For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
It’s one of those things that feels really obvious if you cook a lot of east/south Asian dishes - shrimp sauce and mushroom soy sauce have a pretty similar aftersmell to them because they’re so concentrated