Blood pressure (BP) is the pressure of circulating blood on the walls of blood vessels. Used without further specification, “blood pressure” usually refers to the pressure in large arteries of the systemic circulation. Blood pressure is usually expressed in terms of the systolic pressure (maximum during one heart beat) over diastolic pressure (minimum in between two heart beats) and is measured in millimeters of mercury (mmHg), above the surrounding atmospheric pressure (considered to be zero for convenience). Blood pressure is one of the vital signs, along with respiratory rate, heart rate, oxygen saturation, and body temperature. Normal resting blood pressure in an adult is approximately 120 millimetres of mercury (16 kPa) systolic, and 80 millimetres of mercury (11 kPa) diastolic, abbreviated “120/80 mmHg”. Traditionally, blood pressure was measured non-invasively using a mercury-tube sphygmomanometer, which instrument is still generally considered to be the gold standard of accuracy. More recently other semi-automated methods have become common, largely due to concerns about potential mercury toxicity, although cost and ease of use have also influenced this trend. Early alternatives to mercury-tube sphygmomanometers were often inaccurate, but modern validated devices have similar accuracy to mercury devices. Blood pressure is influenced by cardiac output, total peripheral resistance and arterial stiffness and varies depending on situation, emotional state, activity, and relative health/disease states. In the short term, blood pressure is regulated by baroreceptors which act via the brain to influence nervous and endocrine systems. Blood pressure that is low is called hypotension, and pressure that is consistently high is hypertension. Both have many causes. and may be of sudden onset or of long duration. Long-term hypertension is a risk factor for many diseases, including heart disease, stroke and kidney failure. Long-term hypertension is more common than long-term hypotension, which often goes undetected because of infrequent monitoring and the absence of symptoms.