  We use microscopic linear response theory to derive a set of equations that
provide a complete description of coupled spin and charge diffusive transport
in a two-dimensional electron gas (2DEG) with the Rashba spin-orbit (SO)
interaction. These equations capture a number of interrelated effects including
spin accumulation and diffusion, Dyakonov-Perel spin relaxation,
magnetoelectric, and spin-galvanic effects. They can be used under very general
circumstances to model transport experiments in 2DEG systems that involve
either electrical or optical spin injection. We comment on the relationship
between these equations and the exact spin and charge density operator
equations of motion. As an example of the application of our equations, we
consider a simple electrical spin injection experiment and show that a voltage
will develop between two ferromagnetic contacts if a spin-polarized current is
injected into a 2DEG, that depends on the relative magnetization orientation of
the contacts. This voltage is present even when the separation between the
contacts is larger than the spin diffusion length.
