The human body has a hierarchy of structure and function, ranging from cellular biomolecules, through to organelles and cells, and on to tissues, organs, and the body's various systems. The biological manifestations that occur with aging affect the entire hierarchical structure of living systems. Age-related effects are seen in the accumulation of damaged cellular biomolecules (e.g., advanced gly-cosylation end products, lipid peroxidation products, genetic damage, and mutation), damaged organelles (mitochondria), and loss of cellular function, which contributes to dysfunction of the body's tissues, organs, and systems. These hierarchical changes have paved the way for over 300 theories in an attempt to explain how and why aging occurs. These theories have previously been broadly categorized into: (1) programed or genetic theories; and (2) damage accumulation (stochastic) theories.
However, with ongoing research these categories have not proven to be entirely comprehensive or mutually exclusive and it is more likely that there is a shifting range throughout the life span that reflects a decreasing influence of genetic factors and an increasing influence of stochastic events.
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