Heart Rate is the basic measure which is used to determine the cardio-respiratory fitness of every individual.
Heart rate is the number of times a heart beats per minute(bpm). A healthy adult has a resting heart rate of between 60 - 80 (bpm), where as a well-conditioned individual has a resting heart rate of around 35-50 bpm.
There is a important concept called stroke volume which also helps in determining the health of the heart. Stroke volume is the amount of blood pumped by the heart per beat. A normal stroke volume at rest is between 70 and 80 cm3 per beat. In a well-trained individuals the stroke volume at rest is 100 to 110 cm3 per beat.
The amount of blood pumped by the heart in one minute is called cardiac output. During exercise, the cardiac output increases to meet the body's demand for oxygen.
To state simply cardiac output = stroke volume * heart rate.
Keep tracking your heart rate once every four weeks, you can evaluate how good your cardio-respiratory fitness is. The lower the heart rate the better the fitness level of the person.
For example if an average stroke volume is 71 ml, and an average resting heart rate is 70 bpm then cardiac output will be 4,970 ml per minute or nearly 5 liters of blood.
As stated a key adaptation to a regular exercise program is that the resting heart rate decreases and stroke volume increases. For example, a well trained individual whose stroke volume is 100ml and heart rate is 50, has a cardiac output of 100 ml * 50 bpm = 5000 ml. Cardiac output can be maintained with fewer beats in such individuals. Heart of such a individual will be very healthy and lasts long as it is doing less work.