U.S.Department of Health & Human Service
NIH – National Heart, Lung and Blood Institute
Blood pressure is measured as systolic and diastolic pressures. “Systolic” refers to blood pressure when the heart beats while pumping blood. “Diastolic” refers to blood pressure when the heart is at rest between beats.
You most often will see blood pressure numbers written with the systolic number above or before the diastolic number, such as 120/80 mmHg. (The mmHg is millimeters of mercury—the units used to measure blood pressure.)
Chronic low blood pressure with no symptoms is almost never serious. But health problems can occur when blood pressure drops suddenly and the brain is deprived of an adequate blood supply. This can lead to dizziness or lightheadedness. Sudden drops in blood pressure most commonly occur in someone who’s rising from a lying down or sitting position to standing. This kind of low blood pressure is known as postural hypotension or orthostatic hypotension. Another type of low blood pressure can occur when someone stands for a long period of time. This is called neurally mediated hypotension.
Postural hypotension is considered a failure of the cardiovascular system or nervous system to react appropriately to sudden changes. Normally, when you stand up, some blood pools in your lower extremities. Uncorrected, this would cause your blood pressure to fall. But your body normally compensates by sending messages to your heart to beat faster and to your blood vessels to constrict. This offsets the drop in blood pressure. If this does not happen, or happens too slowly, postural hypotension results.
The risk of both low and high blood pressure normally increases with age due in part to normal changes during aging. In addition, blood flow to the heart muscle and the brain declines with age, often as a result of plaque buildup in blood vessels. An estimated 10% to 20% of people over age 65 have postural hypotension.
- Hormonal problems such as an underactive thyroid (hypothyroidism), diabetes, or low blood sugar (hypoglycemia)
- Some over-the-counter medications
- Some prescription medicines such as for high blood pressure, depression or Parkinson’s disease
- Heart failure
- Heart arrhythmias (abnormal heart rhythms)
- Widening, or dilation, of the blood vessels
- Heat exhaustion or heat stroke
- Liver disease
- Loss of blood from bleeding
- Low body temperature
- High body temperature
- Heart muscle disease causing heart failure
- Sepsis, a severe blood infection
- Severe dehydration from vomiting, diarrhea, or fever
- A reaction to medication or alcohol
- A severe allergic reaction, called anaphylaxis
- Dehydration and electrolyte loss, which may result from diarrhea, vomiting, excessive blood loss during menstruation, or other conditions
- Age-associated decline in blood pressure regulation, which may be worsened by certain health conditions or medications
- Central nervous system disorders, such as Shy-Drager syndrome or multiple system atrophy
Nerve problems, such as peripheral neuropathy or autonomic neuropathy
- Cardiovascular disorders
- Nutritional diseases