KEY A, artery BV, blood vessel CT, connective tissue Ep, transitional epithelium Lym, lymphatic vessel M, muscularis SM(L), longitudinally cut smooth muscle U, ureter V, vein arrows, binucleate cells
OVERVIEW OF THE ENDOCRINE SYSTEM / 740 Hormones and Their Receptors / 740 Regulation of Hormone Secretion and Feedback Mechanism / 742 PITUITARY GLAND (HYPOPHYSIS) / 742 Gross Structure and Development / 743 Blood Supply / 743 Nerve Supply / 745 Structure and Function of the Pituitary Gland / 745 Anterior Lobe of the Pituitary Gland (Adenohypophysis) / 745 Posterior Lobe of the Pituitary Gland (Neurohypophysis) / 748 HYPOTHALAMUS / 751 PINEAL GLAND / 752 THYROID GLAND / 755 PARATHYROID GLANDS / 760 ADRENAL GLANDS / 762 Blood Supply / 762 Cells of the Adrenal Medulla / 764 Zonation of the Adrenal Cortex / 766 Zona Glomerulosa / 766 Zona Fasciculata / 767 Zona Reticularis / 768 Fetal Adrenal Gland / 768 Folder 21.1 Functional Considerations: Regulation of Pituitary Gland Secretion / 743 Folder 21.2 Clinical Correlation: Principles of Endocrine Diseases / 750 Folder 21.3 Clinical Correlation: Pathologies Associated with ADH Secretion / 753 Folder 21.4 Clinical Correlation: Abnormal Thyroid Function / 758 Folder 21.5 Clinical Correlation: Chromaffin Cells and Pheochromocytoma / 766 Folder 21.6 Functional Considerations: Biosynthesis of Adrenal Hormones / 769
The endocrine system produces various secretions called hormones [Gr. hormaein, to set in motion] that serve as effectors to regulate the activities of various cells, tissues, and organs in the body. Its functions are essential in maintaining homeostasis and coordinating body growth and development and are similar to that of the nervous system: Both communicate information to peripheral cells and organs. Communication in the nervous system is through transmission of neural impulses along nerve cell processes and the discharge of neurotransmitter. Communication in the endocrine system is through hormones, which are carried to their destination via connective tissue spaces and the vascular system. These two systems are functionally interrelated. The endocrine system produces a slower and more prolonged response than the nervous system. Both systems may act simultaneously on the same target cells and tissues, and some nerve cells secrete hormones.
In general, a hormone is described as a biologic substance acting on specific target cells.
In the classic definition, a hormone is a secretory product of endocrine cells and organs that passes into the circulatory system (bloodstream) for transport to target cells. For years this endocrine control of target tissues became a central part of endocrinology. Recent research shows that a variety of hormones and hormonally active substances are not always
FIGURE 21.1 • Hormonal control mechanisms. This schematic diagram shows three basic types of control mechanisms. a. In endocrine control, the hormone is discharged from a cell into the bloodstream and is transported to the effector cells. b. In paracrine control, the hormone is secreted from one cell and acts on adjacent cells that express specific receptors. c. In autocrine control, the hormone responds to the receptors located on the cell that produces it.