An example of the use of a control diagram to depict quantifiable interactions in a complex system, here cardiovascular hemodynamics as modeled by Guyton et al. The arrows represent input variables, in this case, cardiovascular regulatory factors, such as arterial pressure (AP), urine output (UO), extracellular fluid volume (ECFV), blood volume (BV), mean systemic filling pressure (MSP), right atrial pressure (RAP), and so on. This figure is reproduced with kind permission from Springer Science+Business Media: Annals of Biomedical Engineering, Critique of a Large-Scale Organ System Model: Guytonian Cardiovascular Model, 3, 1975, 396, K. Sagawa, figure 10, copyright 1975 by Academic Press, Inc.