  Considering the growing interest of the astrophysicist community in the study
of dissipative fluids with the aim of getting a more realistic description of
the universe, we present in this paper a physical analysis of the
energy-momentum tensor of a viscous fluid with heat flux. We introduce the
general form of this tensor and, using the approximation of small velocity
gradients, we relate the stresses of the fluid with the viscosity coefficients,
the shear tensor and the expansion factor. Exploiting these relations, we can
write the stresses in terms of the extrinsic curvature of the normal surface to
the 4-velocity vector of the fluid, and we can also establish a connection
between the perfect fluid and the symmetries of the spacetime. On the other
hand, we calculate the energy conditions for a dissipative fluid through
contractions of the energy-momentum tensor with the 4-velocity vector of an
arbitrary observer. This method is interesting because it allows us to compute
the conditions in a reasonable easy way and without considering any
approximation or restriction on the energy-momentum tensor.
