Chemical Coordination and Integration

19.1 Endocrine Glands and Hormones

Coordination and integration of physiological functions in the body are essential for maintaining homeostasis and proper functioning of organs. The neural system provides rapid point-to-point coordination among organs, but this neural coordination is fast yet short-lived and cannot innervate all cells of the body. Therefore, continuous regulation of cellular functions requires a special kind of coordination provided by hormones. Hormones are chemical messengers secreted by endocrine glands that regulate various physiological activities. Endocrine glands are ductless glands whose secretions, called hormones, are released directly into the bloodstream and transported to distant target organs. The classical definition of hormones as chemicals produced by endocrine glands and transported via blood to target organs has been expanded. Currently, hormones are defined as non-nutrient chemicals produced in trace amounts that act as intercellular messengers. This broader definition includes molecules secreted by organized endocrine glands as well as other tissues. Invertebrates have simple endocrine systems with few hormones, whereas vertebrates have a large number of hormones providing complex coordination. The endocrine system works in conjunction with the neural system to regulate and coordinate physiological functions in the body. **Table on page 13 (6×2)** | Hormones | Target gland | | --- | --- | | (a) Hypothalamic hormones | | | (b) Thyrotrophin (TSH) | | | (c) Corticotrophin (ACTH) | | | (d) Gonadotrophins (LH, FSH) | | | (e) Melanotrophin (MSH) | | **Table on page 13 (5×2)** | Column I | Column II | | --- | --- | | (a) T_{4} | (i) Hypothalamus | | (b) PTH | (ii) Thyroid | | (c) GnRH | (iii) Pituitary | | (d) LH | (iv) Parathyroid |

• Endocrine glands are ductless and secrete hormones directly into the blood.
• Hormones act as chemical messengers regulating physiological functions.
• Neural coordination is fast but short-lived; hormonal coordination is slower but long-lasting.
• Hormones are produced in trace amounts and act on specific target organs.
• Invertebrates have simpler endocrine systems compared to vertebrates.
• Endocrine and neural systems jointly maintain homeostasis.
• 📌 Endocrine glands: Ductless glands that secrete hormones into the bloodstream.
• 📌 Hormones: Chemical messengers produced in trace amounts that regulate physiological activities.
• 📌 Homeostasis: Maintenance of stable internal environment in the body.

19.2 Human Endocrine System

The human endocrine system comprises organized endocrine glands and hormone-producing cells dispersed in various tissues throughout the body. The major endocrine glands include the pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus, and gonads (testis in males and ovary in females). Besides these, some other organs such as the gastrointestinal tract, liver, kidney, and heart also produce hormones contributing to physiological regulation. The endocrine glands secrete hormones that regulate metabolism, growth, development, reproduction, and other vital functions. The hypothalamus, located at the base of the forebrain, acts as a neuroendocrine organ regulating pituitary function and integrating neural and endocrine activities. The pituitary gland, often called the 'master gland', controls several peripheral endocrine glands. This section provides an overview of the structure and functions of the major endocrine glands and their hormones, setting the foundation for detailed study of each gland in subsequent subsections.

• Human endocrine system includes major glands like pituitary, thyroid, adrenal, pancreas, parathyroid, thymus, and gonads.
• Some non-classical endocrine organs like heart, kidney, and gastrointestinal tract also secrete hormones.
• Endocrine glands regulate diverse physiological processes including metabolism, growth, and reproduction.
• Hypothalamus acts as a neuroendocrine organ controlling pituitary secretions.
• Pituitary gland regulates other endocrine glands and is called the master gland.
• Hormones secreted by these glands act on specific target tissues to maintain body functions.
• 📌 Endocrine system: Network of glands producing hormones to regulate body functions.
• 📌 Pituitary gland: Master endocrine gland controlling other glands.
• 📌 Hypothalamus: Brain region regulating pituitary and integrating neural-endocrine functions.