Why kidney disease causes edema?

Why kidney disease causes edema?
Kidney is main organ to excrete water and sodium. Kidney disease would make excretion of water and sodium decrease. Then edema would occur due to the retention of water and sodium. This is called renal edema. Edema is the most common symptom of kidney disease. The reasons cause renal disease include:
1 GFR(glomerular filtration rate) decrease, retention of water and sodium
2 permeability of blood capillary from whole body change, then body fluid would flow into the space between tissues.
3 plasma-albumin level reduce, then the plasma colloid osmotic pressure would lower
4 effective blood volume reduce,this would causes increase of secondary aldosterone.
Clinically , we can classified renal edema into two category according to different pathogenesis:
1 nephritis edema: often come along with acute nephritis, maybe rapidly progressive glomerulonephritis(RPGN),CKD and other glomerulopathy. Main reasons for edema are:
---low GFR
---imbalance of glomerulotubular, when glomerrulus get infected, GFR lower down obviously, but the function of glomerular reabsorption works well.,so the balance between glomerulus and kidney tubules would break. Then the absorption of water and sodium in kidney tubules would increase, so edema occur.
---the increase of capillary hydrostatic pressure would make the fluid inside capillary flow to the space between tissues ,finally edema occur.
---suffering acute nephritis , some patients’ blood volume and blood pressure would go up, this would bring about congestive heat failure. Then retention of water and sodium would worsen.
2 nephrotic edema: it always goes with primary glomerular disease and nephrotic syndrome(NS) which caused by other reasons. The pathogenesises are :
---plasma colloid osmotic pressure decline: when suffer kidney disease, patients would suffer hypoproteinemia because urine protein. Then plasma colloid osmotic pressure decline, and the filtration of body fluid in capillary increase, but the absorption of fluid from space between tissues reduce, finally edema appear
---effective blood volume reduce: the plasma extravasation would reduce effective blood volume and stimulate volume receptor in vessel, and then provoke system of renin-angiotensin-aldosterone. Antidiuretic hormone secretion would increase while diuresis hormone secretion decrease. Glomerulus’ reabsorption of sodium increases. Those lead to serious retention of water and sodium. Finally, edema turns worse.