Dizziness or lightheadedness upon standing is one of the "lesser" symptoms of a worsening stenotic aortic valve. What happens is that while you are seated (or even worse, kneeling) the body settles into a blood supply level to the brain that is adequate. Upon standing, your lower extremities need more blood flow, but your heart cannot pump enough blood through your valve quickly enough to maintain both the needs of your lower body and your brain. It is kind of an exercise in hydraulics. When a system has a fixed amount of fluid within it (like blood in your body), then you quickly enlarge the area you are trying to fill, the pressure in the system drops. So, what is happening is that temporarily your blood pressure is dropping and you get dizzy. Once your body re-regulates itself, things right themselves and you're back to "normal."
Sorry for the rambling explanation, but I hope it helps you to understand the issue. A stenotic valve cannot pass as much blood as a healthy valve, and eventually the heart cannot force hard enough to overcome the restriction. As this symptom progresses, it may be described as "pre-syncope" (almost fainting). Actually fainting (syncope) is one of the "cardinal symptoms" often pointed to in deciding when to have surgery to repair or replace a stenotic aortic valve. (For the record, the cardinal symptoms are usually listed as fainting, shortness of breath or chest pain).
