How do I use the integral function with an m-file that contains a for loop? Integral Function Problems

Hi, I am trying to integrate the following function with respect to T :

Cp= A0 + (A1*T^1) + (A2*T^2) + (A3*T^3) + (A4*T^4) + (A5*T^5) + (A6*T^6) + (A7*T^7) + (A8*T^8)

so that:

h=integral of Cp from Tz=.005 to Tz=.373

I have written an m file to define Cp, but when I use the integral function to call my .m file, I get a value that is way lower than what it should be. The value of the integral comes back less than 1 when it really should be about 341.2. I am including my .m file code and how I tried to use the integral below. Any pointers would be very appreciated. Thank you in advance.

(note: T in the equation for Cp is actually a value Tz where Tz=T/1000)

Parameters:

Ts= 373.15;     %INPUT = DEGREES IN KELVIN
Tz=Ts/1000;

MW_o2 = 31.999;  
R_o2=.260;  

T_ref_K= 0.5;   % KELVIN, temp is NOT CELSIUS
T_ref_C= -272.65;
P_ref=101;   % kPa

matrix_o2 = [ 1.00645, -1.047869, 3.729558, -4.934172, 3.284147, -1.095203, 0.145737, 0 ,0, 0.36979, 0.000491 ];

Tz_low=.5/1000;
Tz_high=Ts/1000
Cp_o2_old=0;    %this would be a function INPUT

m-file

function [ y ] = Cp_o2( Cp_o2_old, matrix_o2, Tz )
%UNTITLED2 Summary of this function goes here
%   Detailed explanation goes here

for i=1:9

value_o2=matrix_o2(i)*(Tz.^(i-1))
Cp_o2_new=Cp_o2_old + value_o2
Cp_o2_old=Cp_o2_new;
Cp_o2_new;
end

y=Cp_o2_new;

end

integral function:

integral(@(Tz)Cp_02(Cp_o2_old, matrix_o2, Tz), Tz_low, Tz_high)

Thank You Again, J. Ryan Kersh