2. Decreased Oncotic Pressure (Low Blood Proteins)
Proteins in the blood, especially albumin, act like sponges that pull water back into the circulation from the tissues. When the level of these proteins falls, fluid is not reabsorbed properly and remains in the tissues, causing edema.
A good example is liver disease, where albumin production is reduced, leading to swelling in the legs and abdomen. In nephrotic syndrome, the kidneys leak large amounts of protein into the urine, resulting in puffy eyes, leg swelling, and sometimes generalized body swelling.
Severe malnutrition, particularly lack of dietary protein, can also reduce oncotic pressure, leading to swollen bellies in children with protein deficiency (often seen in famine-stricken regions).
Similarly, certain intestinal diseases (protein-losing enteropathies) prevent the body from absorbing or retaining protein, which again reduces plasma oncotic pressure and contributes to edema.