  A theoretical analysis of the thermodynamic response functions of the 2D
single-band Hubbard model is realized by means of the composite operator
method. It is shown that all the features of these quantities can be explained
by looking at the dependence of the thermodynamic variables on their conjugate
ones. It is found that the numerical data from quantum Monte Carlo techniques
for the internal energy and electronic specific heat are well reproduced. The
anomalous normal state properties in hole-doped cuprate high Tc superconductors
are also well described. Actually, the results for the linear coefficient of
the electronic specific heat are in agreement with those obtained by using a
pure fermionic theoretical scheme. Finally, we obtain several characteristic
crossing points for the response functions when reported as functions of some
thermodynamic variables. These peculiar features indicate the existence of more
than one energy scale competing with thermal excitations in the 2D Hubbard
model.
