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McKinley/O’Loughlin Human Anatomy, 2nd Edition CHAPTER 20 Answers to “What Did You Learn?” 1. Cells that respond to a specific hormone are called target cells. 2. In a negative feedback loop, the result of a process acts back at an earlier step in the process to slow down or turn off the process. 3. The hypothalamus secretes regulatory hormones that are releasing hormones (RH) and inhibiting hormones (IH) to control the function of the anterior pituitary gland. 4. The pituitary gland lies inferior to the hypothalamus within the sella turcica of the sphenoid bone. It is connected to the hypothalamus by a thin stalk, the infundibulum. 5. The anterior pituitary secretes adrenocorticotropic hormone (ACTH), gonadotropins (FSH and LH), thyroid-stimulating hormone (TSH), growth hormone (GH), prolactin (PRL), and melanocyte-stimulating hormone (MSH). ACTH stimulates the adrenal cortex to produce and secrete some corticosteroid hormones. FSH and LH influence reproductive system activities by regulating hormone synthesis and secretion in both female and male gonads and the production and maturation of gametes in both sexes. TSH regulates the release of thyroid hormone from the thyroid gland. GH affects cellular activities in most body cells, especially growth in the skeletal and muscular systems, and it causes the liver to produce somatomedins, which stimulate growth at the epiphyseal plate of long bones. Although GH affects activities in almost every cell in the body, its strongest effects are upon the growth and development of the skeletal and McKinley/O’Loughlin Human Anatomy, 2nd Edition muscular systems. In females, PRL regulates mammary gland growth and the production of breast milk, and in males, prolactin may play a role in the sensitivity of the interstitial Leydig cells to LH. MSH stimulates both the rate of melanin synthesis by melanocytes in the integument and their distribution through the skin. 6. The anterior pituitary is controlled by regulatory hormones (releasing and inhibiting hormones) released from the hypothalamus. These hormones enter the hypothalamo-hypophyseal portal system and stimulate cells in the anterior pituitary. In contrast, the posterior pituitary is under neural control from the hypothalamus. Axons from neurons in the paraventricular nucleus and supraoptic nucleus terminate in the posterior pituitary. These axons transport the hormones to the posterior pituitary for eventual release. Stimulation of these neurons causes release of hormones from their axon terminals. 7. Colloid is a viscous, protein-rich fluid housed within the lumen of the thyroid follicle. 8. Thyroid hormone stimulates its target cells (many of the cells in the body), so cellular metabolism increases in these cells. Basal body temperature rises as a result of this stimulation. 9. Parathyroid hormone increases calcium levels in body by (1) stimulating osteoclasts to resorb bone, (2) stimulating calcitriol synthesis to increase calcium absorption in the small intestine, and (3) preventing calcium loss during the formation of urine. McKinley/O’Loughlin 10. Human Anatomy, 2nd Edition Aldosterone regulates the sodium (Na+) and potassium (K+) ion ratio by promoting sodium ion retention with the concomitant reabsorption of water and the excretion of potassium from tubular fluid through kidney tubule cells. 11. Stimulation by the sympathetic division of the ANS causes chromaffin cells to secrete epinephrine. 12. The hormones produced by the endocrine cells of the pancreas are glucagon, insulin, somatostatin, and pancreatic polypeptide. 13. The pineal gland is located in the posterior region of the epithalamus. 14. Modified cardiac muscle cells in the wall of the right atrium secrete the hormone atriopeptin. Answers to “Content Review” 1. Hormones are molecules released by endocrine glands into the bloodstream. Only cells with specific receptors for the hormone (so the hormone can bind or attach to the cell or a molecule within the cell) will respond to that hormone. These cells are called target cells and the organs that contain these target cells are called target organs. In contrast, organs, tissues or cells that do not have the specific "receptor" for a hormone do not bind or attach the hormone and do not respond to its stimulating effects. 2. Peptide hormones are formed from chains of amino acids. An example would be growth hormone. Biogenic amines are small molecules produced by altering the structure of an amino acid. An example would be thyroid hormone. Steroid McKinley/O’Loughlin Human Anatomy, 2nd Edition hormones are a type of lipid derived from cholesterol. An example would be testosterone. 