Simple and complex feedback loops in the "endocrine system. A, A simple first-order feedback loop. B, A complex, multilevel feedback loop: the hy-pothalamic-pituitary-target gland axis. Solid lines indicate stimulatory effects,- dashed lines indicate inhibitory, negative-feedback effects.
hormone-effector pairs relative to normal feedback relationships. For example, in the case of anterior pituitary hormones, measuring both the trophic hormone and the target gland hormone concentration provides important information to help determine whether a defect in hormone production exists at the level of the pituitary or at the level of the target gland. Furthermore, most dynamic tests of endocrine function performed clinically are based on our knowledge of these feedback relationships. Dynamic tests involve a prescribed perturbation of the feedback relationship^),- the range of response in a normal individual is well established, while a response outside the normal range is indicative of abnormal function at some level and greatly enhances information gained from static measurements of hormone concentrations (see Clinical Focus Box 31.1).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.