Coinciding with these changes are important functional alterations. The resting cardiac output, defined as the total volume of blood pumped out of the heart in 1 minute while at rest, decreases by 1% per year after age 30." To understand the reasons for this, you must be familiar with the determinants of cardiac output, which is computed by multiplying heart rate (in beats per minute) by stroke volume (the volume of blood pumped out of the ventricle in one contraction). A decline in resting cardiac output with age is primarily caused by a decrease in stroke volume.