In biochemistry, steady state refers to the maintenance of constant internal concentrations of molecules and ions in the cells and organs of living systems. Living organisms remain at a dynamic steady state where their internal composition at both cellular and gross levels are relatively constant, but different from equilibrium concentrations.
Blood glucose
thumb|Figure 2: Regulation of metabolic pathways maintains blood glucose concentration at approximately 5 mM in humans.
Blood glucose levels are maintained at a steady state concentration by balancing the rate of entry of glucose into the blood stream (i.e. by ingestion or released from cells) and the rate of glucose uptake by body tissues. Glucose levels in the brain are also maintained at steady state, and glucose delivery to the brain relies on the balance between the flux of the blood brain barrier and uptake by brain cells. In teleosts, a drop of blood glucose levels below that of steady state decreases the intracellular-extracellular gradient in the bloodstream, limiting glucose metabolism in red blood cells.
Blood lactate
Blood lactate levels are also maintained at steady state. At rest or low levels of exercise, the rate of lactate production in muscle cells and consumption in muscle or blood cells allows lactate to remain in the body at a certain steady state concentration. If a higher level of exercise is sustained, however, blood lactose levels will increase before becoming constant, indicating that a new steady state of elevated concentration has been reached. Maximal lactate steady state (MLSS) refers to the maximum constant concentration of lactase reached during sustained high-activity.
Nitrogen-containing molecules
Metabolic regulation of nitrogen-containing molecules, such as amino acids, is also kept at steady state.
Ions
thumb|Figure 3: Different steady state concentrations of ions on either side of the cell membrane maintain a resting membrane potential.
One main function of plasma and cell membranes is to maintain asymmetric concentrations of inorganic ions in order to maintain an ionic steady state different from electrochemical equilibrium. In other words, there is a differential distribution of ions on either side of the cell membrane - that is, the amount of ions on either side is not equal and therefore a charge separation exists. The maintenance of this steady state gradient, in turn, is used to do electrical and chemical work, when it is dissipated though the passive movement of ions across the membrane. Electrical excitation of the cell results in an influx of sodium ions into the cell, temporarily depolarizing the cell.