The cause of oedema in Nephritic syndrome is-

there is a derangement in the capillary walls of the glomeruli that result in increased permeability to plasma proteins. Any increased permeability resulting from either structural or physicochemical alterations in the GBM allows proteins to escape from the plasma into the glomerular filtrate.With long-standing or extremely heavy proteinuria, serum albumin is decreased, resulting in hypoalbuminemia and adrop in plasma colloid osmotic pressure. Which results in a decrease of intravascular volume and renal blood flow triggers increased release of renin from renal juxtaglomerular cells. Renin, in turn, stimulates the angiotensin-aldosterone axis, which promotes the retention of salt and water by the kidney. This tendency is exacerbated by reductions in the cardiac secretion of natriuretic factors. In the face of continuing proteinuria, thesealterations fuher aggravate the edema. Ref: Robbins basic pathology 9th edition pg no:524