Throughout the day, your blood pressure can vary by between 30-40 mmHg (both systolic and diastolic) depending on what you are doing. Having a stressful week at work, the temperature outside, and even what you had for lunch could affect your blood pressure reading.
Each time that you have your blood pressure measured, it is important that the test is carried out under similar conditions to ensure that the results are consistent. If you have a low blood pressure reading, your GP will first consider the everyday causes that might have affected it, before considering the possible underlying causes.
Many factors have a daily, or sometimes even hourly, effect on your heart and circulation. Below are things that could affect your blood pressure and, in some cases, may cause low blood pressure.
The time of day - your blood pressure falls overnight so it will be low in the morning.
Your age - typically, blood pressure rises as you get older, although postural, or orthostatic, and postprandial hypotension are also more likely in the elderly.
How stressed or relaxed you are - if you are stressed, your heart will beat faster and your blood pressure will increase, and the opposite if you are relaxed.
How much exercise you do - initially, exercise will raise your blood pressure, but if you are healthy and exercise regularly, your blood pressure will be low when you are resting.
Your temperature - if you are cold, your heart beat will slow down and your blood pressure will fall.
If you have recently eaten - blood will be used for digesting food in your stomach, so the blood pressure elsewhere in your body will fall.
If your blood pressure is still considered low after taking into account everyday factors, such as those listed above, there may be another cause. Some possibilities are explained below.
Some medication may cause hypotension as a side effect. This tends to be orthostatic, or postural hypotension (low blood pressure when you stand up, or change position). Examples of medication that can cause hypotension include:
- beta-blockers - which may be prescribed after a problem with your heart,
- alpha-blockers - a medicine that is prescribed to lower blood pressure for people with hypertension (high blood pressure), and
- some antidepressants.
Your GP will discuss any possible side effects with you when prescribing medication. While you are taking medication, your blood pressure will be carefully monitored if you are considered to be at risk of hypotension.
Serious illnesses or conditions
If you have an acute (short-term) illness, your blood pressure will be measured regularly because it is a good indicator of the severity of your illness. A heart condition, such as heart disease, or a heart attack, can also cause low blood pressure, as your heart may not be able to pump blood around your body.
Autonomic disorders affect your autonomic nervous system and they can cause hypotension. Your autonomic nervous system is part of your nervous system (the network of cells that carry information around your body). It controls the bodily functions that you do not actively think about, such as sweating, digestion, and the beating of your heart.
The autonomic nervous system also controls the widening and narrowing of your blood vessels. If there is a problem with it, your blood vessels could remain too wide, causing low blood pressure. In particular, autonomic disorders tend to cause orthostatic hypotension.
Some examples of autonomic disorders are:
- diabetes mellitus (a long-term (chronic) condition caused by too much glucose (sugar) in the blood),
- Parkinson’s disease (a chronic condition that affects the way the brain coordinates body movements), and
- multiple system atrophy (a disorder that causes the brain signals to the muscles and limbs responsible for movement to deteriorate).
The adrenal glands are two small glands that are located just above your kidneys. They produce hormones that control your blood pressure and maintain the balance of salt and water in your body. One of the hormones they produce is called aldosterone, which is responsible for controlling the amount of salt in your body.
If your adrenal glands become damaged - for example through an infection, or a tumour - the production of aldosterone may be reduced, resulting in a loss of salt from your body. This can cause dehydration which, in turn, leads to low blood pressure.
If a problem with your adrenal glands is diagnosed, it can be treated by increasing the amount of aldosterone in your body. This could also be a symptom of Addison’s disease (a condition in which the adrenal glands cannot produce enough of the hormones cortisol and aldosterone). Addison’s disease can also be treated with medication.
Serious injuries and shock
Low blood pressure can also be caused by serious injuries, or burns, particularly if you have lost a lot of blood. This can mean that there is less blood being pumped around your body. Low blood pressure can also occur if you go into shock after having a serious injury.
Other kinds of shock are described below.
Septic shock and toxic shock syndrome
Septic shock and toxic shock syndrome are caused by bacterial infections. The bacteria attack the walls of the small blood vessels, causing them to leak fluid from the blood into the surrounding tissues. This causes a significant drop in blood pressure (severe hypotension).
Anaphylactic shock, or anaphylaxis, is caused by an allergic reaction to something - for example, a wasp sting or a peanut. During an allergic reaction, your body produces a large amount of a chemical called histamine, which causes your blood vessels to widen and leading to a sudden, severe drop in blood pressure.
Cardiogenic shock occurs when your heart cannot supply enough blood to your body, so your blood pressure drops. This can happen during a heart attack.
Other possible causes of low blood pressure are listed below.
Rare nerve conditions - if the nerves in your legs are affected, you may experience a severe drop in blood pressure when you stand up (postural or orthostatic hypotension).
Increasing age - as you get older, your arteries can become stiffer. If they do not constrict (get smaller), your blood pressure may drop, particularly when you stand up.
Pregnancy - during the early to mid stages of pregnancy, low blood pressure is fairly common.
Prolonged bed rest - low blood pressure may possibly occur as a result of moving less and having overall less nervous system activity.
Dehydration - low blood pressure may occur following particularly severe dehydration from vomiting and diarrhoea because the lack of water and salt in your body will reduce the volume of your blood.
Your genes - some research has suggested that low blood pressure is genetic. If your parents have low blood pressure, it is possible that you could inherit it from them.