单选题
Solving a longstanding mystery, scientists have found that the human heart continues to generate new cardiac cells throughout the life span, although the rate of new cell production slows with age.
The finding, published in the April 3 issue of Science, could open a new path for the treatment of heart diseases such as heart failure and heart attack, experts say.
"We find that the beating cells in the heart, cardiomyocytes, are renewed," said lead researcher Dr. Jonas Frisen, a professor of stem cell research at the Karolinska Institute in Stockholm, Sweden. "It has previously not been known whether we were limited to the cardiomyocytes we are born with or if they could be renewed," he said.
The process of renewing these ceils changes over time, Frisen added. In a 20-year-old, about 1 percent of cardiomyocytes are exchanged each year, but the turnover rate decreases with age to only 0.45 percent by age 75.
"If we can understand how the generation of new cardiomyocytes is regulated, it may potentially possible to develop pharmaceuticals that promote this process to stimulate regeneration after, for example, a heart attack," Frisen said.
That could lead to treatment that helps restore damaged hearts. "A lot of people suffer from chronic heart failure," noted co-author Dr. Ratan Bhardwaj, also from the Karolinska Institute. "Chronic heart failure arises from heart cells dying," he said.
With this finding, scientists are "opening the door to potential therapies to having ourselves heal ourselves," Bhardwaj said. "Maybe one could devise a pharmaceutical agent that would make heart cells make new and more cells to overcome the problem they are facing."
But barriers remain. According to Bhardwaj, scientists do not yet know how to increase heart cell production to a rate that would replace cells faster than they are dying off, especially in older patients with heart failure. In addition, the number of new cells the heart produces was estimated using healthy hearts--whether the rate of cell turnover in diseased hearts is the same remains unknown.

单选题 The human heart stops producing cardiac cells ______.
A. when a person is born B. when a person becomes old
C. when a person gets sick D. when a person dies

单选题 The finding could prove to be helpful to ______.
A. the study of longstanding mysteries B. the analysis of cardiac cells
C. the prevention of chronic diseases D. the treatment of heart disease

单选题 The people who're in their mid-70s, only 0.45 percent of cardiomyocytes ______.
A. are still functional B. are replaced each year
C. are lost each year D. are damaged each year

单选题 Chronic heart failure is associated with ______.
A. the death of heart cells B. the life span of a person
C. the effects of pharmaceuticals D. the weight of the patient

单选题 It remains unknown whether the rate of cell turnover in diseased hearts ______.
A. changes over time
B. can be monitored
C. is the same as that in healthy hearts
D. is high enough to replace cells faster than they're dying off