Pure phonetic loan from English 'pyrrole' - bǐ-luò ≈ pyr-rol. The 口 radical on both characters signals 'sound only, no semantic content,' a common Chinese pattern for foreign chemical names (cf. 吗啡 morphine, 咖啡 coffee).
culture
Four pyrrole rings linked together form the porphyrin ring at the heart of hemoglobin (red, in animals) and chlorophyll (green, in plants). The same molecule colors blood and leaves.
