Most living things undergo two types of biological change during their lifetime. One is 16 , an increase in the size or efficiency of an organism. The other is aging, which involves a decrease in size or efficiency.Signs of aging begin to 17 in most people between the age of 30 and40. Heredity determines most of the ways a person changes while aging, but environment also 18 . Graying hair is probably the most common sign of aging. All the senses decline with age. For example, the eyes lose their ability to adapt 19 darkness, and they require brighter light for reading.Movement becomes harder with age. By age 80, about half the muscle cells have been replaced by other kinds of tissue. In women 20 , bones lose calcium and become more likely to 21 . As a person ages, the body’s ability to combat infection declines. This change occurs because the white blood cells of 22 is called the body's immune system lose their 23 function.Many people believe that with advancing age, an individual loses the ability to learn, remember, and make decisions. But 24 disease or injury damages the brain, a healthy elderly person who 25 active probably suffers no serious decline in 26 ability.No one completely understands the aging process. Some scientists 27 that human aging will ever be controlled. Others believe that aging can be 28 . Usually, normal human cells can be kept 29 in a laboratory for only a limited time. But 30 cells, such as various kinds of cancer cells, can live for a long time. If scientists can determine how such cells survive, they may gain an insight into the process of cell aging.(16)