Claude Bernard emphasized the requirement for a stable internal environment - milieu interieur - that would allow biological processes to continue despite changes in the external environment in the 19th century (Bernard, 1878). Walter Cannon, who established the word "homeostasis" to describe how essential physiological variables are maintained within a set range by feedback mechanisms, expanded on Bernard's concept and popularized it (Cannon, 1929). In addition to the internal regulatory systems described by Bernard and Cannon, his contemporary, Curt Richter, broadened the concept of homeostasis to include behavioral responses as an important method by which homeostasis may be managed (Moran and Schulkin, 2000; Richter, 1943)
Homeostasis is a unifying issue in modern physiology, and the molecular processes of homeostatic control have been extensively studied. However, because the term is seemingly simple, it is frequently misused. It is necessary to introduce and revisit several essential definitions and concepts originally created in control theory and systems dynamics theory, but applicable to homeostatic control in biological systems, for the sake of this discussion.
To begin, it is necessary to distinguish between two types of variables found in homeostatic systems. Regulated variables are physiological variables that are kept at a constant level, such as blood glucose or core body temperature. Controlled variables, on the other hand, are the processes' activities, or rates, that contribute to the stability of regulated variables (Cabanac, 2006). Blood calcium concentration, for example, is a regulated variable, but urinary calcium excretion rate is a controlled variable that is changed to regulate blood calcium concentration.
Model of homeostasis based on stock and flow
(A) In homeostasis, the stock and flow model illustrates two types of variables: Homeostasis maintains homeostasis by controlling the quantity of a controlled variable. Flows are the procedures that alter the stock's value. Some, but not all, flow variables and targets for homeostatic control signals are controlled variables and targets for homeostatic control signals (graphically represented here as dials). Clouds represent' sources' and sink for extrinsic to the homeostatic system controlled variables.
(B) A physiologic illustration of a stock and flow model: dietary glucose uptake, hepatic glucose generation, and glucose uptake into adipose and muscle are all flows that keep blood glucose at a constant level.
The stability of a regulated variable is usually aided by the presence of multiple controlled variables. The rates of intestinal calcium absorption and bone resorption, in addition to calcium excretion in the kidney, are also regulated variables that contribute to the maintenance of a constant blood calcium concentration. The rates of intestinal and renal glucose transport, glycogenolysis, gluconeogenesis, glycolysis, glycogenesis, and glucose transport from the blood into tissues are all controlled variables in the case of blood glucose concentration (a regulated variable). Regulated variables are numbers, whereas controlled variables are processes, with process activity or rate as a variable.
In terms of systems dynamics, regulated variables are the system's stocks, whereas controlled variables are the system's flows: they either raise (in-flows) or decrease (out-flows) the regulated variable's value (Figure 1). Notably, not all equities are regulated variables, and not all regulated variables are stocks. Blood glucose, for example, is a controlled variable, whereas blood alcohol is not. Similarly, not all flows are controlled variables, but all controlled variables are flows. As a result, heat loss by sweating may be regulated, whereas heat loss through conduction cannot. We'll use both terms in this talk to stress the important components of homeostasis because they capture various aspects of system activity.
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