Answer
b. blood from the pulmonary and systemic circuits fills atria.
Work Step by Step
During atrial diastole, the heart muscles relaxes and the atria fills with blood from the large veins.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.