I didn't understand the equation. In my opinion extracellular free ligand accumulation depends on;

-Internalization of the receptor ligand complexes(Cause adsorption and desorption pathway is reversible, internalization decreases the possiblity of desorption)

-Recycling

-Degradation

-Complex formation

However in the article of Tzafriri et al(2004) titled "Analysis of compartmental models of ligand induced endocytosis" includes a differential equation like dL/dt= (-kf*L*Rs+ kr*Cs)(n/Nav) which doesn't include anything about these parameters. Also I didn't understand why this equation includes (n/Nav) part. Can anyone help me to understand?

(L: free extracellular ligand, Rs: Surface receptor, Cs: Surface complex, kf:ligand adsorption, kr: ligand desorption, n: cell density(cell/l), Nav: Avagadro number)