hi protein, specially albumen in our blood helps in maintaining oncotic pressure of blood, which is the negative pressure that keeps the contents of blood with in the blood vessels. this is opposed by hydrostatic pressure that tries to drive the blood contents out of vascular compartment into cavities like abdominal cavity etc. when oncotic pressure drops due to low albumen (mean low protein), hydrostatic pressure dominates and throws the fluid into cavities until equilibrium of pressures is attained. hope this helped.