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Q t C a
Q t C vt
Vascular
space
Extracellular
space
Interstitial
space
Intracellular
space
(A)
Extracellular
space
Q t C a
Q t C vt
(B)
Intracellular
space
Q t C a
Q t C vt
(C)
Figure 6.12 Uptake from the vascular space of a tissue compartment into the intracellular space
where the tissue is represented as (A) three distinct compartments, (B) two compartments, or (C) a
single homogeneous compartment.
law of diffusion, which states that the rate of transfer of chemical across a membrane
(flux) is proportional to its concentration gradient across the membrane,
Flux
= PA ∆ ,
(62)
t
where PA t is the tissue membrane permeation area cross product and Δ C is the con-
centration gradient (units of mass/volume) of free chemical across the membrane.
For chemicals that have a perfusion-rate-limited distribution, diffusion of the
chemical across the membrane is very rapid relative to its rate of delivery to the tissue
(i.e., PA t  Q t ), and the rate of uptake by tissues is limited by blood flow rather than
the rate of diffusion across the membrane. For such chemicals, the free chemical con-
centration in the intracellular and extracellular spaces is in equilibrium, and the tissue
compartment can be represented as a single homogeneous compartment as shown in
Figures 6.11 and 6.12C . The mass balance equation for such a tissue compartment is:
V dC
dt
t =
Q C
(
C
),
(63)
t
t
a
vt
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