The proteins of the blood plasma are hydrophilic colloids. They tend to hold the water of the plasma within the blood capillaries by virtue of their osmotic pressure and inability to diffuse through animal membranes, despite the hydrostatic pressure tending to filter the water out of the capillaries.
We understand the osmotic pressure of hydrophilic colloids as being the sum of the pressure exerted by the particles as molecular kinetic units moving freely in the solution volume and the pressure due to the hydration of the hydrophilic colloids. If we knew the number of hydrophilic colloid particles in a solution of given volume we should not know their osmotic pressure because in addition to the properties of molecular kinetic units the particles have an attraction for water and each particle is surrounded by a shell of water. This shell of water reduces the true volume of solvent materially and in
FAHR G, SWANSON WW. THE QUANTITIES OF SERUM ALBUMIN, GLOBULIN AND FIBRINOGEN IN THE BLOOD PLASMA IN ACUTE AND CHRONIC NEPHROPATHIES. Arch Intern Med (Chic). 1926;38(4):510-526. doi:10.1001/archinte.1926.00120280102007