Hardening of the arteries is a gradual process occuring as people grow older. It has been known by various names : arterial stiffening, vascular stiffness, arteriosclerosis and scientists have long thought that it played a role in diseases like atherosclerosis and high blood pressure. But the medical world now has evidence that its impact may reach beyond the blood vessels.

As they sort out the links between the arteries and the heart, the medical community hopes it will also gain insight into exactly how hardening of the arteries relates to disease. Hardening of the arteries has long been considered a normal part of aging in industrialized societies. However, in some people, for reasons not yet understood, this common condition turns into a disease process. Stiffening of the arteries is the major cause of high blood pressure in older people, which in turn is a leading risk factor for stroke, coronary artery disease, heart attack, and heart failure. And now stiffening is suspected of making arteries more prone to the cellular processes that underlie atherosclerosis, another key precursor of heart disease and stroke.

Aging is still considered the major risk factor for these diseases. But some early evidence from the Fleg's studies and those of others suggest that lifestyle may also play a key role. Low-salt diets and regular aerobic exercise may reduce hardening of the arteries.

Age and Hardening of the Arteries

What made scientists think there might be a link between hardening of the arteries and heart function in the first place? It goes back to what they have learned about both over the last few decades, partly through the Baltimore Longitudinal Study of Aging. And the scientists, by comparing younger and older volunteers, have been able to put together a picture of what happens in both heart and blood vessels as people age.

The heart, they have learned, adjusts to age in many subtle and interconnecting ways: It develops thicker walls, and it fills with blood and pumps the blood out in a different pattern and even by somewhat different mechanisms than when young.

The large, elastic arteries that are closest to the heart also change in complex ways. Picture an animated computer graphic of the arteries at, say, age 25, when the walls are compliant. The largest artery in the body, the aorta, leads away from the heart, first up toward the neck, where the carotid artery branches off to take blood to the head and brain, and then down toward the rest of the body. When the aortic valve opens, the aorta receives the rushing pulse of blood from the heart. It also receives pressure spreading from the walls of the heart to its own walls. This pressure travels along the aorta's walls in wave after wave until it reaches the walls of the smaller, branching arteries that take the blood to the rest of the body. There the waves of pressure slow and some are sent back through the aorta walls, becoming what are called wave reflections.

Now add, say, 50 years to this picture. The arteries, including the aorta, grow stiffer, their walls thicker, the diameters larger. The stiffer walls no longer expand as much as blood flows through them. Eventually, the resistance of the stiffer aorta walls increases significantly. Commonly it doubles over a lifespan and contributes to the increase in systolic blood pressure that often accompanies aging.

Along the walls of the stiffer aorta, the pressure waves now move more rapidly, and as a result, the wave reflection occurs sooner than it did before. The timing of the wave reflection, in fact, is one of the effects that hardening of the arteries can be measured noninvasively.

As the blood moves on into the smaller arteries, the hydraulics change. The pulse smooths out, the flow becomes more steady. The opposition to this steady flow is known as peripheral vascular resistance or PVR; so far studies show that among men, resting PVR does not change with normal aging, but that it does rise somewhat in women. In most people with high systolic blood pressure, PVR is elevated.

Next, picture the effects of movement, when a person sits up, stands up, or begins to walk or run. The heart rate increases and blood pressure rises. A group of pressure-sensitive nerves in the aorta respond to the changes in pressure by sending a message to the brain. The brain in turn sends a message back to the heart, which changes its rate and strength of contraction. This aorta/brain/heart message system is called the baroceptor response. Blood vessels also dilate to allow for the extra blood flow. In addition, blood is turned away temporarily from those muscles that don't need it (for instance, the stomach), so that more can be delivered to the working muscles.

Blood flows from the heart through the arteries (red blood vessels) and back to the heart through the veins (blue blood vessels). Age brings changes and hardening of the arteries.

In the older picture, the baroceptor response is blunted, perhaps as a result of stiffer arteries; the nerves in the aorta could be affected by increasing stiffness. Also at maximum exercise, the large arteries do not dilate as much as in the younger picture.

These in brief outline are some of the major changes that occur in blood vessel hydraulics with age. One reason these changes intrigue scientists is that they could have a major impact on heart dynamics. Hardening of the arteries increases the amount of resistance the heart must overcome to eject blood into the arteries, and any resistance to flow places a load on the heart.