I'm not sure why you receive that error, but your code works properly with some tweaks.
There is no need to use global here.
If you are defining the call to ode inside a function you need to assign variables as output and call that function accordingly.
Note that as you have defined a discrete Time span, thus the result be obtained for those particular values.
[T,Y]=HematopoeiticSystemTrial
Here, the first column of the output Y corresponds to x_0, second to x_1, third to x_2 and fourth to x_3.
function [t,dydt1] = HematopoeiticSystemTrial
time = [0:1:4];
%------%
x_0 = 1/110;
x_1 = 1/24;
x_2 = 1/22;
x_3 = 1/10;
% ----- Paremeters -----%
p = [x_0 x_1 x_2 x_3];
%--------------------------------------------------%
[t,dydt1] = ode45(@myODE,time,p);
end
function dydt = myODE(t,y, r);
P0=0.7;
h0=0.0000235;
r0=0.01818;
r1=0.08712;
r2=8.0217;
d3=0.1;
p1=0.4783;
p2=0.4987;
% y(1) = x_0;
% y(2) = x_1;
% y(3) = x_2;
% y(4) = x_3;
p0=P0./(1+h0.*y(1));
tmp1 = r0.*y(1).*(p0-1);
tmp2 = 2.*r0.*y(1).*(1-p0) + r1.*y(2).*(2.*p1-1);
tmp3 = 2.*r1.*y(2).*(1-p1) + r2.*y(2).*(2.*p2-1);
tmp4 =2.*r2.*y(3).*(1-p2) - d3.*y(4);
dydt = [tmp1; tmp2; tmp3; tmp4];
end
