Low blood pressure is also known as hypotension. It is when your blood pressure is lower than normal. Blood pressure measures the pressure in your arteries when the heart beats and pumps blood through your body. Your blood pressure has two numbers. The top number is the systolic number. It measures the highest amount of pressure in the artery when the heart beats. The second number or bottom number is the diastolic number. It is the lowest pressure in the artery when the heart rests. Your blood pressure changes based on your activity level, health, and many other things. You may have low blood pressure all the time or it may be due to a drug or illness. Your care will be based on the reason for your low blood pressure.