Final answer:
After age 45, the rate at which cells break down outpaces the rate at which they are renewed, leading to the gradual decline of tissue function known as aging. Cell regeneration slows and cells may begin to function abnormally, contributing to diseases associated with aging.
Step-by-step explanation:
After age 45 or so, the rate of cell breakdown begins to exceed the rate of cell wall renewal. This is a process that reflects the general pattern of aging at the cellular level. As we age, cells do not regenerate as quickly as they once did, leading to an overall decline in the body's ability to maintain and repair tissues. Changes in the cell membrane make the transport of oxygen and nutrients into the cell and removal of carbon dioxide and waste less efficient, contributing to cellular dysfunction and the presentation of age-related diseases such as arthritis, memory issues, and certain types of cancer.
The longevity of cells in different tissues varies greatly and is influenced by internal and external factors including the specific functions of the tissue. For example, muscle and nerve cells remain in the GO phase of the cell cycle for a lifetime, adipose tissue for years, liver cells for about a year, erythrocytes for months, bone osteoclasts for weeks, leukocytes for days, and epidermal cells for hours. Senescence affects every cell, tissue, and organ in the body, and factors such as genetic makeup and lifestyle contribute to the variability in aging between individuals.