"Blood pressure (BP) is the pressure exerted by circulating blood upon the walls of blood vessels and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation, usually measured at a person's upper arm. A person’s blood pressure is usually expressed in terms of the systolic (maximum) pressure over diastolic (minimum) pressure and is measured in millimeters of mercury (mm Hg). … Blood pressure varies depending on situation, activity, and disease states, and is regulated by the nervous and endocrine systems. … Chronic hypertension is a risk factor for many diseases, including kidney failure, heart attack, and stroke." – Wikipedia. This class represents the measurement (or “clinical observation”) of the patient’s Blood Pressure. Blood pressure is typically expressed in terms of the systolic pressure over diastolic pressure as if this were a single observation, yet in fact these are two separate observations taken moments apart. Therefore, the value of this “blood pressure observation” is made up of a set of one systolic and one diastolic observation. This class also re-defines the generic clinical Observation class to fix the observed characteristic to LOINC 55284-4.