Equilibria in Biology and the Environment Partitioning of a Solute between Compartments

6 Equilibria in Biology and the Environment Partitioning of a Solute between Compartments 

In biology and the environment, the equilibria of interest are those of a substance distributed or partitioned among the major compartments of the system. In biology we usually seek to establish the distribution of a substance between the circulating blood and the separate compartments of fat or lipids, muscle and bone. The substances in question can be toxic or benign (e.g., a drug). 

In the environment, the major compartments of interest are air, water, and soil, and the solute involved is usually toxic or objectionable. An interesting confluence of biology and the environment occurs in the exposure of animals to toxic chemicals contained in the air or in water. The question then becomes one of establishing equilibrium concentrations between environmental and body compartments. In fish, for example, the usual aim is to determine the distribution of the solute between water and animal fat. A description of this case is given in Illustration 6.8. 

The equilibrium concentrations between various compartments are usually taken to be in a constant ratio termed the partition coefficient K. Thus, for a substance distributed between blood and muscle, we have 

The relation is, in other words, a linear one, and the partition coefficient can be viewed as the equivalent of a Henry s constant, relating the two concentrations in question. These concentrations represent hypothetical maximum levels, which may appear briefly during the initial period of exposure. Thereafter, metabolic processes and excretion intervene to reduce the concentrations to new steady-state values. These can still be substantial, as was shown in Illustration 5.7. 

---