In the poem the author uses the conceit, an extended metaphor, of a flea to comment on the relationship between the speaker and a woman. The speaker tries to convince a lady to sleep with him, using the argument that if their blood mingling in the flea is harmless,then if they were to pursue a sexual relation, the mingling would also be just as harmless. The poem actually hinges on the idea of blood and mixing of blood. The idea that blood is powerful and the source of new life in pregnancy. The humor of blood is related to the sanguine temperament, considered to be creative, impulsive, and pleasure seekiing.