Steady state concentration refers to the amount of a substance in the body or a chemical process when no changes occur over time. In humans, this can occur when a drug is given to a patient over a long period, and reaches a constant level in the bloodstream. During chemical reactions, steady state concentration occurs when the amount of raw materials equals the amount of finished product in a continuous reaction.
Drug therapy often requires a patient to receive a continuous level of drug for medical treatment. As drugs enter the body, they are metabolized or consumed by the body, and are eventually removed by the kidneys in the patient's urine. If a constant level of drug is needed, then repeated blood tests are taken over time to determine what amount must be given to the patient to maintain a constant level. These tests determine the steady state concentration of the drug.
Pills do not usually create a steady concentration in the body because they are introduced as separate doses, not given continuously. This is why a doctor may prescribe multiple doses of drug for several days. Drug researchers have tested the drugs and determined how they are removed from the body. Repeated doses can give a level in the blood similar to a steady state concentration.
Chemical processing may use continuous reactors that take raw materials and produce finished products. These systems are different from batch reactors, where all the materials are introduced and reacted until the raw materials are consumed. Continuous reactions rely on steady state concentration, because a specific amount of raw material must be introduced to maintain a constant amount of finished product.
If the raw material rate is increased, more finished product may be created than can be handled downstream. Another effect is the chemical reaction may be different and create the wrong finished products or a different chemical mix. Either way, the steady state concentration is disturbed, and control systems are programmed to correct the flow rates and return the process to chemical equilibrium, or steady state.
All steady state conditions, whether inside the body or in chemical reactors, will vary slightly over time. In the body, changes in body temperature or metabolism, such as might occur with exercise, can cause the body to metabolize a drug differently. If a minimum concentration of drug is needed, the manufacturer may test the effects of different body conditions on the steady state concentration. If a constant drug level is critical for patient treatment, the drugs may need to be administered in a hospital so repeated blood tests can be performed.
Chemical reactions will use instrumented control systems to monitor the process for steady state behavior. As the process changes slightly, the control systems can make adjustments to flow rates, temperatures or pressures. The steady state concentration may vary slightly, which is called variation within limits, but will continue to produce finished product that meets specifications.