3. The hypothalamus has special neurosecretory cells that secrete hormones to influence the secretory activity of the anterior pituitary gland. These hormones are called regulatory hormones because they are molecules secreted into the blood to regulate secretion of most anterior pituitary hormones. Regulatory hormones fall into one of two groups: Releasing hormones [RH] stimulate the production and secretion of specific anterior pituitary hormones, and Inhibiting hormones [IH] deter the production and secretion of specific anterior pituitary hormones. Additionally, the hypothalamus produces two hormones that are transported through axons to the posterior pituitary and stored there until needed. These two hormones are oxytocin and antidiuretic hormone (ADH). 4. The anterior pituitary gland is controlled by regulatory hormones secreted by the hypothalamus. These regulatory hormones reach the anterior pituitary by traveling through a blood vessel network called the hypothalamo-hypophyseal portal system. A portal system is composed of two capillary plexuses interconnected by a portal vein. The hypothalamo-hypophyseal portal system is essentially a venous "shunt" that takes venous blood (carrying regulatory hormones) from the hypothalamus directly to the anterior pituitary first, before the blood returns to the heart. Thus, it provides a pathway for hypothalamic hormones to immediately reach the anterior pituitary. In addition, the veins that drain this portal system also provide a pathway by which the anterior pituitary hormones may be released into the general bloodstream. McKinley/O’Loughlin 5. Human Anatomy, 2nd Edition The anterior pituitary secretes (1) adrenocoticotropic hormone [ACTH] to stimulate the adrenal cortex to produce and secrete some corticosteroid hormones, (2 and 3) gonadotropins (FSH and LH) to stimulate the gonads in males and females both for regulation of reproductive system activities through hormone synthesis and secretion, and the production and maturation of gametes, (4) thyroid-stimulating hormone (TSH) to stimulate the thyroid gland to release thyroid hormones, (5) growth hormone (GH) affects cellular activities in most body cells, especially growth activities in the skeletal and muscular system, and it causes the liver to produce somatomedin, which stimulates growth activities at the epiphyseal plate of long bones, (6) prolactin (PRL) regulates mammary gland growth and the production of breast milk in females, and in males, prolactin may play a role in the sensitivity of the interstitial cells in the testes to LH, and (7) melanocyte-stimulating hormone (MSH), which stimulates both the rate of melanin synthesis by melanocytes in the integument and their distribution through the skin. 6. Parafollicular cells secrete the hormone calcitonin in response to elevated blood calcium levels. Calcitonin acts to reduce blood calcium levels by stimulating osteoblast activity and inhibiting osteoclast activity. New bone matrix formation with the simultaneous deposition of calcium salts onto this matrix, and a decrease in bone matrix resorption. 7. PTH secretion occurs when there is a decrease in calcium ion concentration in the body. Calcium ions are needed for many body activities, such as activity at nerve synapses, muscle contraction, etc. Insufficient PTH secretion may result in McKinley/O’Loughlin Human Anatomy, 2nd Edition adequate levels of calcium in the blood, and consequently some body functions would be interrupted. Symptoms are mostly neuromuscular in nature and range from mild tingling of the fingers and extremities, to marked muscle cramps and contractions (tetany), and in severe cases there may even be convulsions. 8. The adrenal cortex is partitioned into three separate regions or zones. Different functional categories of steroid hormones are synthesized and secreted in the separate zones; the zona glomerulosa (produces mineralocorticoids) , the zona fasciculata (produces glucocorticoids), and the zona reticularis (produces androgens). 9. Four endocrine cell types form the islets: alpha cells secrete glucagon to raise blood glucose levels; beta cells produce insulin causing blood glucose levels to decline; delta cells produce somatostatin to slow down or inhibit digestive activities in the digestive tract; and F cells secrete pancreatic polypeptide to suppress and regulate somatostatin secretion from delta cells. 10. Some of the organs of the GI tract, heart, kidneys, and gonads contain their endocrine cells which secrete their own hormones. These hormones help regulate digestive system activities, blood volume, electrolyte levels in the blood, red blood cell production, blood pressure, and sexual maturation and activity, respectively.