help with this type of ode45

Hi,

I am trying to solve a set of differential equations using ode45, the problem is that i need to define some if conditions and a state that does not depend of the time.

Initial conditions for time dependent states

x0=[0.43 5 0.58 0.14 0.0024 0.01 4];

Initial conditions for non-time dependent state

u(1)=um*((x0(3)/x0(2))/((x0(3)/x0(2))+Ksr))*(1-((x0(3)/x0(2))/Sm)^nk)*((x0(5)/(Kox*x0(1)+x0(5))));

Time definition

tsim=50;

dt=0.0001;

t(1)=0;

i=1;

Ni=round(tsim/dt);

for i=1:Ni

if t(i)<24

          F1=0;

          F2=0;

          elseif t(i)>24 && t(i)<30

          F1=80*(1/1000);

          F2=0;%70*(1/1000);

           else

          F1=0;

          F2=0;

end

dx=@(t,x)    [u(i)*x(1)-((F1+F2)/x(7))*x(1);
             -((Csx*u(i)*x(1))+(Rcsx*x(1))+Csp*((k1*u(i)*x(1))+(k2*x(1))))+(F1/x(7))*Sin-((F1+F2)/x(7))*x(2);
             -((Cnx*u(i)*x(1))+(Rcnx*x(1)))+(F2/x(7))*Nin-((F1+F2)/x(7))*x(3);
             ((k1*u(i)*x(1))+(k2*x(1)))-((F1+F2)/x(7))*x(4);
              ((KL*(O2Leq-x(5)))-(k3*u(i)*x(1))-((k4*k1*u(i)*x(1))+(k4*k2*x(1))))+(F3/x(7))*O2in-((F1+F2)/x(7))*x(5);
              ((alfa1*u(i)+alfa2)*x(1))+(alfa3)-((F1+F2)/x(7))*x(6);
              (F1+F2)];

[t,x]=ode45(dx,tspan,x0);     

u(i+1)=um*((x(3)/x(2))/((x(3)/x(2))+Ksr))*(1-((x(3)/x(2))/Sm)^nk)*((x(5)/(Kox*x(1)+x(4))));

      if u(i+1)<0

          u(i+1)=0;
      end

       t(i+1)=t(i)+dt;

        i=i+1;

end

besides, i need to define some if conditions for time dependent states

if x(3)<0.15

          x(3)=0.15;

      end

if x (5)<0.002432

          x(5)=0.002432;

      end

I´ll appreciate your help with this.

Best Regards.