Table 2.

Definitions of terms for homeostasis paper

Term
Control center (or integrator)	The control center consists of an error detector and controller. It receives signals (information) from sensors, compares information (value of regulated variable) with the set point, integrates information from all sensors, and sends output signals (sends instructions or commands) to increase or decrease the activity of effectors. The control center determines and initiates the appropriate physiological response to any change or disturbance of the internal environment
Controller	The component of the control center that receives signals (information) from the error detector and sends output signals (instructions or commands) to increase or decrease the activity of effectors. The controller initiates the appropriate physiological response to an error signal resulting from a change or disturbance of the regulated (sensed) variable.
Effector	A component whose activity or action contributes to determining the value of any variable the system. In this model, the effectors determine the value of the regulated (sensed) variable.
Error detector	The component in the control center that determines (calculates) the difference between the set point value and the actual value of the regulated (sensed) variable. The error detector generates the error signal that is used to determine the output of the control center.
Error signal	A signal that represents the difference between the set point value and the actual value of the regulated variable. The error signal is one of the input signals to the controller.
External environment	The world outside of the body and its “state.” The state or conditions in the outside world can determine the state of many internal properties of the organism.
Integrator	This is another term for the control center. The integrator processes information from the sensor and those components that determine the set point, determines any error signal present, and sends output signals (instructions or commands) to increase or decrease the activity of effectors.
Internal environment	The internal environment is the extracellular fluid compartment. This is the environment in which the body's cells live. It is what Bernard meant by the “internal milieu.”
Homeostasis	The maintenance of a relatively stable internal environment by an organism in the face of a changing external environment and varying internal activity using negative feedback mechanisms to minimize an error signal.
Negative feedback	A control mechanism where the action of the effector (response) opposes a change in the regulated variable and returns it back toward the set point value.
Nonregulated variable (controlled variable)	A variable whose value changes in response to effector activity but whose value is not directly sensed by the system. Controlled variables contribute to determination of the regulated variable. For example, heart rate and stroke volume (controlled variables) contribute to determining cardiac output (another controlled variable) that contributes to arterial blood pressure (a regulated variable).
Perturbation (disturbance)	Any change in the internal or external environment that causes a change to a homeostatically regulated variable. Physiologically induced changes in the set point would not be considered a perturbation.
Regulated variable (sensed variable)	Any variable for which sensors are present in the system and the value of which is kept within limits by a negative feedback system in the face of perturbations in the system. A regulated variable is any property or condition of the extracellular fluid that is kept relatively constant in the internal environment in order to ensure the viability (survival) of the organism.
Response	The change in the function or action of an effector.
Sensor (Receptor)	A “device” that measures the magnitude of some variable by generating an output signal (neural or hormonal) that is proportional to the magnitude of the stimulus. A sensor is a measuring “device.” For some regulated variables, sensors are specialized sensory cells or “sensory receptors,” e.g., thermoreceptors, baroreceptors, or osmoreceptors. For other regulated variables, sensors are cellular components, e.g., the Ca2+-sensing receptor (a G protein-coupled receptor that senses blood Ca2+ in the parathyroid gland).
Set point	The range of values (range of magnitudes) of the regulated variable that the system attempts to maintain. Set point refers to the “desired value.” The set point is generally not a single value; it is a range of values.

A glossary of terms used in discussing the core concept of homeostasis. The components of a homeostatically regulated system (Fig. 1) are defined here as are some other terms that occur in teaching this concept.