The greatest canal system: your heart

You might think of your circulatory system (heart and blood vessels) as one continuous circular canal composed of multiple locks (valves). These locks regulate the flow of water (blood) through the canal by increasing the pressure (raising the water level behind the lock) prior to opening it. Then when the lock is opened this difference in pressure (higher to lower) allows the barge to pass through easily. In your heart, in addition to the locks (or valves) there is another engineering feat of nature at work; this is the ability of certain chambers (the ventricular chambers) to "squeeze down" in order to eject more volume during times of heavy traffic in the canal (or during "exercise").

This is how it works. The receiving chamber of the right side of your heart, the right atrium, has a lower pressure than your body's veins. This allows deoxygenated blood to flow into it from your veins. Between heartbeats, as blood returning from these veins fills the right atrium, the pressure within the right atrium increases, (the valve or lock separating it from the lower chamber is closed). At the same time the pressure in the pumping chamber (or right ventricle) is decreasing due to relaxation of the cells after the previous contraction (or heartbeat). When the right atrial pressure exceeds that in the right ventricle the lock (or valve) between the atrium and ventricle opens (simply a passive process, due to the differences in pressures between them) and blood from the upper right atrium rushes into the lower right ventricle. During the next heartbeat the ejection process occurs (The Ejection Fraction was described in detail in "How good a pump is your older heart".) The ventricles squeeze down and blood is pumped to your lungs, where it picks up oxygen (color of blood changes from blue to red in the diagram). From the lungs the blood flows to the receiving chamber in the left side of your heart, the left atrium. Again, this flow of oxygenated blood from your lungs to your left atrium can occur because the pressure in your left atrium is lower than in the vessels carrying it from your lungs to the left atrium. Just as occurred in your right heart, the left atrium gets dilated as blood fills it. And, when the left atrial pressure exceeds the left ventricular pressure, the valve between the left atrium and left ventricle opens and oxygenated blood rushes into the left ventricle. During the next heartbeat the newly oxygenated blood is forcefully pumped from the left ventricle out to your body via the aorta, a large blood vessel connected to the left ventricle.

What causes shortness of breath during exercise in healthy individuals?

You have about 6 to 9 liters of blood in your body depending on body size. At rest most of this blood resides in the veins throughout your body, not within your arteries or within your lungs. This is because the journey of blood through your body's veins takes longer than that through your arteries or lungs. Think of these veins as millions of tiny passageways in the body's continuous canal system. The journey through these tiny passageways take a longer time than it does to go through the larger vessels in the heart and lungs. During exercise, however, the time needed for the journey of blood through these tiny vein passageways (billions of canals) is lessened. This is because during exercise blood flowing within your venous canal system is pushed along quicker due to the increased action of your muscles and the influence of nerve impulses, which help to squeeze blood out of your veins. This increase in the rate of flow through these canals thus produces a large increase in the amount of blood returning to your right heart, per unit time, from your body. As we have learned, when this increased amount of blood reaches the right heart is then needs to get pumped to the lungs once again in order to continue its endless journey of reoxygenation and recirculation. Here is where the problem of "shortness of breath" may come in, as you get older!

Everything works fine when the pressure in your left heart stays low, because blood can easily flow from the lungs into it. Remember, it's simply a matter of these pressure differences that makes the canal flow smoothly. However, as a result of aging, the reserve pumping capacity of the left heart decrease. What does this mean? It means, that if blood cannot be pumped from the left heart to keep up with the increased flow of blood from the lungs the blood can't get out to the body to recirculate. Therefore, blood gets backed up in your lungs. Why does this happen? It happens because the left ventricle dilates, then its pressure increases between beats during the filling period. As a result the blood has difficulty in flowing from the lungs into your left heart. This would be similar to water backing up in the canal if a lock fails and won't open when it should to let the barge through. It's a twofold problem in the old heart. First, the older left ventricle can't empty the blood as well as it should. Remember you learned in previous articles that the older heart can't increase its ejection fraction (EF) like the younger ventricle and secondly, it can't increase its rate of pumping like the young ventricle, either. So, the left ventricle dilates because blood gets stuck within it. When this happens the pressure within the left ventricle increases. Blood backs up in the left atrium, causing the left atrial pressure to increase. Remember, a higher left atrial pressure is required to open the valve (lock) between the left atrium and the lower left ventricle to allow the blood to flow through. This higher left atrial pressure prevents blood from exiting the lungs and flowing into it. The lungs become engorged with blood. This excess blood within the lungs sends a signal to the brain, which notifies you that you are "short of breath".

Is "shortness of breath" the same as "heart failure" in older individuals?

"Shortness of breath" a common cause of reduced exercise ability in older individuals, is actually caused by the same mechanisms that are present in sick patients with "heart failure", but is not heart failure per se. Patients with severe heart failure become short of breath, even at rest, because they have problems in the ability of the left ventricle to eject blood and therefore blood backs up into the lungs. With patients in whom heart disease is less severe, shortness of breath does not occur at rest, but with exertion. In these patients the amount of exertion required to cause shortness of breath is dependent on the severity of the reduced pumping capacity of the left ventricle. Thus, this is a muted version of the shortness of breath experienced in patients with heart failure. It commonly occurs during vigorous exercise in otherwise healthy older individuals as they age.

Summary

In the preceding three articles of this series you have learned a great deal about the aging heart. In the first article, "Are You In Shape For Your Age?" you learned that there is an approximate 50% age associated decline in your body's maximum exercise ability. In the second article, "Why Does Aging Cause Your Exercise Ability to Deteriorate?", you learned about some of the causes for this decline, namely the inability of the older heart to increase its EF during exercise like the younger heart, and the inability of the older heart to increase its rate like the younger heart. In article three, "How Good a Pump is Your Older Heart?", we showed you that in spite of the aging changes which cause these declines, the older heart is "smart" and is able to adapt by "doing" a trick which allows it to increase the volume of blood that it pumps in each beat during vigorous exercise to equal that of the younger heart. This fourth article in the series, "Your Older heart May Cause You To Feel Short of Breath" show's you that the "trick" can backfire when the heart becomes larger, and increases its pressures, because this causes shortness of breath. And in this article we have explained why, even if you are healthy, you may experience shortness of breath during exercise that mimics that of patients with heart failure. We have shown you that the reason for this shortness of breath, which limits exercise capacity, is caused by the left ventricular dilatation and higher filling pressure of the left ventricles of the older heart. Blood has difficulty exiting the lungs, which have become engorged with blood causing shortness of breath. Additive to this is a lower heart rate, ("Are You in Shape for Your Age?") and the inability to increase EF during exercise ("How good a pump is your older heart"). What you can you do to lessen the impact of "the aging process" and associated shortness of breath during vigorous exercise will be the topic of a future article.

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