Other declines noted within the cardiovascular system are a decrease in cardiac output (the volume of blood pumped per minute by either ventricle), a decrease in stroke volume (the volume of blood pumped by one side of the heart per beat), a decrease in maximum heart rate (HR), and a likely increase in systolic blood pressure (BP). Cardiac output must rise if either stroke volume or HR rise. It must be able to vary to accommodate differences in pressure and/or resistance, which call for changes in blood flow. For example, blood flow must increase with an increase in resistance, as in exercise, due to vasodilation of the arterioles. If the heart was not able to vary its output, adaptation to a stressor such as exercise would be impossible. With aging, the capacity of the heart to adapt may become somewhat impaired. Cardiac output decreases by approximately 1% per year after 20 years of age in the average sedentary individual. Between the ages of 30 and 70, the average sedentary individual's cardiac output decreases as much as 30% due to a decrease in stroke volume which in turn is largely due to a progressively reduced capacity of the left ventricle.