Throughout history man has had to accept the fact that all living things must die, but people now live longer than they used to. Yet, all【B1】things still show the effect of aging, which will eventually【B2】death. The body and the【B3】, they do not function as well as they【B4】in childhood and adolescence(青春期). The body provides less【B5】against disease and is more prone(易于……的)【B6】accident. A number of related causes may【B7】to aging. Some cells of the body have a【B8】long life, but they are not【B9】when they die. As a person ages,【B10】of brain cells and muscle cells decreases. Other body cells did and are replaced by new cells. In an aging person the【B11】cells may not be as viable(能生存的) or as capable【B12】growth as those of a young person. Another factor in aging may be changes within the cells【B13】. Some of the protein chemicals in cells【B14】known to change【B15】age and become less elastic. This is why the skin of old people wrinkles and hangs loose. This is also the reason old people shrink in【B16】. There may be other more important chemical changes in the cells. Some complex cell chemicals,【B17】DNA and RNA, store and【B18】information that the cells need. Aging may【B19】this process and change the information carrying molecules【B20】they do not transmit the information as well. 【B1】