3. Increased Capillary Permeability
Sometimes the blood vessel walls themselves become “leaky,” allowing more fluid and proteins to pass into the surrounding tissues. This is usually due to inflammation or injury.
For example, if someone has cellulitis (a skin infection), the area becomes red, hot, and swollen due to fluid leakage. Allergic reactions can also make blood vessels leaky, leading to sudden swelling of the lips, eyes, or throat, known as angioedema.
In severe infections or sepsis, the body’s immune response releases chemicals that increase permeability, often causing widespread edema.
Burns or trauma also damage the vessel walls, resulting in rapid and severe fluid leakage. Although this type of edema may sometimes be localized to one area, in critical illnesses it can become generalized, making the body appear swollen.