Human Anatomy - Kotsan I. Y. 2009

Main stages of organism development in ontogeny
Postnatal development

Following the birth of a child, their development undergoes a qualitative shift. Prenatal development takes place within the mother's body, which essentially acts as the external environment for the embryo. This primary external environment exhibits little Variability, meaning that embryonic development is predominantly influenced by hereditary factors encoded in the chromosomal apparatus of the fertilized egg.

From the moment of birth, the child is immediately exposed to the natural external environment, which exerts a profound influence on the developing Organism. The body begins adapting to exogenous conditions—a process that takes various forms throughout the individual's entire life. Newly acquired traits are superimposed upon inherited ones, resulting in complex transformations within the organism.

After birth, the growth rate of the organism gradually decreases. While the weight of a newborn is 32 million times greater than that of a fertilized egg, the weight of an adult human exceeds that of a newborn by only 20–25 times.

Physical development is characterized by body weight, height, and the dimensions of individual body parts. During postnatal development, these indicators change unevenly. Accelerated growth is observed during early childhood (from 1 to 3 years), at ages 5 to 7, and during Puberty (13–16 years). Concurrently with somatic growth, age-related changes occur in all Organs and systems. Human growth ceases at approximately 20–25 years of age, ushering in a relatively stable period of life, the duration of which varies among individuals.

The entire human life cycle can be divided into distinct periods, each characterized by specific features related to the Structure and function of the organism, its individual systems, and organs. For age assessment, great importance is attached to the degree of ossification of individual skeletal bones, the state of the Endocrine glands, dental development, Hair growth, and other indicators. Almost all morphological traits are subject to significant individual variation, largely depending on living conditions, past illnesses, hereditary factors, and the like. Therefore, a distinction must be made between chronological (calendar) age and biological age, which frequently differ markedly from one another.

As the organism ages, sclerotic changes occur in A number of organs, often associated with the proliferation of Connective Tissue. These changes, in turn, lead to a decline in the Functional Properties of organs and Tissues, resulting in a reduction of their regenerative capacity—that is, The ability to replace dying Cells with new elements.

Engaging in physical culture and sports, maintaining A balanced diet, and leading a healthy lifestyle help preserve health, counteract biological Aging, enhance working capacity, and prolong human life.