U.S. scientists say they have discovered a new human organ – one of the largest in the body, and one that was there the whole time.

The new “organ” is called the interstitium and it’s found everywhere in our bodies.

It’s the tissue that lines our digestive tracts, lungs, and urinary systems, surrounds our arteries and veins, and supports the tissue between our muscles.

Scientists have long known about the body’s various interstitial spaces between cells, but its importance was never quite understood. The interstitium was simply thought to be dense connective tissue that enclosed fluid-filled spaces.

Now, a team from New York University's School of Medicine, is asking for a re-think of the “previously unappreciated” tissue layers of the interstitium.

They say the interstitium is not just a bunch of collagen walls between cells; instead, they are dynamic, fluid-filled spaces that aid in important functions.

According to their report in the journal Scientific Reports, interstitial spaces are organized by a collagen “mesh” and can shrink and expand “and may thus serve as shock absorbers.”

These “dynamically compressible and distensible sinuses” act as thoroughfares to transport critical fluids within organs and around the body.

They say the interstitium plays an important role in carrying lymph, the clear fluid that also travels through lymphatic vessels and that supports immunity.

But it might also play a role in some of our body’s less desirable functions.

It may be an important part of a sudden swelling called edema. It may also help in the spread of cancer cells during metastasis. And it may have an important function during fibrosis, or the stiffening of certain tissues.

Because of its “dynamic” role in all these functions, the researchers say the interstitium should be called an organ in its own right. (The human body already has 80 known organs, which are tissues that have a specific structure and a specific function.)

The team says their findings mean that that many of the normal and abnormal activities of different organs need to be reconsidered to include the newly discovered role that the nearby interstitium appears to play.