If the normal range of blood pressure falls below 90/60mmHg, it indicates low blood pressure (hypotension). The causes associated with low blood pressure include reduced cardiac output, dilation of blood vessels, decrease in blood volume, inhibition of brain centers that control blood pressure, impairment of the autonomic nervous system, etc. Low blood pressure occurs when the arterial pressure (force on arterial walls in the heart) reduces due to lack of proper blood flow (due to plaque, hemorrhage, hormonal abnormality, etc). This reduces the blood flow to the brain (called brain perfusion).
The image shows a watercolor drawing of a pregnant woman lying flat on her back, causing the aorta and inferior vena cava to compress and reduce circulation. The enlargement of uterus in women creates pressure on the blood vessels which causes hypotension. Supine hypotension is mostly experienced in pregnancy when you lie down on your back in later trimestors mainly. Low blood pressure in the first trimester occurs mainly due to hormonal changes that cause dilation of the blood vessels. [Image used under Creative Commons License from Wikimedia]
Symptoms of hypotension can include such as fatigue, lightheadedness, dizziness, nausea, clammy skin etc. Generally, if the blood pressure reading is lower than 90/60 mm Hg, it is abnormally low and is referred to as hypotension. There are some adults who regularly have a blood pressure (BP) in the hypotensive range, but they do not have any symptoms at all and do not require treatment.
Blood pressure is the pressure of circulating blood on the walls of vessels. When used without further specification, blood pressure usually refers to the arterial pressure in systemic circulation. A healthy blood pressure is 120/80 or less, but the lower you can get it, the better it is.
Use this blood pressure chart to learn what your blood pressure numbers mean. If your systolic and diastolic readings fall into two different categories, your correct blood pressure category is the higher category. For example, if the blood pressure reading is 125/85 millimeters of mercury, you have stage 1 hypertension. High blood pressure is called hypertension and low blood pressure is called hypotension.