What causes the accumulation of fluids in tissue?

Answer 1

When our capillaries exchange fluids with the tissues in our body, a balance of hydrostatic pressure (fluid leaving vessels) and osmotic pressure occurs (fluid entering vessels).

So as the blood flows into the capillary, the hydrostatic pressure is usually strong and so fluid and all dissolved materials leaves the capillaries into the tissues of our body, leaving big stuff like protein still in the capillaries. Now once, the blood finishes it's "tour" of the tissue and reaches the end of capillary bed, the osmotic pressure is much stronger and so fluid flows back into the capillaries, which is high in solute (like proteins) but low in fluids.

However, sometimes, say by a lack of proteins within the blood, this latter reabsorption is unable to occur, leading to an accumulation of fluid in that particular tissue. This swelling is called "edema".

So now the lymphatic system comes into play, absorbing all this pooled up blood and properly redistributing it throughout the body, among other things such as doing a "security check" on the blood and fighting any dangerous foreign materials. The blood that enters this lymphatic system is called lymph, which eventually becomes mixed up with the rest of our blood at the subclavian vein..

Answer 2

If the lymph system can not carry fluids back to the blood for some reason, there will be a fluid build up called edema.