Why do we age?

The mystery of human biology

Aging is, along with sleep, one of the fundamental mysteries of human biology. What causes the body to slow down, its cells to stop dividing, and its organs to fall prey to increasing disease and disability? No one has definitive answers to these questions, but theories can be grouped into two camps: gradual damage over time and genetic programming.

The first group of theories holds that the body ages due to the wear and tear that accumulates in tissues over the years. Waste products accumulate in cells, backup systems fail, repair mechanisms gradually break down, and the body simply wears out like an old car.

The second group argues that aging is determined by genes, by an internal molecular clock that adjusts to a specific schedule for each species. This theory is supported by animal studies: scientists have managed to increase the lifespan of some animals by altering a single gene. Biologists point out that, from an evolutionary perspective, the effects of natural selection diminish greatly after reproductive age. Evolution favors genes that are beneficial early in life, as they dedicate the organism's resources to reproduction and leave fewer available for long-term maintenance.

Changes

Aging affects almost every system in the body: the senses, the digestive organs, the cardiovascular system, the immune system, bones, and muscles. Interestingly, the central nervous system (the brain and spinal cord) is one of the least affected by age. In most tissues, the decline in function is not drastic. Only in situations of stress or illness does it become apparent that the aging body is having trouble coping.

Changes in bones and muscles affect an older person's daily life perhaps more than anything else. Between the ages of 30 and 60, bone density decreases in both men and women. Muscles also change over time. Between the ages of 30 and 75, approximately half of the body's muscle mass disappears, while the amount of fat doubles.

The heart, blood vessels, and lungs are durable structures, built for a long life. The fact that so many older people develop heart and lung problems has less to do with the aging process than with lifestyle factors such as smoking, obesity, and lack of exercise. These systems do change somewhat over time: the heart's valves and walls become thicker and stiffer, making it harder for the heart to pump blood. Arterial walls also become harder and thicker, which can contribute to high blood pressure. Lung tissues lose elasticity, and more importantly, the immune system weakens, making us more vulnerable to infections.

Some neurotransmitters decrease with age, as does cerebral blood flow.

These changes can produce subtle alterations over time. They may affect short-term memory, verbal fluency, and learning ability, but they do not necessarily significantly impair intellectual functioning.

Whether driven by an internal clock in our genes or the wear and tear of a lifetime, aging can often be mitigated through our lifestyle choices.

Parts of this article, originally published in English at nationalgeographic.com, have previously appeared in the book 'Your Body: A User's Guide' by Patricia S. Daniels. Copyright © 2023 National Geographic Partners, LLC.