Blood pressure is the pressure exerted by the blood on the walls of the blood vessels. Unless indicated otherwise, blood pressure is understood to mean arterial blood pressure, i.e. the pressure in the large arteries, such as the brachial artery (in the arm). The pressure of the blood in other vessels is lower than the arterial pressure.

Within limits, the lower your blood pressure reading is, the better. In most people blood pressure isn't too low until it causes symptoms such as lightheadedness or fainting.