List of Terms
βc	= Intravascular dispersive Peclet number, Eq. 33.
βp	= A permeation Peclet number, Eq. 32.
C	= Concentration (moles or mmoles/ml).
ΔC	= Concentration difference between two locations.
Cin(t)	= Concentration in the plasma of the inflowing blood.
Cout(t)	= Concentration in the plasma of the venous outflow.
Cc(x, t)	= Concentration in the capillary plasma.
CE(x, t)	= Concentration in the extravascular volume of distribution, υEVE.
c	= Capillary region.
Dc, DE	= Diffusion coefficients in the capillary plasma and the extravascular region, respectively (cm2/sec).
E(t)	= Instantaneous extraction calculated from outflow concentrations (dimensionless).
e	= Subscript denoting extravascular region.
Emax	= Maximum value of the instantaneous extraction.
E(tp)	= Value of E(t) obtained at the time of the peak of the curve for the nonpermeating tracer, hN(t).
FER	= Fractional escape rate = (d Q/dt)/Q = d(loge Q)/dt.
Fa	= Total flow through the organ (ml/sec).
Fc	= Flow through the individual capillary (ml/sec).
Fs′	= Flow of solute-containing mother fluid (ml/g sec−1), Eq. 13.
h(t)	= Frequency distribution of transit times (sec−1).
ha(t)	= h(t) for the bed lying between the injection site and the arterial inflow end of the capillary.
hcap(t)	= h(t) for the capillary tissue model for either a permeant or a nonpermeant solute.
hυ(t)	= h(t) for the vascular bed lying between the capillary outflow and a downstream sampling point in the vein.
hN(t)	= h(t) for a nonpermeant substance.
hD(t)	= h(t) for a permeating solute.
Hct	= Hematocrit, the fraction of the blood volume which is erythrocytes.
j	= Index of the segment axially along the capillary-tissue hexagon.
K	= Exchange coefficient (ml/sec), Eq. 18.
Krj	= Radial exchange coefficient in the j segments across the capillary membrane, Eq. 19.
Kxc	= Axial exchange coefficient in the capillary, Eq. 20.
KxE	= Axial exchange coefficient in the extravascular region. Eq. 21.
L	= Capillary length (cm).
Nc	= Number of capillaries in the whole organ, Eq. 9.
nx	= Number of segments axially in which the capillary-tissue hexagon is divided for numerical purposes, usually 20–60.
P, P(x)	= Capillary permeability at position x(cm) along the capillary (cm/sec).
Po, PL	= Permeabilities at the arterial and venous ends of the capillary, respectively.
Q	= Total tracer contained in an organ.
Δq	= Quantity of solute (mmoles) transferred from one region to another in one time step in a solution.
Δqr, Δqx	= Reference to amounts transferred in the r and the x directions, Eqs. 25–28.
R	= Half of the intercapillary distance with a hexagonal arrangement of parallel capillaries (cm).
Rc	= Capillary radius (cm).
r	= Radial distance from axis of capillary (cm).
ρ	= Density (g/ml) of organ tissue excluding the blood contained in large vessels.
S	= Capillary surface area (cm2/g) in tissue devoid of large nonexchanging vessels, Eq. 12.
Sc	= Surface area of a single capillary (cm2) = 2π RcL or ρ VhS.
t	= Time (sec).
σ, τ, tc	= Parameters defining a lagged normal density curve (sec) (27).
t̄c	= Capillary mean transit time (sec).
t̄E	= Mean transit time of tracer in extravascular region when blood-tissue exchange is barrier limited, Eq. 32b.
t+	= Time infinitely soon after the beginning of a time step occurring just after the shift of the capillary fluid downstream by one segment.
t−, t + Δt−	= Times just before a downstream shift of the intracapillary contents occurs at the end of each Δt in the numerical solution.
tp	= Time of the peak of hN(t).
Δt	= Duration of time step (sec).
υ	= A fractional volume of distribution (ml/ml).
υc	= Fractional volume of distribution (ml/ml of accessible intravascular “space” of blood) of permeant tracer in the blood = 1 − Hct(1 − υrbc).
υE	= Fractional volume of distribution (ml of accessible extravascular space/ml of total extravascular tissue) in the extravascular region, which is identical to the extravascular tissue-plasma partition coefficient.
υrbc	= Fractional volume of distribution of permeant tracer in the erythrocyte (ml/ml of erythrocyte), which is the erythrocyte-plasma partition coefficient.
υV	= Fraction of the organ volume which is made up of blood in large nonexchanging vessels (ml blood/ml organ).
V	= An actual volume (ml).
Vh	= Volume (ml) of a single capillary-tissue hexagon of length L.
Vc	= Volume (ml) of a single capillary of length L.
VE	= Volume (ml) of extravascular tissue in a single capillary-tissue hexagon = Vh – Vc.
W	= Total organ weight (g).
x	= Distance along the capillary from inflow, x = 0, to outflow, x = L.
ζc, ζE	= Tortuosity coefficients (dimensionless) in capillary and extravascular region, respectively; ζc = 1 + 0.5 Hct(1 − υrbc).