A rise in hydrostatic pressure occurs in cardiac failure. A fall in oncotic pressure occurs in nephrotic syndrome and liver failure. It is commonly thought that these facts explain the occurrence of edema in these conditions. However, it has been known since the 1950s that the situation is more complex and it is still far from completely understood.
Causes of edema which are generalized to the whole body can cause edema in multiple organs and peripherally. For example, severe heart failure can cause pulmonary edema, pleural effusions, ascites and peripheral edema, the last of which effects can also derive from less serious causes.
Although a low plasma oncotic pressure is widely cited for the edema of nephrotic syndrome, most physicians note that the edema may occur before there is any significant loss of protein in the urine or fall in plasma protein level. Fortunately there is another explanation available. Most forms of nephrotic syndrome are due to biochemical and structural changes in the basement membrane of capillaries in the kidney glomerulae, and these changes occur, if to a lesser degree, in the vessels of most other tissues of the body. Thus the resulting increase in permeability that leads to protein in the urine can explain the edema if all other vessels are more permeable as well.
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