DAVINA: In Shakespeare’s time, it was thought that a healthy person had thin blood that allowed both physical and emotional elements to move through the body.

RALPH: By wishing her blood to thicken, Lady Macbeth wants to block her "compunctious visitings of nature", or her natural feelings of conscience, from flowing through her body and stopping her cruel intentions. She doesn’t want her feelings for others to keep the peace between her cruel intention and the carrying out of that intention. In other words, thick blood will keep her conscience from getting in the way of murder.