Anatomy & Physiology

Anatomy & Physiology Study Guide concentrations of several other hormones, including TSH, thyroid hormones, ADH, PTH, prolactin, and glucocorticoids, remain unchangedwith advancing age. Although circulating hormone levels may remain within normal limits, some endocrine tissues become less responsive to stimulation. For example, in elderly individuals, smaller amounts of GH and insulin are secreted after a carbohydrate- rich meal. The reduction in levels of GH and other tropic hormones affects tissues throughout the body; these hormonal effects are associated with the reductions in bone density and muscle mass. 17.14 Chapter Seventeen Review Homeostasis is preserved through intercellular communication: • In general, the nervous system performs short-term “crisis management,” whereas the endocrine system regulates longer-term, ongoing metabolic processes. • Paracrine communication involves the utilization of chemical signals to transfer data from cell to cell within a single tissue. • Endocrine communication occurs when chemicals, also known as hormones, are sent into circulation by endocrine cells. The hormones change the metabolic activities of many tissues and organs simultaneously by modifying the activities of target cells. The endocrine system regulates physiological processes through the binding of hormones to receptors: • The endocrine system includes all the cells and endocrine tissues of the body that produce hormones or paracrine factors. • Hormones can be categorized into three groups: amino acid derivatives; peptide hormones; and lipid derivatives, including steroid hormones and eicosanoids. • Hormones may circulate freely or bound to transport proteins. Free hormones are rapidly removed from the bloodstream. • Receptors for catecholamines, peptide hormones, and eicosanoids are in the plasma membranes of target cells. Thyroid and steroid hormones cross the plasma membrane and bind to receptors in the cytoplasm or nucleus, activating or inactivating specific genes. • Endocrine reflexes are the counterparts of neural reflexes. • The hypothalamus regulates the actions of the nervous and endocrine systems by secreting regulatory hormones, which control the activities of endocrine cells in the adenohypophysis; acting as an endocrine organ by releasing hormones into the bloodstream at the neurohypophysis and exerting direct neural control over the endocrine cells of the suprarenal medullae. The bilobed pituitary gland is an endocrine gland that releases nine peptide hormones: • The pituitary gland, or hypophysis, releases nine important peptide hormones; all bind to membrane receptors and use cyclic-AMP as a second messenger. • The adenohypophysis, or anterior lobe, of the pituitary gland, is subdivided into three pars: pars distalis, pars intermedia, and pars tuberalis. Achieve Page 217 of 368 ©2018