Biology of Aging | National Institute on Aging

http://www.nia.nih.gov/health/publication/aging-under-microscope/what-aging

What is aging?

In the broadest sense, aging reflects all the changes that occur over the course of life. You grow. You develop. You reach maturity. To the young, aging is exciting—it leads to later bedtimes and curfews, and more independence. By middle age, another candle seems to fill up the top of the birthday cake. It’s hard not to notice some harmless cosmetic changes like gray hair and wrinkles. Middle age also is the time when people begin to notice a fair amount of physical decline. Even the most athletically fit cannot escape these changes. Take marathon runners, for example. An NIA-funded study found that their record times increased with age—aging literally slowed down the runners. Although some physical decline may be a normal result of aging, the reasons for these changes are of particular interest to gerontologists.
Gerontologists look for what distinguishes normal aging from disease, as well as explore why older adults are increasingly vulnerable to disease and disability. They also try to understand why these health threats take a higher toll on older bodies. Since 1958, NIA’s Baltimore Longitudinal Study of Aging (BLSA) has been observing and reporting on these kinds of questions. As with any longitudinal study, the BLSA repeatedly evaluates people over time rather than comparing a group of young people to a group of old people, as in a cross-sectional study. Using this approach, BLSA scientists have observed, for example, that people who have no evidence of ear problems or noise-induced hearing loss still lose some of their hearing with age—that’s normal. Using brain scans to learn if cognitive changes can be related to structural changes in the brain, BLSA scientists discovered that even people who remain healthy and maintain good brain function late in life lose a significant amount of brain volume during normal aging.
However, some changes that we have long thought of as normal aging can be, in fact, the signs of a potential disease. Take, for example, sudden changes in personality. A common belief is that people become cranky, depressed, and withdrawn as they get older. But an analysis of long-term data from the BLSA showed that an adult’s personality generally does not change much after age 30. People who are cheerful and assertive when they are younger will likely be the same when they are age 80. The BLSA finding suggests that significant changes in personality are not due to normal aging, but instead may be early signs of disease or dementia.
The rate and progression of cellular aging can vary greatly from person to person. But generally, over time, aging affects the cells of every major organ of the body. Changes can start early. Some impact our health and function more seriously than others. For instance, around the age of 20, lung tissue starts to lose elasticity, and the muscles of the rib cage slowly begin to shrink. As a result, the maximum amount of air you can inhale decreases. In the gut, production of digestive enzymes diminishes, affecting your ability to absorb foods properly and maintain a nutritional balance. Blood vessels in your heart accumulate fatty deposits and lose flexibility to varying degrees, resulting in what used to be called “hardening of the arteries” or atherosclerosis. Over time, women’s vaginal fluid production decreases, and sexual tissues atrophy. In men, aging decreases sperm production, and the prostate can become enlarged.
Scientists are increasingly successful at detailing these age-related differences because of studies like the BLSA. Yet studies that observe aging do not identify the reasons for age-related changes, and, therefore, can only go so far toward explaining aging. Questions remain at the most basic level about what triggers aging in our tissues and cells, why it occurs, and what are the biological processes underlying these changes. Scientists look deep into our cells and the cells of laboratory animals to find answers. What they learn today about aging at the cellular and molecular levels may, ultimately, lead to new and better ways to live a longer, healthier life.

Possible Pathways Leading to Aging

Illustration and information adapted from www.genome.com

To answer questions about why and how we age, some scientists look for mechanisms or pathways in the body that lead to aging. Our cells constantly receive cues from both inside and outside the body, prompted by such things as injury, infection, stress, or even food. To react and adjust to these cues, cells send and receive signals through biological pathways. Some of the most common are involved in metabolism, the regulation of genes, and the transmission of signals. These pathways may also be important to aging.