Cardiac reserve refers to the heart's ability to quickly
adjust to immediate demands placed upon it. The maximum
percentage that the cardiac output can increase above normal is
called the cardiac reserve. In the normal young adult the
cardiac reserve is 300 to 400 percent. For example, running to
catch a bus would cause an increase in oxygen demand which must
be balanced by increased blood circulation. This 
increase in
cardiac output is achieved by an increase in either stroke
volume
or heart rate or both. In the athletically trained person,
cardiac reserve is occasionally as high as 500 to 600 percent.
Normal cardiac reserve in nonathletic persons is approximately
four times their normal cardiac output. In the weak or elderly
person, it may be as low as 200 percent.