If someone showed you the above picture without context, you'd probably think it was an intact blood vessel. Readers, it is not. It is a perfect cast of the branching air passages in the right lung of a man dying of heart failure, formed from coagulated blood that had been accumulating therein.

The man's doctors were absolutely gobsmacked by the thing, which they published in the New England Journal of Medicine.

First, let's get one thing clear. It's not actually part of his lung, as some news headlines are claiming. It's not possible to cough up a lung (although you can cough so hard a lung herniates through your ribs. That's not fun, so try to avoid it).

In fact, bronchial tree clots - called casts - aren't uncommon at all. Several blood clot casts have been reported in the last few years - coughed up by a 57-year-old-woman with lupus in 2010, a 25-year-old pregnant woman in 2005, and an 80-year-old man in 2015.

And if you head on over to medical image-sharing social network Figure 1 and search for 'bronchial cast', you'll see a variety of casts, formed from coagulated blood, or from mucus that accumulates in the lungs in certain medical conditions.

What makes this particular cast so intriguing is not that it happened at all, but that it's absolutely enormous - and the patient coughed it up in one piece, without it breaking.

(Woodward et al./NEJM)

"We were astonished," the man's doctor, Georg Wieselthaler, told The Atlantic. "It's a curiosity you can't imagine - I mean, this is very, very, very rare."

The patient, an anonymous 36-year-old man, coughed up the cast while being treated for acute end-stage heart failure in the ICU, after a long history of heart failure.

His doctors connected his heart to a device to help it pump blood around his body. But because these devices can also cause blood clots, they had to give him a continuous infusion of an anticoagulant called Heparin to try and prevent this from happening.

Except coagulation is a necessary part of the body's self-repair system, working to keep blood vessels from developing tiny tears that will result in internal bleeding - or, if they occur in the blood vessels that transport blood around the lungs, from leaking into the air passages and accumulating there.

Sadly, this is what happened with the patient. Over the course of the week after his doctors implemented the Impella device and the Heparin treatment, he started to cough up smaller blood clots, culminating in an extreme fit of coughing during which he brought up a giant mass.

When the doctors unfolded it, they saw a cast so perfect they were able to clearly identify it as the man's right bronchial tree.

They think what held it together could have been a protein called fibrinogen, which is vital to the clotting process. Although the patient was on anticoagulants, his infection caused an elevated level of fibrinogen in his blood - this could have held the clot together while he coughed it up.

Sadly, although he felt better after the clot was out of his lungs, and coughed up no further clots, the condition of his heart was too severe. He died just over a week later from complications of heart failure.

The case was reported in the New England Journal of Medicine.