Da'irar Willis wani tsari ne na jini a cikin kwakwalwa wanda ke samuwa ta hanyar tsarin zobe mai kama da arteries. Ya kasance a gindin kwakwalwa kuma yana haɗa manyan tsarin jijiyoyin jini guda biyu waɗanda ke ba da jini ga kwakwalwa: arteries carotid na ciki da kuma jijiyoyin vertebral. Da'irar Willis na taimakawa wajen tabbatar da cewa jini yana gudana zuwa kwakwalwa ko da daya daga cikin arteries ya toshe ko lalacewa, kuma yana zama wurin daidaita yanayin jini da hawan jini a cikin kwakwalwa.