person stands up from a relaxed position, raising blood pressure and allowing more blood to flow to the brain. In a person with orthostatic hypotension, this constriction is delayed, allowing more blood flow towards the muscles of the legs. The blood collects in the legs, decreasing cardiac output. It can be diagnosed by taking a patient's blood pressure while lying down, then immediately again once they stand up, then again a few minutes after they stand up.
The condition is common with the elderly, people with chronic low blood pressure, and those on vasodilators. It is also a common symptom of many diseases, including Addison's Disease, diabetes mellitus and anorexia nervosa. However, it can affect anyone, although the feeling of lightheadedness often passes almost immediately. 
The condition has numerous possible treatments. One of the more common is to drink more water to increase blood volume. Compression stockings can also prevent blood from pooling in the legs. In severe cases, medication can be prescribed. 
Orthostatic hypotension at Wikipedia