The key thing to remember is that this happens by diffusion.
Blood plasma is forced out of capillaries due to high pressures at the arterial end (the end where the arteries connect) - it diffuses out.
Then, when it becomes tissue fluid, it bathes the body cells. Oxygen and glucose diffuse into the body cells by the tissue fluid and waste and urea diffuse out.
The tissue fluid seeps back into the capillary at lower pressures (the vein end of the capillary).