α0, α1, αn	Moments about zero for a probability density  function. (Units are tn when t is the inde-  pendent variable.) [α0 = area; α1 = mean;  for the density function h(t), ${\alpha }_n={\int }_{-0}^{+\infty }{t}^{n}h\left(t\right)\mathrm{d}t$. See central moments, μ
β n–2	Dimensionless parameters of shape of density  function calculated from the central mo-  ments, $={\mu }_n∕{\mathrm{SD}}^n={\mu }_n∕{\mu }_2^{n∕2}$. “β1” is skewness  (or asymmetry); β1 is zero for all symmet-  rical functions, positive for right skewness.  “β2” is kurtosis (or flatness). β2 = 3.0 for  normal density function; β2 > 3 for lepto-  kurtosis (highpeakedness), and <3 for  platykurtosis
γ	Ratio of interstitial volume of distribution  to intracapillary volume of distribution,  ${\mathrm{V}}_{\mathrm{I}}^{\prime }∕{\mathrm{V}}_{\text{cap}}^{\prime }$
Δ	Difference
δ(t)	Unit impulse func-  tion, or Dirac delta function, has unity area, an infinite amplitude  at t = 0, and is zero at all other times. It is  the limit of any symmetrical unimodal den-  sity function of unity area as its width ap-  proaches zero. For delta function occurring  at a nonzero time t0, it is written δ(t – t0)
∊	Epsilon, vanishingly small difference
ζ	Tortuosity of diffusion pathway. ζ is ratio of  apparent path length to measured length of  diffusion pathway, dimensionless; thus the  effective diffusion coefficient, D = D0/ζ2  where D is the free aqueous diffusion coef-  ficient
η	Viscosity, poise (P) = dyn/cm2 = g·s−1·cm−1.  Water viscosity = 0.01002 P at 20°C.  Plasma viscosity ≈ 0.011
η(t)	Equals h(t)/R(t) (fraction/s); the emergence  function, the specific fractional escape rate  following an impulse input. Of the particles  residing in the system for t seconds after  entering, η(t) is the fraction that will depart  or escape in the tth second. In chemical  engineering it is known as the intensity  function (7), and in population statistics as  the risk function, the death rate of those  living at age t. Also, η(t) = (dR/dt)/R(t) =  −d logeR(t)/dt. See FER(t)
Θ	Ratio of intracellular volume of distribution  to intracapillary volume of distribution,  ${\mathrm{V}}_{\text{cell}}^{\prime }∕{\mathrm{V}}_{\text{cap}}^{\prime }$, dimensionless
λ, λij	Partition coefficient, a dimensionless ratio  of Bunsen solubility coefficients in two  phases. λij is the ratio of solubility in region  or solvent i to the solubility in region j. The  reference region j is usually the plasma. At  equilibrium, λij is the ratio of concentra-  tions
μ	Chemical potential for a solute in a solution,  N·m−2; μ = μ0 + RT In a, where the activity  a is a concentration times an activity coef-  ficient and μ0 is the potential at a reference  state of temperature and pressure
μ n	nth central moment of a density function,  h(t), a moment around the mean, $\stackrel{‒}{t}$. ${\mu }_n={\int }_{-\infty }^{\infty }{(t-\stackrel{‒}{t})}^{n}h\left(t\right)\mathrm{d}t$. Units are those of t to  the nth power
μ2, μ3, μ4	μ2 is variance, the second moment of a density  function around the mean, $={\alpha }_2-{\alpha }_1^2$. Also  ${\mu }_3={\alpha }_3-3{\alpha }_1{\alpha }_2+2{\alpha }_1^3$ and ${\mu }_4={\alpha }_4-4{\alpha }_1{\alpha }_3+6{\alpha }_1^2{\alpha }_2$. See also βn
π	Osmotic pressure, Pa or N·m−2 or mmHg, is  the pressure that would have to be exerted  on a solution to prevent pure water from  entering it from across an ideal semiperme-  able membrane, i.e., a membrane permeable  to solvent only. π = CRT is Van’t Hoff’s  law for ideal dilute solutions, and across a  membrane impermeable to solute. π =  φCRT is preferred to account for activity  coefficients less than unity. When the sol-  ute can permeate the membrane, the effec-  tive π = σφCRT. Osmotic pressure, a colli-  gative property of solutions, is related to  actual pressure rn the same fashion as a  freezing point is to actual temperature. On-  cotic pressure is a term, now obsolete al-  though historically useful, for the osmotic  pressure associated with the presence of  large, relatively impermeant molecules such  as plasma proteins. It should now be re-  placed by more exact terms, e.g., across  some specific membrane the effective Δπ  equals $RT{\phi }_{i=1}^{i=N}{\sigma }_i{\varphi }_i\Delta {\mathrm{C}}_i$, where the effects of  concentration differences for a set of N  solutes are summed.
ρ	Density, g·cmm−3. (Specific gravity is density  relative to density of water)
σ	Reflection coefficient, in notation of irrevers-  ible thermodynamics, dimensionless; σ =  –LpD/Lp or, experimentally, σ = –JD/Jv for  ΔC8 = 0. The effective osmotic pressure  across a membrane is σΔπ, mmHg; i.e., σ =  (observed osmotic pressure)/CRT
τC	Capillary mean transit time, t̄c, used in Krogh  cylinder capillary-tissue models with plug  flow velocity profiles
φ	Activity coefficient, the ratio of apparent  chemically effective concentration to the  actual concentration in a solution, in the  absence of chemical binding, dimensionless.  The osmotic activity coefficient φ = π/CRT
ψ	Electrical potential, V
ω	Solute permeability coefficient, ω = P/RT,  mol·cm−2. s−1 ·(mmHg)−1. In the notation of  irreversible thermodynamics ω = (LD – RT/F  σ2Lp)C̄s, where C̄s, is the average solute con-  centration across the membrane