steady state: one construct, many systems

In the theory of open systems, a steady state (Fließgleichgewicht) is a condition in which a system's composition stays constant not because nothing moves but because inflows and outflows exactly balance. Von Bertalanffy's 1950 The theory of open systems in physics and biology distinguished this flowing balance from true thermodynamic equilibrium, where nothing flows at all. The referent is a constant state amid throughput, the same construct that reappears as a drug plateau and as a growth path, recognised by unchanging state variables despite ongoing fluxes.[1]

Unit

Constant state variables held by balanced inflow and outflow

Recovery means

Not applicable

Measured by

Identified when state variables stop changing (time-derivatives near zero) while fluxes continue

In growth economics, the steady state is the long-run path on which capital per worker and output per worker stop changing because saving-driven investment just offsets depreciation and labour-force growth. Solow's 1956 A contribution to the theory of economic growth showed an economy converging to such a balanced path regardless of its starting capital. The referent is a constant per-capita configuration sustained by balanced flows of investment and depreciation, the same balance-amid-throughput idea as an open biological system or a dosed drug, here realised in an economy.[2]

Unit

Constant capital- and output-per-worker on a balanced growth path

Recovery means

Not applicable

Measured by

Located where net investment per worker is zero, so the capital-labour ratio is stationary

In pharmacokinetics, steady state is reached during repeated or continuous dosing when the rate of drug entering the body equals the rate of elimination, so the average plasma concentration plateaus and stops rising from dose to dose. Gibaldi and Perrier's Pharmacokinetics shows it is approached over roughly four to five elimination half-lives and sets the target concentration for maintenance dosing. The referent is a constant average drug level held by balanced input and clearance, the identical balanced-flow construct seen in open systems and in growth economics, realised in the body.[3]

Unit

A constant average drug concentration held by balanced dosing and clearance

Recovery means

Not applicable

Measured by

Taken when pre-dose (trough) concentrations stop rising across doses, reached after about four to five half-lives