These terms refer only to direction not to the actual mechanism. Mechanisms may be diffusional or carrier mediated. Relation of between filtration, reabsorption secretion and excretion. See figure 15-6. First, it is important to realize that each of these is controlled to maintained homeostases.
Filtration is dependent on the net filtration pressure.
This is determined by the forces we discussed previously as Starling Forces. However, in this case, filtration is favored. Therefore, reabsorption is a big deal here. Reabsorption of water causes concentration to increase in tubule lumen. Now concentration in tubule is higher than that in capillary and diffusion occurs.