Homogeneous compartment

On some occasions, the body does not behave as a single homogeneous compartment, and multicompartment pharmacokinetics are required to describe the time course of drug concentrations. In other instances certain pharmacokinetic processes may not obey first-order kinetics and saturable or nonlinear models may be required. Additionally, advanced pharmacokinetic analyses require the use of various computer programs, such as those listed on the website http //www.boomer.org/pkin/soft.html. 

The Wagner-Nelson method of calculation does not require a model assumption concerning the absorption process. It does require the assumption that (a) the body behaves as a single homogeneous compartment and (b) drug elimination obeys first-order kinetics. The working equations for this calculation are developed next. 

The first-order one-compartment model [43,58], which considers the organism as one homogeneous compartment surrounded by a homogeneous medium, provides an acceptable estimation of surfactants bioconcentration, and has been adopted by the OECD and EPA in their guidelines [3-5]. The BCF can be determined as a ratio of the concentrations of the chemical in the organism (Ca) and the medium (Cw) under equilibrium state or as a ratio of the uptake and elimination constants (ki and, respectively). 

First, at the compartmental approach per se grounded on the assumption of homogeneous compartments. Compartmental models are in fact appropriate when there is an obvious partitioning of the material in the process... 

Strategy. Let us assume that the Earth s stratosphere is a large homogeneous compartment and that the flows of O2,0, and O3 are given by the four Chapman equations. The concentration of M (N2 and 02) is sufficiently high so that it is virtually a constant. To solve this problem, let us first set up the equations for the steady-state concentrations of O and 03 in other words, we will set up the equations for the rates of formation of O and O3 and set these rates equal to zero. Using these two expressions, we will then calculate the value of the 03 to... 

Volume of distribution represents a complex combination of multiple chemical and biochemical phenomena. It is a measure of the relative partitioning of a drug between plasma (the central compartment) and the tissues. Thus, the VD term considers all of the tissues as a single homogeneous compartment. 

In TDM, it is preferable to work with concentrations of a drug in this homogeneous compartment, so D, (amount of drug in the compartment) is divided by a volume term. This volume term is called apparent volume of distribution) or V. It is not a real volume in the physiological sense, but instead is a proportionahty constant to translate the absolute amount of drug present in the compartment into its concentration relative to a volume. 

The relative polarity of a POP appears to determine whether it partitions preferentially into polar or non-polar lipid classes62,95 and may explain, at least in part, deviations from predicted values based on single, homogenous-compartment models. 

It is also assumed that ionic species or their ionic pairs cannot diffuse in the membrane phase without intervention by the carrier. The membrane is formally divided into n homogeneous compartments which contain the carrier in the form C2A, CqB, and CH the local concentrations of which are equal to [C2A]j, [C2B]., and [CH], with k= 1, 2,..In the case of bulk agitated or flowing liquid membranes, the model should be modified by adding a large central compartment (layer). 

We have developed a number of multi-media models based on our (Higinal idea, and validated their predictability for evaluating environmental fate and exposure [2-9]. In our models, it is assumed that the environment consists of phases which are composed of several homogenous compartments. Also, the models assume that rates of intraphase transfer processes are faster than those of interphase transfer, transport and transformation processes (local equilibrium). 

These evaluative models divide the environment into homogeneous compartments, air, soil, water, and sediment and define the distribution among, and loss from these compartments. The models are based on the properties of the compound that define its distribution and transformation as well as characteristics of the environment under study, such as size of, and advective rates through compartments. Three levels, of increasing complexity, have been defined (Fig. 10.9) ... 

Thermodynamic quantities which have the same value throughout a homogeneous compartment (temperature, T pressure, p) are called intensive. Those which are proportional to the amount of component i in that compartment are called extensive (enthalpy, H entropy. S free-energy, G). An extensive quantity may be converted to an intensive one by dividing by the amount of component i present. For example, the partial molal free-energy or chemical potential. 

When distribution equilibrium is attained at a finite time (from a few minutes to a few hours), build up of drug in the tissue compartment stops and the body begins to behave as a single homogeneous compartment, with drug in both the central and peripheral compartments declining exponentially with the same rate constant p. 

Let the ecosystem be a fish in an aquarium. The uptake and clearance of compounds by fish can be modelled in the simplest way by a two-compartment model treating the fish as a single homogeneous compartment. Assuming that ... 

The body fluids do not constitute a homogeneous solution of electrolytes. Body fluids are classically categorized as intracellular and extracellular. The extracellular compartment is further divided into an intra- and extravascular compartment. The intravascular fluid constitutes the blood plasma, but even blood plasma does not form a homogeneous compartment. The composition of plasma varies with anatomical location and physiological conditions. The electrolyte compositions of plasma obtained from venous and arterial blood differ, and there are diurnal variations in the electrolyte concentrations of the plasma. 

Fig. 6 Blending chamber silo with elevated homogenizing compartment (schematic)...

Fig. 7 Blending chambersilo with integral homogenizing compartment...

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