Odema occurs when plasma protein level is below-

Under normal circumstances albumin accounts for almost half of the total plasma protein. Therefore conditions in which albumin is either lost from the circulation or synthesized in inadequate amounts are common causes of reduced plasma osmotic pressure. In nephrotic syndrome , damaged glomerular capillaries become leaky, leading to the loss of albumin (and other plasma proteins) in the urine and the development of generalized edema. Reduced albumin synthesis occurs in the setting of severe liver disease (e.g., cirrhosis) and protein malnutrition . Regardless of cause, low albumin levels lead in a stepwise fashion to edema, reduced intravascular volume, renal hypoperfusion, and secondary hyperaldosteronism. Unfounately, increased salt and water retention by the kidney not only fails to correct the plasma volume deficit but also exacerbates the edema, since the primary defect--low serum protein--persists. Robbins 9 th edition page 77