The purpose of this research is to learn more about how renin (a blood test) is affected by cardiopulmonary bypass, the heart-lung machine used during open heart surgery. Renin is a protein that may be elevated in response to low blood pressure or situations where organs do not receive sufficient oxygen. Renin may potentially be used as an indicator for specific treatments aimed to increase the blood pressure. This study will evaluate blood samples for renin concentration throughout the course of open heart surgery.