To answer this question, let's find out how the human body maintains the blood pressure to a normal level when changing position from sitting or lying to standing.
When a person stands up from sitting or lying down, the body must work to adjust to that change in position. The gravity causes the blood pools down to the legs. It is especially important for the body to push blood upward and supply the brain with oxygen. If the body fails to do this adequately, blood pressure falls, and a person may feel lightheaded or even pass out. Orthostatic hypotension is the term used to describe the fall in blood pressure when a person stands.
(orthostatic= upright posture of the body; hypo= less + tension=pressure).
Adequate blood supply to the body's organs depends upon 4 factors, which is a co-work of body organs & nerve system:
- a heart strong enough to pump,
- arteries and veins that are able to constrict or squeeze,
- enough blood and fluid within the vessels,
- a good nerve system that passes the signals correctly
When the body changes position, a variety of actions occur involving all parts of the cardiovascular system as well as the autonomic nervous system that helps regulate their function.
The autonomic nervous system
can be considered to "run in the background" of the body, regulating body processes that we take for granted. There is a balance between the sympathetic system
(adrenergic nerves), that speed things up, and the parasympathetic system
(cholinergic nerves) that slow things down. These names are based on the type of chemical that is used to transmit signals at the nerve endings.
- Adrenaline (from the sympathetic nervous system) allows the body to respond to stress. Imagine seeing a bear in the woods; your heart beats faster, your palms get sweaty, your eyes dilate, and your hair stands on end.
- Acetylcholine is the chemical that is the anti-adrenaline and is involved in the parasympathetic nervous system.
These two systems are in balance, and yet need to respond to routine changes in the body that happens throughout the day.
- When the body moves to a standing position, pressure monitors (baroreceptor cells) located in the carotid arteries and the aorta sense a subtle drop in blood pressure because of gravity, which causes blood to flow towards the legs.
- Almost immediately, the sympathetic system is stimulated, causing the heart rate to increase, the heart muscle to contract or squeeze more forcefully, and blood vessels to constrict or narrow.
- All of these actions serve to increase the blood pressure so that an adequate amount of blood can still be pumped to the brain and other organs.
- Without these changes, gravity would cause the blood to remain in the lowest part of the body and away from the brain, causing symptoms of lightheadedness or even passing out.
So what may cause orthostatic hypotension? It could be one or more of a heart not strong enough, arteries and veins that can't squeeze effectively, dehydration, and autonomic nervous system disorders.