Rose Namajunas says she once beat down an opponent so ferociously, the blood spilled like a river ... and she describes exactly what it smelled like.

Yeah, gross ... and yet, awesome.

Of course, we all remember the bloodbath when Rose dominated Paige VanZant -- but Namajunas says she's been in way bloodier fights ... and describes it in ultra-gory detail. 

Rose is taking on Michelle Waterson at "UFC on FOX" this weekend -- and Waterson has also fought (and beat) Paige.  

Namajunas says her own PVZ victory is the most impressive because, you guessed it -- more blood.