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.
Source: https://www.sciencealert.com/a-man-coughed-up-this-perfect-blood-clot-and-doctors-don-t-understand-how