The chapter explores various factors affecting the movement of small, water-soluble solutes through capillary membranes, including permeability, concentration gradients, diffusion distance, and exchange surface area. It highlights the role of microcirculation and factors such as Starling forces in explaining the movement of fluids and dissolved substances across capillaries. The discussion also covers experiments demonstrating the impact of volume and concentration outside the capillary on fluid exchange and the pressure gradients involved in pushing fluids out of capillaries.