Rates of solute absorption from tissue depots: Theoretical considerations

Summary

Equations are derived for describing rates of solute absorption from tissue depots by capillary blood. One form of the generalized equation is:

$$\frac{{\dot M}}{M} = - \frac{n}{\lambda }\left[ {1 - \exp ( - \frac{{fb k a}}{b}} \right]$$

where

M=amount of a reference solute present in a tissue at any time;

$\dot M$=rate at whichM is changing;

n=number of actively perfused capillaries per unit volume of tissue;

b=blood flow per actively perfused capillary;

λ=f _b/f_e, wheref _b andf _e represent activity coefficients for the solute in blood and extravascular tissue respectively;

a=capillary surface area available for solute absorption per capillary;

k=permeability constant of the capillary wall for the permeant species of the reference solute.

The symbolsk, a andb represent appropriately weighted means characteristic of the entire population of perfused capillaries within each and every elemental volume of tissue, as defined formally in the derivation. The equation is examined in reference to experimental data on the absorption of radioxenon and sodium radioiodide injected into the gastrocnemius muscle of anesthetized cats. The results indicate that rates of radioidide absorption from this tissue site are considerably diffusion- or permeability-limited even at resting levels of blood flow.