Page 131 - The Toxicology of Fishes

Toxicokinetics in Fishes 111

8
FLOW-LIMITED
7 MODEL F I L M
UPTAKE COEFFICIENT (L/kg hr) 5 D E 4 5 G H J K COMPLETE N
6

4

3
MODEL
2

1 C 1 2 3 O
A
B
0
0 1 2 3 4 5 6 7 8
CHEMICAL LOG K OW
FIGURE 3.25 Observed and predicted uptake of chemicals across the gills of adult rainbow trout as a function of chemical
log K ow . Branchial uptake predicted by the flow-limited gill model is shown as a solid line; that predicted by the gill model
with flow and diffusion limitations is shown as a dashed line. (Adapted from Erickson, R.J. and McKim, J.M., Aquat.
Toxicol., 18, 175–198, 1990.)

the lamellae in a posterior-to-anterior direction, providing for highly efficient counter-current exchange
with inspired water.
The principal limitations on chemical uptake at fish gills were reviewed earlier in this chapter (see
Compartmental Models for Fish and Branchial Clearance sections). If the permeability of the gill
epithelium to a compound is low, diffusion can limit exchange, and uptake flux is controlled by the
product of gill permeability and the surface area for diffusion. If, on the other hand, the permeability of
the epithelium is high, uptake may be limited by the chemical capacities of blood and water flowing to
the gills. A blood flow limitation exists if more chemical is delivered to the gills in respiratory water
than can be carried away in blood. A water flow limitation exists if the capacity of blood to carry chemical
away from the gills exceeds the chemical capacity of respiratory water.
Ignoring diffusion limitations on uptake, Erickson and McKim (1990a) developed a model for the
branchial flux (F ) of organic chemicals based on flow limitations and the behavior of counter-current
g
exchange systems. The complete flow-limited model may be stated as:
[
F g = Min Q Q P bw]   C w −( C P bw)  (3.96)
w ,
v
c
where Q is the flow of inspired water that exchanges with blood, Q is the total cardiac output, P is
c
bw
w
an equilibrium blood–water partition coefficient, and C is the chemical concentration in venous blood
v
entering the gills. In this equation, F is calculated as the product of the concentration gradient between
g
inspired water and venous blood entering the gills (second term in brackets) and the chemical capacity
of blood or respiratory water (first term), whichever is less. When the flow-limited model was parame-
terized using data for rainbow trout it reproduced the well-known dependence of branchial uptake
efficiency on chemical log K for compounds of low to intermediate hydrophobicity but did not predict
ow
the observed decline in uptake for high log K compounds (Figure 3.25). In addition, the model tended
ow
to overestimate the maximal rate of uptake by about 20%.
Subsequently, Erickson and McKim (1990b) published a more comprehensive gill model that included
both flow and diffusion limitations, as well as binding of high log K compounds to dissolved organic
ow
material. The full model, which included advective (direction of fluid flow) and diffusive (perpendicular