The Components of the Endocrine System The components of the endocrine system are introduced in the above diagram. Some of these organs, such as the pituitary gland, have endocrine secretion as a primary function others, such as the pancreas. Others have other functions in addition to endocrine secretion. Other endocrine organs include the hypothalamus ,the adrenal medullae, the heart, the thymus, the pancreas and digestive tract , the kidneys, the reproductive organs), and placenta . Paracrine factors enter the bloodstream, but the concentrations are usually so low that distant cells and tissues are not affected. However, some paracrine factors, including several of the prostaglandin’s and related chemicals, have primary effects in their tissues of origin and 63HEM 604 BASIC ANATOMY AND PHYSIOLOGY OF HUMAN BODY secondary effects in other tissues and organs. When secondary effects occur, the paracrine factors are also acting as hormones. SELF-ASSESSMENT EXERCISE 1 i. What are endocrine cells? ii. List the four endocrine organs that you know. 3.3 Hormones Hormones are chemical messengers that are released in one tissue and transported in the bloodstream to reach specific cells in other tissues. Whereas most cells release paracrine factors, typical hormones are produced only by specialized cells. In intercellular communications, hormones are letters and the circulatory system is the postal service. A hormone released into the bloodstream will be distributed throughout the body. Each hormone has target cells, specific cells that respond to its presence. These cells posses the receptors needs to bind and “read” the hormonal message. Although every cell in the body is exposed to the mixture of hormones in circulation at any moment, each individual cell will respond to only a few of the hormones present. The other hormones are treated like junk mail and ignored, because the cell lacks the receptors to read the messages they contain. The use of hormones to coordinate cellular activities in tissues in distant portions of the body is called endocrine communications Because the target cells can be anywhere in the body, a single hormone can alter the metabolic activities of multiple tissues and organs simultaneously. These effects may be slow to appear, but they typically persist for days. Consequently, hormones are effective in coordinating cell, tissue, and organ activities on a sustained, long-term basis. For example, circulating hormones keep body water content and levels of electrolytes and organic nutrients within normal limits 24 hours a day throughout our entire lives. While the effects of a single hormone persist, a cell may receive additional instructions from other hormones. The result will be a further modification of cellular operations. Gradual changes in the quantities and identities of circulating hormones can produce complex changes in physical structure and physiological capabilities. Examples include the processes of embryological and fetal development, growth, and puberty. Hormones can be divided into three groups on the basis of chemical structure (1) amino acid derivatives, (2) peptide hormones, and (3) lipid derivatives. 64HEM 604 BASIC ANATOMY AND PHYSIOLOGY OF HUMAN BODY 3.4 Integration between the Endocrine System and the Nervous System The nervous system also relies primarily on chemical communication, but it does not use the bloodstream communications for message delivery like the endocrine system. Instead, neurons release a neurotransmitter at a synapse very close to the target cells that bear the appropriate receptors. The command to release the neurotransmitter rapidly travels from one location to another in the form of action potentials propagated along axons. The nervous system thus acts like a telephone company, carrying high-speed “messages” from one location in the body to another and delivering them to a specific destination. The effects of neural stimulation are generally short lived, and they tend to be restricted to specific target cells – primarily because the neurotransmitter is rapidly broken down or recycled. This form of synaptic communications is ideal for crisis management: if you are in danger of being hit by a speeding bus, the nervous system can coordinate and direct your leap to safety. Once the crisis is over and the neural circuit quiets down, things soon return to normal. When viewed from a general perspective the differences between the nervous and endocrine systems seem relatively clear. In fact, these broad organizational and functional distinctions are the basis for treating them as two separate systems. Yet, when we consider them in detail, the two systems are organized along parallel lines. For example: Both systems rely on the release of chemicals that bind to specific receptors on their target cells. The two systems share many chemical messengers for example; nor epinephrine and epinephrine are called hormones when released into the bloodstream but neurotransmitters when released across synapses. Both systems are regulated primarily by negative feed back control mechanisms. The two systems share a common goal: to preserve homeostasis by coordinating and regulating he activities of other cells, tissues, organs, and systems. SELF-ASSESSMENT EXERCISE 2 i. STATE TRUE OR FALSE: Saliva is a hormone. Please explain your answer. 65HEM 604 BASIC ANATOMY AND PHYSIOLOGY OF HUMAN BODY ii. Hormones can be classified into 3 groups -------------------------, ---------------- and ---------------------------. 4.0 CONCLUSION You have been able to learn that the endocrine system includes all the endocrine cells, and tissues of the body. They have glandular secretory cells that release their secretions into the extra cellular fluid. The main function of the endocrine system is to preserve homeostasis by coordinating and regulating the activities of other cells, tissues, organs, and systems. 5.0 SUMMARY In this unit we have learnt that: • Endocrine cells are different from exocrine cells; the later secrete their products onto epithelia’s surfaces generally by way of duct. • There are several similarities as well as distinction between the endocrine system and the nervous system. 6.0 TUTOR-MARKED ASSIGNMENT 1. Define endocrine communication. 2. List the similarities between the endocrine system and the nervous system 3. What is the difference between endocrine cells and exocrine cells? ANSWERS TO SELF-ASSESSMENT EXERCISE SELF-ASSESSMENT EXERCISE 1 i. Endocrine cells are glandular secretory cells that release their secretions into the extracellular fluid. ii. Four endocrine organs include hypothalamus, the adrenal medullae, the heart, the thymus, the pancreas and digestive tract, the kidneys, the reproductive organs, and placenta. SELF-ASSESSMENT EXERCISE 2 i. FALSE: Saliva is not an hormone because hormones are chemical messengers that are released in one tissue and transported in the bloodstream to reach specific cells in other tissues. ii. Hormones can be classified into (1) amino acid derivatives, (2)
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