Anatomy & Physiology

Anatomy & Physiology Study Guide

Chapter 17: The Endocrine System For the preservation of homeostasis, cellular activities must be coordinated throughout the body. Neurons monitor or control specific cells or groups of cells. However, the number of cells innervated is only a small fraction of the total number of cells in the body, and the commands issued are very specific and of relatively brief duration. The nervous system cannot regulate long-term processes such as growth, development, or reproduction, which involve or affect metabolic activities in virtually every cell and tissue. This type of regulation is provided by the endocrine system, which uses chemical messengers to relay information and instructions between cells. 17.1 Hormones The use of chemical messengers to transfer information from cell to cell within a single tissue is called paracrine communication . The chemicals involved are called paracrine factors, also known as local hormones. Paracrine factors enter the bloodstream, but their concentrations are usually so low that distant cells and tissues are not affected. However, some paracrine factors, including several of the prostaglandins and related chemicals, have primary effects in their tissues of origin and secondary effects in other tissues and organs. When secondary effects occur, the paracrine factors are also acting as hormones —chemical messengers that are released in one tissue and transported in the bloodstream to alter the activities of specific cells in other tissues. Paracrine factors can diffuse out of their tissue of origin and have widespread effects, and hormones can affect their tissues of origin as well as distant cells. A hormone released into the bloodstream is dispersed throughout the body. Each hormone has target cells, specific cells that possess the receptors needed to bind and “read” the hormonal message when it arrives. However, all cells throughout the body are exposed to them whether or not they have the necessary receptors. At any moment, each cell can respond to only a few of the hormones present. The other hormones are ignored because the cell lacks the receptors to read the messages they contain. The activity of hormones coordinating cellular activities in tissues in distant portions of the body is called endocrine communication . Hormones alter the operations of target cells by changing the types, quantities, or activities of important enzymes and structural proteins. In other words, a hormone may stimulate the synthesis of an enzyme or a structural protein not already present in the cytoplasm by activating appropriate genes in the cell nucleus. They can also raise or lower the rate of synthesis of a particular enzyme or other protein by changing the rate of transcription or translation or turn an existing enzyme or membrane channel “on” or “off” by changing its shape or structure. Through one or more of these mechanisms, a hormone can modify the physical structure or biochemical properties of its target cells. Cells can simultaneously respond to several different hormones. Classes of Hormones Hormones can be divided into three groups by their chemical structure: amino acid derivatives, peptide hormones, and lipid derivatives.

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