Our Hidden Circulatory System

Introduction

For hundreds of years, we have recognized only two circulatory systems: the lymphatic system, which balances body fluid levels, and the cardiovascular system, which pumps blood and vital substances throughout our body. However, a recently discovered circulatory system, called the interstitium, is increasingly recognized as a vital anatomical structure.

What is the Interstitium?

Interstitial is defined as occurring in or being an interval or intervening space (Merriam-Webster). The interstitium does just that; it is a “continuous, fluid-filled compartment that exists in and between essentially every tissue in the body, through which interstitial fluid flows from capillaries toward lymphatic capillaries” (Gordon). In addition to circulating nutrients, fluids, and waste, the interstitium provides structural support for cells, keeping cells and tissues healthy and functional.

The interstitium was first discovered in 2015 by gastroenterologists Dr. David Carr-Locke and Dr. Petros Benias while examining a patient's bile duct for cancer. Using confocal laser endomicroscopy (pCLE) of the duct and a fluorescein injection, they found a widespread fluid-filled space within the tissues with no anatomical correlate (Benias).

Later in March 2018, Dr. Neil Theise expanded on Carr-Locke and Benias’s findings by identifying the interstitium, long thought to be mere connective tissue, as a continuous, fluid-filled organ. His justification for this label is, “The interstitium is large and present in many places… Wherever you examine it, the structure is unitary and unique, which is one qualification for being an organ” (NYU Langone Health). This classification sparked massive debate, prompting Theise to later specify the interstitium as a system rather than an organ because it is a full-body network.

How it stayed hidden for so long

Although the interstitium is found throughout the body and scientists and doctors encounter it quite often, they were taught to ignore it. For decades, anatomy textbooks described these regions as ordinary connective tissue because tissue samples were routinely fixed with chemicals before being examined under a microscope. During this preparation, the fluid drained away, and the tiny spaces collapsed, creating the illusion of solid tissue (Cordis). Since these collapsed structures appeared consistently under the microscope, generations of researchers accepted them as normal anatomy. It was only with newer imaging technologies that scientists realized these spaces existed in living tissue and formed an extensive network.

Implications for illnesses and treatments

The formal classification of the interstitium could potentially change how scientists understand many diseases. Because it surrounds nearly every organ and tissue, researchers believe it may play an important role in inflammation, fluid balance, and the spread of disease throughout the body.

One condition that may be linked to the interstitium is fibromyalgia, a chronic disorder that causes pain, fatigue, and sensitivity throughout the body. Fibromyalgia has no single known cause, and scientists continue to debate its underlying mechanisms. Because the interstitium supports tissues and transports fluid, some researchers have proposed that abnormalities within this network could contribute to the symptoms experienced by patients. Theise suggests, “Maybe the reason why fibromyalgia has been opaque is because we haven’t recognized the compartment of the body that’s affected” (NYU Langone Health).

The interstitium may also give insight into how cancer spreads throughout the body. Cancer cells can circulate the body through interstitial spaces (Cleveland Clinic). By understanding how the interstitium works, researchers can develop preventive measures for cancer.

Conclusion

If the interstitium truly functions as a body-wide network alongside the cardiovascular and lymphatic systems, then scientists may need to rethink many aspects of human anatomy and physiology. Continued research could lead to new and improved diagnostic tools, more targeted drug delivery methods, and a deeper understanding of diseases that have remained poorly understood for decades.

Works Cited

Benias, Petros C., et al. “Structure and Distribution of an Unrecognized Interstitium in Human Tissues.” Scientific Reports, 27 Mar. 2018, www.nature.com/articles/s41598-018-23062-6.

Cleveland Clinic. What Is the Interstitium?, 2 Dec. 2025, my.clevelandclinic.org/health/body/interstitium.

Cordis. Trending Science: Say Hello to Interstitium, What’s Likely the Newest Organ in Your Body, 5 Apr. 2018, cordis.europa.eu/article/id/123187-trending-science-say-hello-to-interstitium-whats-likely-the-newest-organ-in-your-body.

Gordon, Jacob. “The Interstitium: The Body’s Third Circulatory System.” MyBioHack, 6 Apr. 2026, www.mybiohack.com/blog/interstitium.

Merriam-Webster. “Interstitial.” Merriam-Webster, www.merriam-webster.com/dictionary/interstitial.

NYU Langone Health Magazine. An NYU School of Medicine Pathologist Uncovers a Potential New Organ, Setting off a Fiery Debate | NYU Langone News, 2018, nyulangone.org/news/nyu-school-medicine-pathologist-uncovers-potential-new-organ-setting-fiery-debate.