05/11/2026
It’s all connected
We’ve long known about two systems in the human body that circulate fluids. A physician in Italy observed the first one, the lymphatic system, which removes excess fluid from tissues, in 1622. Six years later, an English doctor described the second, the cardiovascular system, which pumps blood through our arteries, veins and capillaries. (It was a great decade for science.)
Now, scientists think they may have come across a third. In 2021, after examining the skin of people with tattoos, researchers saw in their biopsies that ink particles had traveled deeper into the body than they expected, through the skin into an interstitial space beneath it — and from that space into the fascia, the connective material below.
The discovery — a hidden pathway between two layers of tissue not known to connect in this way — was a surprise. It has far-reaching implications for our understanding of the human body and for our health. Because that interstitial space doesn’t just exist between the skin and the fascia, researchers discovered. There are spaces like it throughout the body, forming pathways between organs and allowing fluids, cells and molecules to move between them before re-entering the lymphatic and cardiovascular systems.
Scientists call this large interconnected network the interstitium. It’s the subject of an incredible story in The New York Times Magazine by Avraham Z. Cooper, an associate professor of medicine at Ohio State University.
West meets East
An animation of acupuncture
Jérôme Berthier
The idea of a third circulatory system will not come as a surprise to anyone who practices traditional Chinese medicine. “This knowledge is actually quite ancient,” one professor told Cooper. “It’s something that other systems of medicine have been offering for a long time, but they didn’t have microscopes.” Mention the interstitium to an acupuncturist and you might get an eye roll, like, “No kidding.” (Ask me how I know.)
Acupuncture works, of course. The studies are clear. People seek it out for treatment of all sorts of ills, from chronic pain and migraines to anxiety and insomnia. The discovery of the interstitium, Cooper writes, may help us better understand how it works.
Traditional Chinese medicine holds that acupuncture is a way to balance the flow of energy — known as chi — through one of the body’s 12 main meridians. Acupuncturists insert thin needles into specific points along those meridians to enhance the flow of chi.
Those specific acupuncture points are within the same areas of connective tissue where fluid flows through the interstitium, researchers found. And when they injected dye into acupuncture points on the forearms of volunteers, it slowly migrated up the arm along a meridian.
“This pathway doesn’t go in the veins, it doesn’t go superficially,” one researcher said. Instead, he told Cooper, it flows through the interstitium between the muscles: “When I saw that, I said: ‘We’re onto something. This truly has to do with acupuncture.’”
The future is past
There will need to be a lot more research before we fully grasp the implications of an interconnected interstitium. But things that are good for you (healthy cells, for instance) move through it. So do bad ones (like metastatic cells). Cooper says there are already promising possibilities in that: in how the interstitium might inform the treatment of diabetes, gut health and cancer, among others.
It may also help us understand other biological systems.
Tiny freshwater invertebrates have a kind of interstitium, for instance. Plants appear to have one, too, that moves water and nutrients outside of cell membranes. Indeed, he writes, “fluid moving through interstitial spaces might have represented the first circulatory systems to develop in the earliest forms of complex multicellular plant and animal life, hundreds of millions of years ago.”
