Formulating and evaluating quantitative models of control of sodium stores

Numerous factors that influence sodium handling have been identified, and many have been studied in minute detail; however, relatively little information is available regarding either the steady-state relationship between dietary sodium intake and sodium stores or the transient response of intact animals to challenges to sodium homeostasis. In this paper the principles of elementary feedback control theory have been used both to obtain and analyze quantitative models of the feedback control of sodium stores. It has been assumed that the sodium content of the body determines the rate of urinary sodium excretion, and a mass balance has been used to obtain differential equations that describe the dynamics of sodium stores. Both first- and second-order models are considered, and their predictions for both steady states and transients are compared critically with observations from the literature, using data from human studies whenever possible. The results indicate that a relatively simple proportional feedback controller describes most available data well; however, gaps in the available information are identified, and opportunities for future experimental investigation are described.