Blood cells are made in bone marrow, which explains why Sans bleeds. Why Papyrus doesnt, I can only assume either A, we just knock his head off his neck, or B, there is no bone marrow in the neck.
SANS' BLOOD IS KETCHUP; PEOPLE WHO REMEMBER SMALL DETAILS IN UNDERTALE; MONSTER FOOD IS ABSORBED INSTANTLY, SO IF IT WAS KETCHUP, WE WOULD'VE ALSO SEEN A BROKEN BOTTLE