"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. 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. In some situations only one of these two are needed and recorded, and some medical devices record only one (either the diastolic or the systolic). Therefore, this abstract class defines those properties that are common to individual systolic and diastolic observations as well as the classic “blood pressure” observation that contains both systolic and diastolic values as if it were one observation. This class defines the bodySite and method that may be needed in order to interpret the observation, as well as pointers to the four qualifier observations that may affect the interpretation.