Anatomy & Physiology I and II

Anatomy & Physiology Study Guide

©2018 Achieve Test Prep Page 215 of 367 17.9 Cell Response to Hormones Cells never respond to only one hormone; instead, they respond to multiple hormones simultaneously. When a cell receives instructions from two hormones at the same time, four outcomes are possible: The two hormones may have antagonistic (opposing) effects. The observed effects are weaker than those produced by either hormone acting unopposed. The two hormones may have additive effects; thus, the net result is greater than the effect that each would produce acting alone. One hormone can have a permissive influence on another one. In those cases, the first hormone is needed for the second to produce its effect. Finally, hormones may produce different but complementary results in specific tissues and organs. 17.10 Role of Hormones in Growth Normal growth requires the cooperation of several endocrine organs. Several hormones—GH, thyroid hormones, insulin, PTH, calcitriol, and reproductive hormones—are especially important, although many others have secondary effects on growth. Growth Hormone (GH) The effects of GH on protein synthesis and cellular growth are most apparent in children, in whom GH supports muscular and skeletal development. In adults, growth hormone assists in the maintenance of normal blood glucose concentrations and the mobilization of lipid reserves stored in adipose tissue. It is not the primary hormone involved, and an adult with a GH deficiency but normal levels of thyroxine (T4), insulin, and glucocorticoids will have no physiological problems. Thyroid Hormones Normal growth also requires appropriate levels of thyroid hormones. If these hormones are not present during fetal development, or for the first year after birth, the nervous system will fail to develop normally, and mental retardation will result. If T4 concentrations decline later in life, but before puberty, normal skeletal development will not continue. Insulin Growing cells require adequate supplies of energy and nutrients. Without insulin, the passage of glucose and amino acids across plasma membranes will be drastically reduced or eliminated. Parathyroid Hormone (PTH) and Calcitriol Parathyroid hormone and calcitriol promote the absorption of calcium salts for subsequent deposition in bone. Without adequate levels of both hormones, bones can still enlarge but will be poorly mineralized, weak, and flexible. For example, in rickets, a condition typically caused by inadequate calcitriol production as a result of vitamin D deficiency in growing children, the lower limb bones are so weak that they bend under the body’s weight. Reproductive Hormones The activity of osteoblasts in key locations and the growth of specific cell populations are affected by the presence or absence of reproductive hormones (androgens in males, estrogens in females). These sex hormones stimulate cell growth and differentiation in their target tissues. The targets differ for androgens and estrogens, and the differential growth induced by each account for gender-related differences in skeletal proportions and secondary sex characteristics.