The endocrine system consists of a widely distributed group of glands that secretes regulatory substances called hormones. Because these substances are released directly into the blood, the endocrine glands are known as the ductless glands. Despite the fact that hormones in the blood reach all parts of the body, only certain tissues respond. The tissue that is influenced by a specific hormone is called the target tissue. The cells that make up this tissue have specific receptors on their membranes to which the hormone attaches, enabling it to act on the cells.
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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.