The image of blood is invoked as a symbol of either someone’s lineage or their passion. When the Cardinal and Ferdinand are discussing the Duchess’s secret marriage and children, they invoke the corrupting of blood, but each uses it to symbolize something different. The Cardinal is concerned with blood as a matter of nobility: He says, “Shall our blood / The royal blood of Aragon and Castile / Be thus attainted?” (II.5.21-23, emphasis added). His concerns are primarily legal: any children of the Duchess’s, who might inherit her property and fortune, will also partially be of a bloodline less than hers. The Cardinal believes that this “attainted” blood affects their entire family by mixing them with someone of a lower social status.

However, Ferdinand misunderstands him. He tells the Cardinal to “apply desperate physic” (II.5.24), heating cupping glasses “[t]o purge infected blood, such blood as hers” (II.5.26). Early modern belief associated blood with the humors, and so an excess of blood could cause an imbalance that affected someone’s actions. Ferdinand thus interprets blood physically, bodily, and humorally, as it relates to someone’s sexual passions and desires. This misunderstanding about what blood symbolizes is characteristic of the brothers: While the Cardinal’s motivations are cold and calculated, Ferdinand’s are bodily and violent.