CHAPTER SEVENTEEN
THE ENDOCRINE SYSTEM
17.1 An Introduction to the Endocrine System and Hormones. The endocrine system works with or in harmony with the nervous system to control and coordinate all the activities of the body and to maintain homeostasis. Endocrinology - field of medicine that focuses on the treatment of endocrine system disorders. 1. The endocrine system and nervous system are similar yet different: 1. Both systems rely on the release of chemicals that bind to specific receptors on their target cells. 2. Both share many chemical messengers; when released into the bloodstream they are called hormones but when released into a synapse, they are called neurotransmitters. 3. Both systems are regulated primarily by negative feedback control. 4. Both share a common goal: to preserve homeostasis by coordinating and regulating the activities of other cells, tissues, organs, and systems. 2. Mechanisms of Intercellular Communication 1. Direct communication - via gap junctions; use ions, small solutes, and other lipid-soluble materials as chemical mediators; effects are usually limited to adjacent cells of the same type that are interconnected by connexons. 2. Paracrine communication -- via extracellular fluids; use paracrine factors as chemical mediators; effects are primarily limited to the local area where paracrine factor concentrations are relatively high; target cells must have appropriate receptors. 3. Endocrine communication -- via the bloodstream; use hormones as chemical mediators; effects are on target cells located in other tissues or organs; target cells must have an appropriate receptor. 4. Neural communication - via synaptic clefts; use neurotransmitters as chemical mediators; effects are limited to very specific areas; target cells must have appropriate receptors. 3. Structures of the endocrine system: 1. Thalamus 2. Pineal gland 3. Pituitary gland 4. Thyroid 5. Thymus 6. Adrenal 7. Pancreas 8. Ovaries 9. Testes
17.2 Hormones are chemical messengers released by endocrine cells/glands into bloodstream to be transported throughout the body to regulate the metabolic functions and activities of other cells of the body. See table at end of outline for list of endocrine glands and their related hormones. A. Hormones and paracrine factors of the body can be divided into groups: 1. Amino acid derivatives -- hormones derived from a single amino acid. Examples include: the thyroid hormones such as thyroxine and triiodothyronine; the catecholamines such as epinephrine, norepinephrine, and dopamine; and melatonin 2. Peptide hormones -- chains of amino acids. Examples include: a. Polypeptides: antidiuretic hormone (9 amino acids) and oxytocin (9 amino acids)
b. Small proteins: insulin (51 amino acids), growth hormone (191 amino acids) and prolactin (198 amino acids) c. Glycoproteins: thyroid-stimulating hormone, luteinizing hormone, and follicle- stimulating hormone
3. Lipid derivatives - consist of carbon rings and side chains built either from fatty acids chains or cholesterol. a. Eicosanoids -- built from fatty acid chains and include: leukotrienes and prostaglandins. b. Steroid hormones - built from cholesterol molecules and include: testosterone, estrogen and progesterone, corticosteroids, and calcitriol. B. Pathways of Hormone Action 1. Lipid-soluble = A steroid hormone directly initiates the production of proteins withi