Our Nervous system is much more than consciousness and free will. Our neural networks automatically go about the job of keeping us alive without entering our awareness- unless it fails to perform.
This is true of our intestinal system, and especially our cardiovascular system. We would not have time for anything else if we had to consciously take each breath, command each heart beat, and for this discussion, open up (dilate) the blood vessels to our brain, and constrict the blood vessels in our legs whenever we stand up.
Our brains demand a constant supply of Oxygen and Glucose in order to perform their complex duties. Between 20% and 40% of the 100 Watts of energy our bodies consume is in our brains.
Whenever we stand up, the force of gravity “wants” to force our blood to our legs and away from our head. Unless that tendency is counteracted, our brains would be deprived of essential factors, and we would all have orthostatic (upright position) hypotension every time we stand up.
Normally, when the blood pressure drops from ANY CAUSE, receptors near the base of the heart and great vessels signal the sympathetic branch of the autonomic nervous system to cause a speeding up of the heart and a dilation of the blood vessels in the brain. This compensates for the drop in blood pressure, which is determined by the blood flow and resistance to that flow.
The sympathetic nervous system is a reaction to STRESS, to a “flight or fight” decision. In the short term, it is beneficial, or adaptive. You may know the feeling, heart racing, cold hands (due to constriction of blood vessels), breathing rate increases, and sweating.
The asthmatic feels this from a shot of adrenalin, which delivers the sympathetic reaction to the whole body through the blood stream. The musculature of the bronchial tubes are relaxed, improving breathing, and improving the asthma.
What is adaptive in the short haul may be deleterious if it continues, felt as Anxiety if it lasts too long.
ORTHOSTATIC (postural) HYPOTENSION may be made worse by a variety of other influences, such as an abnormally slow heart, rate, dehydration, blood loss, certain medications and standing still too long.
Since the brain needs both Oxygen and Glucose, High altitude or a
low blood sugar will also facilitate faintness. If you have a tendency toward lightheadedness when you stand up, be careful lest you fall and injure yourself.
If you are unable to cope with these spells, a checkup with your Doctor may be in order; perhaps you have an underlying problem, such as with your heart, which is the next subject of discussion.