Why is my blood pressure low?
It is wonder that when we all stand up that we don’t faint and fall over. To prevent us from falling over every time we stand up, our bodies had developed mechanisms to pump blood from the legs all the way up to the heart and to brain. Without this, gravity works by pulling blood all the way into our feet and without blood flow to our brain we will all faint and fall to the ground whenever we stand up.
To understand why people faint or feel light headed, we first need to understand how blood pressure is regulated.
The heart pumps blood around the body through our blood vessels. When we are lying down and horizontal the heart does not need to pump against gravity and it is very easy for the blood to return from your legs and arms back to your heart so that it can be pumped to your brain.
When you stand however, gravity now works on our blood and we therefore need a way to the blood to return to heart and onto the brain. When we stand we instantaneously pool ~500-750ml of blood in our legs. There are nerves in the heart and also around the major blood vessels to the brain that sense a drop in pressure when we stand. Normally with the slight drop in pressure there will be an automatic reflex that sends signals to nerves that control the blood vessels to instantaneously tighten up the blood vessels so blood pressure can be maintained. This also signals the heart to beat faster to help supply more blood to your brain and other organs.
When blood pools in your legs, the muscles in your legs help squeeze the blood back up to the heart, especially when you move. This also relies on your blood vessels to be rigid enough so that when blood is squeezed through them they go up the body back to your brain.
If there is a lack of blood (from dehydration, blood loss, abnormalities in the way the kidneys control blood volume), or problems with any of the above control mechanisms then there will be a lack of blood being returned to to heart and subsequently the brain which causes many of the symptoms of low blood pressure.
Low blood volume due to dehydration, having sluggish autonomic reflexes (either due to age or damage to the nerves for example due to diabetes, Parkinson disease or autoimmune disease), deconditioned muscles or stretchy blood vessels can all contribute to feeling lightheaded on standing. Medications that are used for BP control and the time around periods with hormonal changes can also affect blood pressure control.
As we can see, there are many inputs that control our blood pressure and in some people there can be more than one contributing factor as to why their blood pressure is low. By addressing each one of these issues we can then try to improve symptoms.