Why are the cells, tissues, and organs collectively called endocrine glands? 2. 3. What substances do endocrine glands secrete?_hormones 4. The substances secreted by endocrine glands act on cells. 5. Define: 6. Autocrine: 7. Paracrine: 8. Both the and systems communicate using chemical signals that bind to receptor molecules. 9. Most endocrine secretions are regulated by negative feedback loops. Summarize how negative feedback works. 10. In your text it describes three control sources for endocrine secretions (three ways to control how hormones are secreted. List them below (simple list, please do not write everything in those sections, just read and simplify). a I b. C Focus