Anatomy & Physiology

Anatomy & Physiology Study Guide

©2018 Achieve Page 357 Anatomical and physiological changes occur as the fetus completes the transition to the newborn stage, or neonate . Prior to delivery, nutrients, wastes, hormones, dissolved gases, and antibodies were transferred across the placenta, and the fetus was dependent on the maternal circulation. At birth, the neonate becomes relatively self-sufficient. It uses its own organs for respiration, digestion, and excretion. The neonatal period consists of the newborn receiving all nutrients from milk (typically breast milk secreted by the maternal mammary glands). Infancy and Childhood The most rapid growth occurs during prenatal development, and the growth rate declines after delivery. Circulating hormones, notably growth hormone, adrenal steroids, and thyroid hormones, direct growth from birth through childhood. These hormones affect each tissue and organ in specific ways, depending on the sensitivities of the individual cells. As a result, growth does not occur uniformly, and body proportions gradually change. The head, for example, is relatively large at birth but decreases in proportion with the rest of the body as the child grows to adulthood. Adoles enc and Maturi y Puberty marks the beginning of adolescence and is the period of sexual maturation, ending when growth is completed. Three major hormonal events interact at the onset of puberty. The hypothalamus increases its production of gonadotropin-releasing hormone (GnRH). Endocrine cells in the adenohypophysis become more sensitive to the presence of GnRH. Circulating levels of FSH and LH rise rapidly, and ovarian or testicular cells become more sensitive to FSH and LH, initiating gamete production, the secretion of sex hormones that stimulate the appearance of secondary sex characteristics and behaviors, and a sudden acceleration in the growth rate. The age at which puberty begins varies. In the United States today, puberty generally starts at about age 12 in boys and 11 in girls, but the normal ranges are broad (10–15 in boys, 9–14 in girls). of 368 26.2 Postnatal The course of postnatal development typically includes five life stages: the neonatal period, infancy, childhood, adolescence, and maturity. Each stage is typified by a distinctive combination of characteristics and abilities. The Neonatal Period, Infancy, and Childhood The neonatal period extends from birth to 28 days old. Infancy then continues to up to 12 months of age. The toddler stage begins at one year old (when walking generally occurs) and extends until three years old. Childhood lasts until puberty, which marks a period of physical and sexual maturation, when the child reaches the adolescent stage. During these developmental stages, the organ systems become operational and gradually acquire the functional characteristics of adult structures. The individual grows rapidly, and body proportions change significantly. Pe iatrics is the medical specialty that focuses on postnatal development from infancy through adolescence. Infants and young children cannot describe the problems they are experiencing; thus, pediatricians and parents must be skilled observers. Standardized tests are used to assess developmental progress relative to average values. The Neonatal Period