Edema occurs when excess fluid leaks from blood or lymph vessels (such as capillaries) and accumulates between cells. Depending on its composition, this excess fluid is classified as either a transudate or an exudate.
In a healthy capillary, the vessel wall functions like a fine sieve, allowing fluid to move in both directions. Under normal conditions, oxygen and nutrients are delivered from the blood to the cells, while metabolic waste and carbon dioxide are reabsorbed back into the blood.
This fluid exchange usually maintains a balance, meaning the amount of fluid leaking out of the vessel is equal to the amount returning.
