01/03/2011 - Articles

The Older Heart Has Trouble Pumping Blood During Exertion - Part V

By: Ed G. Lakatta, MD

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In the preceding article 'Your Older Heart May Cause You to Feel Short of Breath', we compared the circulatory system to a 'great continuous canal'. You learned that the.

To read this article with accompanying illustrations, as well as 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 .

Introduction

In the preceding article "Your Older Heart May Cause You to Feel Short of Breath", we compared the circulatory system to a "great continuous canal". You learned that the flow of blood through this canal system is dependent upon regional differences in pressures throughout the heart and vascular system. This article, the fifth in the series, will explain some age-related changes that affect the heart's ability to pump out blood.

The labor of the heart

The left heart's pumping chamber (left ventricle) is connected to your body's arteries by the aorta. The aorta is the main trunk of the arterial system, arising from the base of the left ventricle. It is through the aorta that oxygenated blood flows out to the body's tree of arteries within the body's organs. The heart can eject blood through the aorta and into those arteries only if the pressure within its cavity exceeds that in the aorta and arteries. Remember, circulation is dependent upon differences in pressures. Put very simply, blood flows from areas of higher pressure to areas of lower pressure. This is how it works. Between heartbeats, the left ventricular pressure drops to very low levels, so that it can fill with blood coming from the lungs. The pressure drops because the muscle within the ventricular wall relaxes. At the onset of a subsequent heartbeat the cardiac cells that make up the heart's ventricular wall get a signal to contract. Most people are familiar with labor contractions during childbirth, so lets compare the heart's action with labor. During labor the mother's uterus gets a signal at timed intervals to contract. Each contraction produces a force, which squeezes the uterus down and makes its cavity smaller. This squeezing down produces the force that propels the infant from the uterus through the birth canal to the outside world. The uterus is able to become smaller during each contraction because the contraction tightens (shortens) each cell that makes up the muscle of the uterus.

The heart muscle contraction is similar to uterine muscle (or labor) contractions. At timed intervals the cells within the heart's ventricle wall synchronously contract and produce a force. This causes the ventricle to squeeze down on the blood within it. The pressure within the ventricular cavity increases. When the ventricular cavity pressure exceeds that of the pressure in the aorta, blood is propelled from the heart into the aorta. As some blood leaves the heart the heart cavity shrinks in order to continue to put pressure on the remaining blood. This maintains the pressure difference between the ventricle and the aorta and maintains the ejection of blood into the arterial system and to your body's organs. However, as the figure below shows, as blood flows into the aorta, the aortic pressure increases. This impedes blood flow from the heart. A war of opposing pressures ensues.

Thus, two factors are working here to determine how much blood is ejected from the heart during each beat: (1) How hard the heart muscle can squeeze down to maintain pressure within the ventricle and (2) the level to which aortic pressure increases as blood is pumped into it.

What are the cummulative factors which prevent blood from getting pumped out of the older heart ?

The strength of a heart cell's contraction can vary. At rest, just like the mother's uterus when not in labor, the heart muscle works at only a fraction of its maximum ability to contract. But, during vigorous exercise (labor) it is required to contract harder. As you age the maximum contraction ability of the muscle cells composing the heart wall declines. This adversely affects the ability of the heart to contract (pump harder) to get this blood out to the body. As we learned in Article #3, "How Good a Pump is Your Older Heart?" the maximum ejection fraction during vigorous exercise, which reflects the heart's ability to force out the blood that it contains, is reduced in older vs. younger persons during vigorous exercise. This age-associated reduction in the maximum contraction capacity of the ensemble of cells within your heart's wall is one reason why the maximum ejection fraction declines with age. The other factor, which can further interfere with ejection of blood from your heart and significantly reduce its maximum ejection capacity during exertion, is an excessive increase in pressure within your arteries.

Understanding pressures in your arteries

As you age the pressure inside your arteries often increases. From what you have learned about the body's "great canal system", which is dependent on pressures flowing (from higher to lower), you understand that increased arterial pressure is not good for getting blood out of the heart to the body. Remember that for the blood to flow out of the left ventricle, through the aorta (THE LARGEST CANAL) and into the arteries to the body, the pressure in the arteries needs to be lower than the pressure in the ventricle. In addition, the completeness of ejection of blood from the heart requires that the arterial pressure not increase too much as blood is expelled from the heart into the arterial tree. To understand how and why the arterial pressure increases with aging you need to first understand what "measuring blood pressure" means.

Age related changes in blood pressure

At rest, in the absence of what doctors diagnose as "high blood pressure" or "hypertension", arterial pressures, on average, are relatively low, say between 110-140 systolic and 60-90 diastolic millimeters of pressure (mmHg). As we age, the systolic pressure when we are at rest, on average, progressively increases. This is due to "hardening" or "stiffening" of your arteries (to be discussed in detail in a future article). During exercise the systolic pressure increases to a greater extent in older vs. younger persons. What does this mean in terms of our "canal system" and the pressure differences needed for the canal to flow smoothly? It means that the older heart must pump blood against the higher systolic pressures in the arteries to get the blood out to the body. We have learned that the maximum contraction ability of the cells composing the heart wall declines with aging. So, these two age-associated changes, one in your arterial tree, which increases the pressure which the heart has to pump against, and the other a decrease of the maximum contractile ability of cells comprising the heart wall, prevent the ejection fraction (EF) increasing during vigorous exercise in older persons, as previously discussed in article #3 ("How Good a Pump is Your Older Heart?")
In future articles we will go into more detail and discuss specific reasons as to why, on average, the arterial pressure increases and the maximum contractile ability of heart cells decreases as you age. And, we will show you that there are things that you can do to keep these age related changes to a minimum and thus maintain your exercise capacity as you age.

Dr. Ed is a physician/scientist, who is internationally recognized for studies that range from humans to molecules on how the heart and blood vessels work in health and disease as the body ages.

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Created on: 12/13/2002
Reviewed on: 01/03/2011

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