In general, doctors will say you have low blood pressure if your blood pressure is below 90/60. However the truth is that there are people with such blood pressure (and below) who live perfectly normal lives, so that in the end the presence of symptoms is what really determines whether you suffer from low blood pressure or not. That’s because in itself low-ish blood pressure does not pose a direct threat to your health.
Also, there are other patients with blood pressures somewhere between 120/80 and 90/60 who do have the signs of low blood pressure. If you want reliable readings of blood pressure, use Omron BP785 blood pressure monitor.
There is a whole range of daily factors that can temporarily induce a low blood pressure such as stress, age, temperature, time of the day (e.g. evening) and time since the last meal. Also, staying in bed for a very long time causes a temporary drop in blood pressure.
These don’t make for huge differences in blood pressure but explain why we feel dizzy or sleepy in such moments or when we make an effort (e.g. when getting out of bed).
When conditions and special circumstances (e.g accident) are involved, there are more possible causes of low blood pressure:
Pregnancy, especially during the first six months, when the body’s circulatory system is expanding and blood pressure isn’t adjusted yet;
Heart disease (e.g. a slow heart rate known as bradycardia) that lowers blood pressure, often because some malfunction in the heart causes it to pump not enough blood into the body – for example after a heart attack;
Endocrine disorder, i.e. hormonal disease due to a gland malfunction (e.g. thyroid or kidneys) – in some cases diabetes can cause low blood pressure too;
Dehydration, most often caused by vomiting, diarrhoea, fever and heat strokes lead to huge loss of water. Mild dehydration may cause you to feel dizzy or even faint when doing an effort. If prolonged this can lead to shock and much more serious conditions;
Blood loss, due to an injury, internal bleeding or severe dehydration (less water in the blood) that result in a decrease in blood volume;
Septic shock, when a bacterial infection reaches the bloodstream from the infected organ;
Severe allergic reaction (breathing problems, hives, itching, swollen throat), usually resulting from certain foods, venomous insect sting or penicillin. This can cause a dangerous sudden drop in blood pressure;
Lack of essential vitamins in your diet will cause anaemia (not enough red blood cells) which is a direct cause of low blood pressure.
As you can see, what causes low blood pressure is a long list of unrelated conditions, as opposed to what causes high blood pressure which is normally a mixture of lifestyle factors, family background and underlying conditions.