Aging and death is a natural process that all living organisms undergo at different rates. Human beings, in particular, experience many physical and mental changes as they age. Life expectancy is defined as the average length of time that a member of a population can expect to live. In the US, life expectancy has increased by Choose...10203040 years since 1900. And the maximum life span for a person today is about 120 years. The branch of science that studies aging and the factors that affect it is called Choose...oncologygerontology epidemiology statistics . The countries with the longest life expectancy for both men and women combined are Choose...United States and Switzerland Canada and Iceland Great Britain and Germany Australia and Japan . Different species of mammals such as dogs, elephants, horses, and human beings age at different rates and have different maximum life spans. The amount of energy expended per day per gram of body weight is called Choose...specific metabolic rate basal metabolism obesitycalories and this value correlates with the maximum expected life span of the species. All animals appear to have a biological clock that determines their rate of aging and the maximum life span for the species. However, environmental factors also play a significant role in the rate of aging and the time of death. The most important environmental factor in extending life span in animals is Choose...amount of pollution calorie restriction diet consumption of protein consumption of fat . Certain diseases occur primarily among older people. One of the most prevalent diseases among the elderly that affects their mental functions is Choose...osteoporosis Parkinson's disease rheumatoid arthritis Alzheimer's disease and some people are genetically more susceptible to this disease than are others. The second most common disease of the elderly that mainly affects physical movements is Choose...rheumatoid arthritis Alzheimer's disease osteoporosisParkinson's disease . Choose...osteoporosis Parkinson's disease Alzheimer's disease rheumatoid arthritis is a disease that affects more older women than men and is primarily due to poor diet and lack of exercise. In the final stages of dying, some people seek assistance from others to help them end their suffering at a time of their choosing. The act of helping another person die is called Choose...hospice living palliative therapy euthanasia making a living will . Many factors under a person's control contribute to healthy aging. Persons surviving to age 65 in the US today can expect to live another Choose...10152025 years. A healthy life style while young will contribute significantly to healthy aging and reduce disabilities when you become old.