what is low blood pressure

what is low blood pressure

Normal blood pressure with respect to cardiovascular risk is less than 120/80 mm Hg. Unusually low readings should be evaluated to rule out medical causes.

If the Yamomamo Indians have an average blood pressure of 96/62 mmHg and are extremely fit and well, it is difficult to define what is meant by low blood pressure. Most people with blood pressures at these levels, ie, around 90/60 mmHg are completely normal and are very lucky that for some reason they are protected from all the bad things we do that increase our blood pressure, like eating too much salt, not eating enough fruit and vegetables, being overweight and inactive etc. However, there are a few people with a blood pressure at these levels who have an underlying cause for their low blood pressure. In nearly all of these cases, blood pressure is much lower when you stand up, particularly if you stand up suddenly. This fall in blood pressure on standing is likely to cause symptoms of fainting and/or dizziness.

Normally, when you stand up blood tends to pool in the veins in your legs. This is stopped by nerves, which contract the veins in your legs 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.

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. This causes symptoms of dizziness and some people may faint. 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.