Every year that you age, your heart shrinks and its ability to pump blood through your body decreases by up to 5 percent, a new study finds.

Researchers at Johns Hopkins University analyzed MRIs (magnetic resonance images) of the structure and pumping function of the hearts of 5,004 men and women between the ages of 45 and 84 to look for minute changes. The patients were of various ethnic backgrounds and had no existing symptoms of heart disease.

The doctors found that the heart's muscle mass gradually shrinks an average of about 0.3 grams per year.

They also found that for each year that people age, the time it takes for the heart muscles to squeeze and relax grows longer, by 2 to 5 percent. The actual amount of blood pumped out of the heart falls by about 9 milliliters per year, the researchers said.

"Our results demonstrate just how the heart plays a losing game of catch-up as people age," said study leader Susan Cheng, a former medicine resident at Hopkins.

The results, presented at a meeting of the American Heart Association on Nov. 4, provide insight into the risk of developing heart failure, which occurs when the heart can't fill up with or pump enough blood through the body, the researchers say.

"We already knew that the heart is constantly trying to adapt to risk factors, but now we know that this task gets more difficult as the heart ages and loses a little bit of its pumping capacity every year," Cheng said.

Obesity and high blood pressure are also risk factors for developing congestive heart failure, which is marked by symptoms such as shortness of breath and fatigue. Estimates show that more than 5 million Americans have some form of congestive heart failure.