When The New England Journal of Medicine last week shared a photo "of a mysterious, branchlike structure" that a patient had coughed up, several people replied that it looked like a piece of the patient's lung, Haley Weiss reports for The Atlantic. The guesses weren't so far-fetched, Weiss reports, but doctors said it's still extremely rare: a perfectly intact, "six-inch-wide clot of human blood in the exact shape of the right bronchial tree."

The New England Journal of Medicine shared the photo of the root-like blood clot, as part of its series of fascinating clinical images.

About the patient

The patient, a 36-year-old male, was admitted to the ICU with aggressive end-stage heart failure, according to Georg Wieselthaler, a transplant and pulmonary surgeon at the University of California-San Francisco (UCSF).

While the patient felt better after coughing up the clot, he died a week later from heart failure complications, according to Wieselthaler. But his story and the fascinating image allow us to "recogniz[e] the beautiful anatomy of the human body," Gavitt Woodard, a clinical fellow at UCSF's thoracic surgery department.

Doctors connected the patient to a ventricular-assist device to increase blood flow through his body. The "high turbulence inside the pumps ... can cause clots to form inside," Wieselthaler explained. "So with all these patients, you have to give them anticoagulants to make the blood thinner and prevent clots from forming."

For Wieselthaler's patient, the anticoagulants caused blood to flow out of his pulmonary network and into the bronchial tree of the right lung, causing him to cough up blood clots for days, Weiss reports. Eventually, with one deep cough, the patient spit out a large, branched blood clot, which, upon further inspection, doctors recognized as the right bronchial tree "based solely on the number of branches and their alignment," Weiss writes.

The doctors "were astonished," according to Wieselthaler. "It's a curiosity you can't imagine."

How did it happen?

While Wieselthaler noted that the case is "very rare," there are a handful of similar documented cases on record. For example, Weiss reports the European Journal of Cardio-Thoracic Surgery in 2005 published an image of a bronchial-tree shaped blood clot a 25-year old woman coughed out after developing a disorder that restricted her body's clotting abilities.

But in most other cases, blood clots don't survive intact, Weiss reports. The structures often are comprised of mucus or lymph fluids and fall apart easily because they are softer and less sticky than hardened lymph fluids or mucus.

So how did Wieselthaler's patient manage to cough up a perfectly intact clot in such a distinct shape?

According to, Wieselthaler his patient's condition caused his blood to have a higher concentration of fibrinogen: A blood plasma protein that forms blood clots by causing platelets to stick together, Weiss reports.

In the patient's case, the fibrinogen might have acted as a "glue" that made the blood in the patient's lungs to become exceptionally thick, causing the blood to form a large intricately shaped clot.

And while the structure's size would seem to present an obstacle, Woodard said that the size may explain why the patient was able to cough up the mass in the first place. Woodard explained that the patient "was able to generate enough force from an entire right side of his thorax to push this up and out." Had the clot been smaller, "he might not have been able to generate the force" to expel it, Woodard said (Weiss, The Atlantic, 12/6).

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