What causes aging?

Aging Biology: How do cells age?

Cells, the most basic body unit, are at the center of any discussion about aging. You have trillions of cells, and they're organized into different tissues that make up organs, such as your brain, heart, and skin.

Some cells, such as those that line the gastrointestinal tract, reproduce continuously; others, such as the cells on the inside of arteries, lie dormant but are capable of replicating in response to injury. Still others, including cells of the heart, nerves, and muscles, cannot reproduce. Some of these non-reproducing cells have short life spans and must be continually replaced by other cells in the body. (Red and white blood cells are examples.)

Others, such as heart and nerve cells, live for years or even decades. Over time, cell death outpaces cell production, leaving us with fewer cells. As a result, we are less capable of repairing wear and tear on the body, and our immune system is compromised. We become more susceptible to infections and less proficient at seeking out and destroying mutant cells that could cause cancerous tumors. In fact, many older adults succumb to conditions they could have resisted in their youth.

Though cell death is the basis for understanding the aging process, it is not the only factor. The aging process is incredibly complicated, and it's often difficult to distinguish between changes that are the result of time marching on and those that come with common medical conditions, including high blood pressure and heart disease.

Aging is the inevitable decline in the body's resiliency, which ultimately leads to dwindling powers, both mental and physical. Some aging changes affect us all. For example, diminished eyesight that necessitates reading glasses is considered normal, primarily because it affects everyone who lives long enough.

On the other hand, cataracts, which are formations on the lens of the eye that cloud your vision, can be prevented and are not considered part of the aging process, despite their prevalence in older people. To further complicate matters, organs age at different speeds. That's why a 50-year-old may hear as well as someone twenty years his junior, but may have arthritis or high blood pressure.

Theories abound about the underlying cause of aging. Some maintain that aging is preprogrammed into our cells, while others contend that aging is primarily the result of environmental damage to our cells. Although none of the theories can fully explain the process, they do help us better understand how we age. On the next page, we'll explore the most popular aging theories.