When person stand from supine position , due to gravitational forces blood accumulate in the lower extremities . Veins are more compliant than arteries therefore veins distend and blood pool in the lower extremities . And thoracic venous blood volume reduced resulting reduced central venous pressure . Due to venous pooling in the lower extremities venous return to heart reduced and then venous filling reduced and stroke volume reduced resulting reduced cardiac output .

Reduced cardiac output leads to reduced arterial blood pressure . If this blood pressure significantly lowers, cerebral blood flow reduced and make person dizziness. this is known as postural hypotension  but due to baroreceptor reflex , sympathetic nervous system stimulated and Increase cardiac contractility, increase heart rate, vasoconstriction produced. then blood pressure bring to normal but patient with autonomic dysfunction and  volume loss cant utilize the above  compensatory mechanism