Anatomy & Physiology

Anatomy & Physiology Study Guide • At the median eminence of the hypothalamus, neurons release regulatory factors, either releasing hormones, RH, or inhibiting hormones, IH, into the nearby interstitial fluids through fenestrated capillaries. • The hypophyseal portal system ensures that these regulatory factors reach the intended target cells before they enter the general circulation. • Thyroid-stimulating hormone (TSH) triggers the release of thyroid hormones. Thyrotropin- releasing hormone (TRH) promotes the secretion of TSH. • Adrenocorticotropic hormone (ACTH) promotes the release of glucocorticoids by the suprarenal cortex. Corticotropin-releasing hormone (CRH) causes the secretion of ACTH. • Follicle-stimulating hormone (FSH) stimulates follicle development and estrogen secretion in females and sperm production in males. Luteinizing hormone (LH) causes ovulation and progestin production in females and androgen production in males. Gonadotropin-releasing hormone (GnRH) promotes the secretion of both FSH and LH. • Prolactin (PRL), together with other hormones, stimulates both the development of the mammary glands and milk production. • Growth hormone (GH, or somatotropin) stimulates cell growth and replication through the release of somatomedins or IGFs from liver cells. The production of GH is regulated by growth hormone–releasing hormone (GH–RH) and growth hormone–inhibiting hormone (GH–IH). • Melanocyte-stimulating hormone (MSH) can be released by the pars intermedia during fetal development, early childhood, pregnancy, or certain diseases. This hormone stimulates melanocytes to produce melanin. • The neurohypophysis, or posterior lobe of the pituitary gland, contains the unmyelinated axons of hypothalamic neurons. Neurons of the supraoptic and paraventricular nuclei manufacture antidiuretic hormone (ADH) and oxytocin, respectively. ADH decreases the quantity of water lost at the kidneys and, in higher concentrations, elevates blood pressure. In women, oxytocin stimulates contractile cells in the mammary glands and has a stimulatory effect on smooth muscles in the uterus. The thyroid gland lies inferior to the larynx and requires iodine for hormone synthesis: • The thyroid gland lies anterior to the thyroid cartilage of the larynx and consists of two lobes connected by a narrow isthmus. • The thyroid gland contains numerous thyroid follicles. Thyroid follicles release several hormones, including thyroxine and triiodothyronine. • Most of the thyroid hormones entering the bloodstream are attached to special thyroid- binding globulins (TBGs); the rest are attached to either transthyretin or albumin. • Thyroid hormones are held in storage, bound to mitochondria (thereby increasing ATP production), or bound to receptors activating genes that control energy utilization. They also exert a calorigenic effect. • The C cells of the thyroid follicles produce calcitonin (CT), which helps regulate Ca2+ concentrations in body fluids, especially during childhood and pregnancy. The four parathyroid glands, embedded in the posterior surface of the thyroid, secrete a parathyroid hormone to elevate plasma Ca2+: Achieve Page 218 of 368 ©2018