Overview
We are born, we grow up, we age, and then we die. Unless disease or trauma occurs, most humans go through the various stages of life described above. Human Development is the
process of growing to maturity. Traditionally, theories that explain senescence have generally been divided between the programmed and stochastic theories of aging. Programmed theories imply that aging is regulated by biological clocks operating throughout the life span. This regulation would depend on changes in gene expression that affect the systems responsible for maintenance, repair and defense responses. Stochastic theories blame environmental impacts on living organisms that induce cumulative damage at various levels as the cause of aging.
Examples of environmental impacts range from damage to deoxyribonucleic acid (DNA),
damage to tissues and cells by oxygen radicals (widely known as free radicals countered by
the even more well known antioxidants), and cross-linking. However, aging is now seen as a combination of genetic and environmental processes; a progressive failure of homeostatic mechanisms involving maintenance and repair genes, stochastic events leading to molecular damage and molecular heterogeneity, and chance events determining the probability of death.
Homeostasis, as we have seen throughout this book, is maintained through complex and
interacting systems, and aging is considered to be a progressive shrinkage of homeostatic
capabilities, mainly due to increased molecular heterogeneity. In this chapter we explore
the physiology of all stages of human development, with a particular emphasis on the aging process.

Source: Cray MI, Textbook of Human Physiology and Biophysics, V#1 , Atlanta Ga: IVMS 2014:485