Fin=klA (pCO2/H - CO2[t]);

r1=k1 ((FLB[t])^2)((CO2[t])^0.5);
r2=k2 FLBT[t] ZHU[t];
r3=(k2/KK)FLB[t] ZLA[t];
r4=k3 FLB[t]((ZHU[t])^2) CO2[t];
r5=k4 FLBZHU[t]((CO2[t])^0.5);

eqns={FLB'[t]==-2 r1+r2-r3-r4,CO2'[t]==-0.5 r1-r4-0.5r5+Fin,FLBT'[t]==r1-r2+r3,ZHU'[t]==-r2+r3-2 r4,ZLA'[t]==r2-r3+r5};

eqEqn=Ks FLB[t] ZHU[t]==FLBZHU[t];

params={k1->18.7,k2->0.58,k3->0.09,k4->0.42,KK->34.4,klA->3.3,Ks->115.83,pCO2->0.9,H->737};

ics={FLB[0]==0.444,CO2[0]==0.00123,FLBT[0]==0,ZHU[0]==0.007,ZLA[0]==0};

sol=NDSolve[{eqns,eqEqn,ics}/.params,{FLB,ZHU,,CO2,ZLA},{t,0,200}];

Row[Plot[Evaluate[#[t]/.sol],{t,0,200}, PlotRange->All,PlotLabel->#[t]]& /@ {FLB,ZHU,CO2,ZLA}]