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Unformatted text preview: Chapter 30. INTEGRATI VE ENDOCRINE CONTROL SYSTEMS This chapter highlights major differences between endocrine and neural regulation by describing three major categories of endocrine control systems. Examples of each type will be provided, and in some cases endocrinopathies that expose disturbances to these control systems will be described. The full impact of this material may not be evident until much later in the text, when the various physiologic systems will have been examined in detail and integrated with each other. A. Endocrine versus neural controls The time-course of the two control systems, relating input to output, is usually strikingly different, with endocrine regulation providing for a more prolonged response to a single input event. In endocrine regulation, the selectivity of targets obviously depends on the tissue distribution of receptors rather than on the neuroanatomic determinants (patterns of innervation) that underlie neural selectivity. In most cases, hormones enter the blood and are distributed throughout the circulation, yet only those tissues with receptors can respond to the signal carried by the hormone. An important specialization occurs in portal vascular systems, where hormone action is indeed largely restricted. The well-known portal systems provide for venous drainage from a primary capillary bed to pass to a second tissue with a set of blood sinuses before blood is returned to the general venous pool. Examples include the vascular link between the hypothalamus and anterior pituitary (see Figure 29-1), and between the viscera and the liver, to be described in Chapter 47. Hormones that are secreted into these portal systems are present in exceedingly high concentrations when delivered to their immediate targets, but they are drastically diluted when blood reaches the general circulation. The major actions of the hypothalamic releasing factors are obtained in the anterior pituitary. However, because some non- hypothalamic tissues, e.g. the placenta, are able to synthesize the same peptides, the full importance of this vascular proximity is not known. The visceral-hepatic portal system is an important means of selectively delivering materials to the liver, as will be made clear in the parts of this book concerned with digestion and metabolism. This portal system collects venous blood that has come from the digestive tract and the accessory digestive organs and passes it totally to the liver for initial processing. The whole array of gastrointestinal hormones and, most importantly, the pancreatic hormones insulin and glucagon, are all secreted into this portal bed. The liver is the most important target tissue for insulin and glucagon, although both hormones can and do exert significant actions in peripheral tissues after they have gained access to the general circulation....
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- Fall '08