Postural changes, such as moving from lying to standing or sitting to standing, cause 300 to 800 ml of blood to pool in the lower limbs due to gravity. Despite the decrease in blood returning to the heart, your body can sustain your blood pressure by activating relevant compensatory changes.  If you are dehydrated, your blood volume is already low. Postural changes reduce the volume of blood returning to the heart beyond the body’s ability to compensate. This leads to a drop in blood pressure (25).