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How does low serum albumin lead to edema or swelling?

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Blood plasma — blood Blood sediment in test tubes, showing plasma (clear fluid), red blood cells (red), and the release of hemoglobin into the surrounding plasma (pink).

Serum albumin is a critical protein that helps maintain osmotic pressure within the blood vessels. This pressure acts as a vital force that keeps fluid contained inside the vascular system, preventing it from leaking into surrounding tissues.12

When serum albumin levels are low, this osmotic balance is disrupted. Because albumin is a large molecule that cannot easily pass through the semipermeable membranes of the capillaries, its presence creates a pressure gradient that holds water within the bloodstream. If albumin concentrations drop, this gradient weakens, allowing water to migrate out of the blood vessels and into the tissue spaces. This accumulation of excess fluid in the tissues results in edema, or swelling, which can occur in the extremities and vital organs, including the lungs.21