The exchange of molecules between the blood and the tissues occurs across the walls of the capillaries. This process is facilitated by the fact that capillary walls are only one cell thick and virtually every cell in the body is no more than 20 microns away from a capillary. This is the movement of soluble substances down their concentration gradients. The pressure inside the capillary is greater than that of the surrounding tissue. Hence, water and dissolved solutes are forced out of endothelial cell gaps by hydrostatic pressure.
Bulk flow is opposed by colloid osmotic pressure.
The word 'colloid' means 'in between suspension and solution'. These particles 'dilute' the blood water, which creates a diffusion gradient for water into the capillary. This 'pressure' causes water to flow back into the capillaries. The colloid osmotic pressure is directed into the capillary. Its value is about -28 mmHg, and is constant along the length of the capillary.