When you stand after sitting or lying down for sometime, you feel dizzy. Like how you feel dizzy after spinning several times. When you are lying down, the flow of blood to your brain decreases. Aorta is one of the several major arteries that we have in side our body. The arteries in your body take various positions as we take various positions like sitting, lying down etc. these arteries become straight when you stand up. As soon as you stand, due to the gravitational pull, a lot of blood is attracted towards the toes all of a sudden. The body has its defence mechanism; it responds in the way of pushing blood towards the brain through the circulatory system, however, for this to work, one's blood pressure has to be in perfect sync otherwise the blood pressure falls short and we feel dizzy as we stand up. If it happens often and dizziness persists, you might want to check with a doctor for your blood pressure.
