In physiology and medicine, hypotension is abnormally low blood pressure, especially in the arteries of the systemic circulation. It is best understood as a physiologic state, rather than a disease. It is often associated with shock, though not necessarily indicative of it. Hypotension is the opposite of hypertension, which is high blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. If it is lower than normal then it is called low blood pressure or hypotension. Hypotension is generally considered as systolic blood pressure less than 90 millimeters of mercury (mm Hg) or diastolic less than 60 mm Hg. However in practice, blood pressure is considered too low only if noticeable symptoms are present.