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The substances introduced into the circulating blood are divided into two groups. Substances such as, for instance, glucose are termed "reactive," the others, "nonreactive." The elimination of the nonreactive substances from the circulation proceeds according to definite laws, according to an exponential function, or very approximately so. This is not the case with reactive substances, which are subject to a variety of interrelated metabolic reactions and to organ regulations. The physical chemical factors determining the concentration of a substance in the organism are discussed. Attention is called to the error of using the terms "plasma level," "serum level," or "blood level" synonymously, for obvious reasons. In the chapter dealing with the kinetics of the concentration of a nonreactive substance in the blood, the significance of the flow equilibrium (Fliessgleichgewicht) or the steady state is discussed. The term "invasion" comprises the resorption and distribution of a given substance in the organism
The Blood Level: Kinesis of the Concentration Changes in the Circulating Fluid.. AMA Am J Dis Child. 1954;87(5):658. doi:10.1001/archpedi.1954.02050090646025