Biological balance of sodium and potassium: a control system with oscillating correcting variable

The rhythm of renal sodium and potassium excretion was measured in 4-h-intervals in 12 subjects. Each person exhibited clear circadian variations of each variable with a maximum between 8 a.m. and 4 p.m. In each subject and for both circadian rhythms the oscillation mean was correlated to the range of oscillation (amplitude). Increase in sodium or potassium excretion during 1 day resulted in an increase of oscillation range. The oscillation means of sodium and potassium periodicity did not correlate. The properties of biological control systems with oscillating correcting variables are comparable to those of technical control systems. The significance of circadian rhythm for the control of electrolyte balance is indicated.