The pressure of the blood against the arteriolar walls while the heart is pumping is called blood pressure. Low blood pressure, or hypotension, is when this pressure is below normal.

Blood pressure between the range of 90/60 mmHg to 120/80 mmHg is considered normal for adults. Lower than this is considered hypotension.  In the measurement, the upper reading is the systolic blood pressure, and the lower reading is the diastolic blood pressure. Systolic blood pressure, or a systole occurs when the heart is beating while pumping out blood. Diastolic blood pressure, or diastole is the resting state of the heart between beats.

Hypotension can be caused by medical states such as pregnancy, heart problems, endocrine problems or dehydration. It can also occur when an individual is taking medications such as Diuretics, Alpha blockers, Beta blockers, medicine for Parkinson's disease, antidepressants and Viagra. Hypotension causes dizziness, fainting, concentration deficiency, blurred vision, nausea, hyperventilation, fatigue, thirst, depression, and in extreme cases, even shock. The last is caused mainly when hypotension occurs due to blood loss, septicemia, anaphylaxis or nutrient deficiency in diet.

Often a person can have low blood pressure without any of the symptoms of hypotension. This is then considered the normal blood pressure for that person, since it varies. However hypotension with symptoms requires medical advice for caution.