Fig. 2

Stock and flow representation of simple homeostatic systems. A–B Basic elements of system’s dynamics models. Stocks represent variables and solid arrows indicate inflows (positive flows) and outflows (negative flows), processes that increase and decrease the value of variables respectively (A). Biflows allow for either positive or negative flows from a stock. The direction of the flow is determined by the biflow’s sign (B). C–D Demand-driven homeostatic systems. The value of the regulated variable changes under the influence of an unregulated external outflow. Control signals may balance this effect by upregulating (C) or inhibiting (D) a homeostatic inflow. In turn, the value of the regulated variable is used by controllers to increase or decrease the levels of the control signal. This effect is modeled as a biflow to indicate that it may be positive or negative depending on the particular expression used to model the dynamics of the control signal. E–F Supply-driven homeostatic systems. In these models, the value of the regulated variable is determined by the balance between an external inflow and a homeostatic flow, inhibited (E) or upregulated (F) by the control signal. Controllers and actuators are implicitly considered through their resultant influence on the dynamics of the system. Dashed arrows represent the flow of information. Blunt and pointed arrows indicate inhibition and upregulation respectively. H: homeostatic flow; E: external flow