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(Tissue) Edema is caused by:

1. that which happens in the capillaries: controls capillary fluid exchange

• the four possible reasons are found here: http://www.cvphysiology.com/Microcirculation/M010.htm
  • note the interesting fact that heart failure can cause capillary hydrostatic pressure to increase
• here’s the link on the formula on fluid flux (see also EFP aka effective filtratin pressure): http://www.cvphysiology.com/Microcirculation/M011.htm
• and further definition of hydrostatic and oncotic pressures: http://www.cvphysiology.com/Microcirculation/M012.htm

in any case, edema happens when the increase in net filtration (i.e. filtration - reabsorption) much more than lymph flow can handle.
2. that which happens in the kidneys: i.e. sodium-water retention when the glomerulus-tubule balance gets out of balance

• it’s either
  • GFR dropping, or
    • e.g. glomerulus nephritis in nephritic syndrome or nephrosis when less water passes through
    • e.g. hyperemic heart failure, 肾病综合症，cirrhosis cum ascites when effective blood flowing through is reduced, causing further constriction of renal blood vessels
  • tubules increasing reabsorption
    • aldo increases
    • adh increases
    • anp falls
    • ff increase (more than 20%) due to efferent vessel constricting moe than usual
    • redistribution of blood in kidneys, increasing renin and angiotensin production and secretion

note1: most problems come about when the glomerulus-tubule balance gets out of balance (i.e. of the water passing the glomerulus, a higher fraction - e.g. sodium absorption will be more than the normal 65-70% - is reabsorbed). normally, 99% of the water that passes the glomerulus is reabsorbed in the tubules. in the case of sodium-water retention, it could be more.

note2: that also tells us that glomerulus problems may not show edematous signs in the early stages of a kidney problem, cos as a parameter, its weightage in water retention is so much less that that of reabsorption.