v1=Va a[t]/(a[t]+Ka); v2=Vb b[t]/(b[t]+Kb);

v3N=(Vcfw*c[t]/(Kcfw)-Vcr*d[t]/(Kcr))/(1+c[t]/(Kcfw)+d[t]/(Kcr));

v4=Vd d[t]/(d[t]+Kd); v5=Ve e[t]/(e[t]+Ke); v6=Vf f[t]/(f[t]+Kf);

Va=2; Ka=3; Vb=7; Kb=10; Vcfw=6; Kcfw=0.5; Vcr=4; Kcr=3; Vd=2; Kd=5; Ve=1; Ke=4; Vf=3; Kf=3;

NDSolve[{a'[t]==v6-v1,b'[t]==v1-v2,c'[t]==v2-v3,d'[t]==v3-v4,e'[t]==v4-v5,f'[t]==v5-v6,a[0]==1,b[0]==0,c[0]==0,d[0]==0,e[0]==0,f[0]==0},{a,b,c,d,e,f},{t,0,30}];

Plot[{Evaluate[a[t]/.%],Evaluate[b[t]/.%],Evaluate[c[t]/.%],Evaluate[d[t]/.%],Evaluate[e[t]/.%],Evaluate[f[t]/.%]},{t,0,30},PlotRange->{0,0.5},PlotStyle->{Gray,Thickness[0.01],Dashed,Thickness[0.01],Dashed,Thickness[0.01]}]