When you stand up, your blood immediately tries to pool in the veins in your legs. If allowed to occur, this would be a bad thing, because there would be a shortage of blood in the rest of your body.
This pooling of blood in the leg veins is stopped by nerves, which contract the leg veins ensuring that enough blood returns to your heart and that there is no reduction in the amount of blood that your heart pumps. This maintains the blood supply to your brain and other parts of your body.
However, if there is pooling of blood in your veins, less blood returns to your heart and less is pumped out which means a reduction in the amount of blood going to your brain and other parts of your body. This causes symptoms of dizziness and some people may faint as a result.
This is exactly what happens to guards on parade when they faint because they have been standing in one position for too long. You can mimic this effect when you squat down for a while, for instance when looking at books on a low shelf, and then stand up suddenly.
Therefore, if you have a low blood pressure and you also have symptoms of dizziness or faintness when standing up suddenly you should have your blood pressure checked both when you are lying down and when you then stand up. This drop in blood pressure when you stand up is called postural hypotension.
The commonest causes are:
An acute illness, such as severe blood loss or sudden severe infection or damage to the heart may cause low blood pressure and very low blood pressures when sitting up or standing. When this happens blood pressure levels are an important measure of the severity of the illness and that is why blood pressure is measured so often in acutely ill people.
Blood pressure lowering drugs. Nearly all of the modern blood pressure lowering drugs does not cause a drop in blood pressure when you stand up. The only class of drugs that is commonly used nowadays that may cause a drop in standing pressure are alpha blockers, such as doxazosin.
If you are older, and particularly if you have diabetes, there may already be a tendency for your blood pressure to fall when you stand, but your high blood pressure tablets may make this slightly worse.
If you get severe food poisoning and are sick and have diarrhoea, you may lose large amounts of fluid from your body. This will make any high blood pressure tablets lower your blood pressure further. If you do get severe food poisoning it is important to check your blood pressure and stop the tablets if your blood pressure is low, such as less than 110/70 mmHg.
If you have a drop in blood pressure when you stand up, but you still need to continue with the tablets for high blood pressure, then it will help if you get up slowly. For example, when you are lying flat in bed, sit up slowly, then stand up slowly and avoid sudden attempts to stand up without support.
Rare diseases of the nerves that control the reflexes in the veins can prevent the veins from contracting when they stand up, resulting in a severe drop in blood pressure. This is very rare. If you have a severe drop in blood pressure when you stand and you are otherwise well (ie, not on any blood pressure lowering drugs) then you should have further investigations.
Diabetes can also cause damage to the nerves supplying blood vessels and can therefore be associated with a drop in pressure when standing up.
Increasing age. As you get older, your arteries stiffen and are less supple and this may occasionally cause a fall in blood pressure when you stand.
Failure of the adrenal gland. The adrenal glands are two very small glands just above the kidney that produce important hormones, one of which is aldosterone. This hormone is very important in controlling the amount of salt in your body.
If your adrenal glands become damaged this can mean that there is not enough aldosterone in your body. This leads to a loss of salt from your body and this may cause low blood pressure, with dizziness or faintness when you stand up suddenly. This condition is very rare, but important to pick up as; it can easily be treated by replacing the missing hormones.
Investigations are only conducted if you have symptoms that suggest a fall in blood pressure when you stand up, such as dizziness or faintness. If you do have these symptoms then your doctor should conduct further tests or refer you to a specialist.
This will mean having a tilt test where you will be strapped to a table and tilted, with careful measurements of heart rate and blood pressure taken. Hormone levels in your blood may also be measured to check that your adrenal glands and nerves are working normally.
If you are taking blood pressure lowering drugs they may need to be changed, particularly if you are taking an alpha blocker such as doxazosin. You will need to discuss this with your doctor or practice nurse. Many people find that by getting up slowly their symptoms can be avoided.
Failure of the adrenal glands can be treated by replacement of the missing hormones. If you have a disease of the nerves then this can be more difficult to treat, but you may respond to drugs that stimulate the nervous system. You may also find that wearing elastic stockings or an anti-gravity suit, or taking hormones that cause retention of salt and/or eating more salt can help. Talk to your doctor for more information.