It takes place through these tiny pores and holes we call junctions and slits that exist between our endothelial cells of the capillary wall. Now, the next question is what causes this movement of blood plasma across the wall of the capillary? And in what direction does this fluid actually move? Does it move out of the capillary or into our capillary? Or does it move in both directions? So, to answer this question, we have to discuss the pressure that exists between the inside of the capillary and the outside, the tissue side of the capillary.