What causes edema in the body?

Edema in the body occurs when there is an imbalance between the processes of filtration and reabsorption within the capillaries. Specifically, when filtration is greater than reabsorption, the net movement of fluid out of the capillaries into the interstitial spaces exceeds the ability of the lymphatic system and the blood vessels to return that fluid. This excess fluid accumulation in the tissues leads to swelling, or edema.

In a healthy state, filtration and reabsorption should be balanced, meaning that the amount of fluid leaving the capillaries matches the amount being reabsorbed. However, when conditions arise that increase filtration or decrease reabsorption—such as increased hydrostatic pressure, loss of protein in the blood, or obstruction of lymphatic drainage—edema can develop.

Inflammation can also contribute to edema, as it typically increases blood vessel permeability, allowing more fluid to escape into the tissues, but the primary factor regarding the physiological balance of fluid dynamics is the relationship between filtration and reabsorption. Thus, the assertion that filtration exceeds reabsorption provides a clear and direct explanation for the development of edema.