To read all the articles in the series, you can go to the mini-site: "Aging of Your Heart and Blood Vessels is Risky" by clicking here .

In the first article, "Are You in Shape for Your Age?" of our Series, "Aging of Your Heart and Blood Vessels is Risky!" we showed you that the body's ability to perform vigorous exercise decreases by about 50% between the ages of 20 and 80 years. This was determined from an analysis of standardized treadmill tests on over 10,000 men and women of varying ages, which produced a table showing the number of minutes they could exercise to physical exhaustion.

EXERCISE ABILITY (MINUTES)

Age	20-29	30-39	40-49	50-59	60-69	70-79
Men	13	12	11	10	9	7
Women	11	10	9	8	7	5

Now, we are going to look at why this decline happens. During exertion, which would be any type of exercise that requires the body to work hard, such as walking up a flight of stairs or running, your heart must increase its pumping ability to meet the body's increased demand for oxygen. Why? Because, oxygen is the fuel needed for the muscles to work. Compare your body to a steam driven locomotive engine. If you are reading this article, you probably remember the steam locomotive! To go faster, or up an incline, the fireman would need to shovel and burn more coal to increase the steam to fuel the engine. Coal was the fuel. Steam was the output. And, energy to push the engine was the result.
The heart works very much like the locomotive. But, its pumping blood, not steam. The amount of blood pumped each minute is referred to as the cardiac output. It is measured by multiplying the heart rate per minute by the volume of blood pumped with each beat (the stroke volume, which is about a cup of blood). (heart rate/minute X volume/beat = cardiac output) At rest, cardiac output averages about 4 liters of blood per minute and is not much altered by age. On average, younger people (20-29) can easily increase cardiac output during exercise to three and half times over resting level; but at age 80, this ability to increase cardiac output during exercise declines to about two to two and a half times over the resting level.
To understand why the old differ from the young in this ability to increase cardiac output you need to know how cardiac output is increased. If you look again at the formula for cardiac output (heart rate/minute X volume/beat = cardiac output) you can see that cardiac output could theoretically be increased in two ways:
The heart could beat faster or the volume of blood pumped in each beat could increase.

volume of blood pumped out of the heart in each heartbeat and the maximum volume of blood pumped by the heart with each beat is generally unchanged between 20-80 years of age in healthy persons. Therefore, the volume per beat is not affecting cardiac output in healthy older persons differently than in younger persons. This tells us then that the reason is that younger individuals can increase their heartrates to higher valves during vigorous exercise than older persons.
The older locomotive is now going up a mountain climb and begins to slow down. Alex, the engineer, yells back to the fireman, "Why are we stalling?" Jabby, the fireman yells back, "The engine is old, Al. It just can't increase its steam pumping rate like it could when it came off the assembly line!" Like the older locomotive, the main factor that underlies the decline in function with vigorous exertion is the inability of the older heart to increase the number of times it pumps per minute as it could when it was younger.
This table shows the average maximum heart rate at varying ages measured in over 10,000 men and women during standardized exercise treadmill testing.

HEART RATE DURING EXHAUSTIVE EXERCISE

Age(yrs)	20-29	30-39	40-49	50-59	60-69	70-79
M	185	180	178	165	155	145
F	182	176	169	165	155	145

You can see that during vigorous exercise a young man in his twenties can increase his heart rate to approximately 185 beats per minute until exhaustion sets in. This is called the maximum heart rate. This ability then declines by about 25% from between 20 and 80 years, so that a man in his seventies might only be able to get his heart rate up to 145 beats per minute as he becomes exhausted.
Can you locate on the table above what your maximum heart rate should be? Even without the table you can figure it out easily. There is a very simple formula to determine approximately what a person's maximum heart rate should be at the point of exhaustion. Just subtract your age from the number 220. Doctors and exercise physiologists use an estimated maximum heart rate to monitor you when you are being tested on the treadmill and to prescribe various exercise programs at levels somewhat below a person's maximum rate. We'll explain more about this later in an article on aerobic exercise.
Remember that in the First Article of our Series, "Are You In Shape For Your Age", we said that the decline in our bodies exercise capacity is about 50% between the ages of 20-80 years, yet in the table above, the decline in what we call the heart's capacity during exercise is only decreasing by 25 %. So, something else in addition to reduction in heart function must be going on with aging as well, to account for the observed age associated reduction in exercise capacity. In other words, things outside of the heart must be affecting your ability to exercise change with aging.

Once again, compare the aging heart again to the old locomotive, which can produce steam, but, because some of the nuts, bolts, and valves carrying this steam to the engine are leaking, not all of it gets to the engine. Some of it escapes into the cabin. To run like a new engine requires getting all the steam to the engine to push the engine harder and faster. In the human body, once the blood (the liquid carrying the fuel, oxygen) leaves the heart, it travels many routes to reach all the body's organs where it "feeds" them with oxygen according to their needs. But, during strenuous exercise oxygen becomes more selective about where it's going to travel. Since more fuel is needed for the exercising muscles, blood is diverted away from organs whose function is not needed during exercise, such as the stomach. This blood is, instead, channeled to the muscles that need extra fuel (oxygen) to work. And once at the muscles, yet another thing must happen: These muscles must be able to use the oxygen! Getting blood to the muscles and the ability of these muscles to use the oxygen supplied by this blood for working the muscles are referred to as the peripheral reserve capacity. These peripheral (outside of the heart) mechanisms also decline with age.

Why does this peripheral decline happen? It happens, in part, because, like old railroads, switches on the tracks often don't work as efficiently as newer ones (a peripheral determinate, meaning not the train itself, but helping mechanisms not working as well); and, in part, because there are changes in the body's composition as we age. The amount of lean muscle tissue decreases. A smaller amount of muscle cannot extract and burn as much fuel as a larger mass of muscle. Similarly Al and Jabby's locomotive will work less efficiently as its ability to burn coal (fuel) decreases as it gets older. We will be visiting the topic of decreased muscle mass with aging in a future article on body composition, which will include information on fat verses muscle, muscle strength, and how these affects exercise.

So, where are we? In the first article, "Are You in Shape for Your Age", you learned that an approximate 50% age associated decline in your body's maximum oxygen utilization, as reflected in a similar decline in your exercise capacity, can be estimated by the number of minutes you can stay on the treadmill. In this article, you have learned

1. about cardiac output, and that it is a major factor that determines your exercise capacity,
2. that an inability of your heart to beat as fast as it did when it was younger is one of the problems with reduced exercise capacity at older ages,
3. how to calculate your maximum heart rate, and
4. that a decline in peripheral oxygen utilization or ability to transport and use all the oxygen due to changes in vessels (the transport system) and body composition, or lean muscle mass, are other additional factors that lower the body's exercise capacity as it ages.

It's not all bad, though. The aging heart has ways to compensate for some of the above results of the aging process. Coming up next week we will talk about "the trick" your old heart uses to keep the volume of blood that it pumps with each beat equal to that of a younger heart, in spite of the fact that its actual pumping ability weakens.