Standing has little effect on blood flows to the brain in most animals because their brains are roughly horizontal to their hearts. Humans, however, have to deal with the gravitational inevitability of large flows of blood rushing away from the brain every time they stand.
Every time you stand from @300-800 mls. of blood rush from the trunk of your body to your legs causing blood to pool in the legs and producing about a 40% reduction in the output of blood pumped by the heart. Unless something is quickly done reduced flows of oxygen to the brain result in dizziness, fatigue, etc.
That ‘something’ involves a complex series of reactions involving the heart, the blood vessels, the muscles and hormones and neurotransmitters. As you’ll read in this series uncovering the cause of dizziness, fainting, fatigue and other symptoms that can occur during standing in chronic fatigue syndrome can be difficult. Several studies wrapping up in 2012, should, however, shed some light on both causes and treatments.