. What do you mean by Endocrine Glands and Hormones? Explain.

Endocrine glands lack ducts and are hence, called ductless glands. Their secretions are called hormones. The endocrine system includes the cells, tissues, and organs that secrete hormones into body fluids (the internal environment). In contrast, exocrine structures secrete into tubes or ducts, which leads to internal or external body surfaces. The thyroid and parathyroid glands, for example, secrete hormones into the blood and are therefore endocrine (ductless) glands, but sweat glands and sebaceous (oil) glands in the skin are exocrine. Two other patterns of secretion are paracrine, which affects neighboring cells, and autocrine, which affects the secreting cell itself. An endocrine gland has the following characteristics: 1. it secretes chemicals called hormones 2. it has no duct (a ductless gland), instead, the hormone is secreted directly into the bloodstream 3. it has a rich supply of blood with a relatively large number of blood vessels. Some glands have both endocrine and exocrine functions, such as the pancreas, which secretes the hormones insulin and glucagon, and also secretes pancreatic juice containing digestive enzymes into the pancreatic duct that leads to the gut. A hormone is a chemical messenger. It has the following properties: i. it travels in the blood ii. it has its effect at a site different from the site where it is made, called the target, hence the term messenger iii. it fits precisely into receptor molecules in the target like a key in a lock – it is therefore specific for a particular target iv. it is a small soluble organic molecule v. it is effective in low concentrations. Although the effects of hormones are many and varied, their actions can be categorized into seven broad areas. Hormones 1. regulate the chemical composition and volume of the internal environment (extracellular fluid). 2. help regulate metabolism and energy balance. 3. help regulate contraction of smooth and cardiac muscle fibers and secretion by glands. 4. help maintain body activities despite disruptions such as infection, trauma, emotional stress, dehydration, starvation, hemorrhage, and temperature extremes. 5. regulate certain activities of the immune system. 6. play a role in the smooth, sequential integration of growth and development. 7. contribute to the basic processes of reproduction, including gamete (egg and sperm) production, fertilization, nourishment of the embryo and fetus, delivery, and nourishment of the newborn. Hormones secreted by endocrine glands belong to four different chemical categories: 1. Polypeptides are composed fo chains of amino acids that are shorter than about 100 amino acids. Some important examples include insulin and antidiuretic hormone (ADH). 2. Glycoproteins are composed of a polypeptide significantly longer than 100 amino acids to which is attached a carbohydrate. Examples include follicle-stimulating hormone (FSH) and luteinizing hormone (LH). 3. Amines, derived from the amino acids tyrosine and tryptopha, include hormones secreted by the adrenal medulla, thyroid,a nd pineal glands. 4. Steroids are lipids derived from cholesterol, and include the hormones testosterone, estradiol, progesterone, aldosterone, and cortisol. Steroid hormones can be subdivided into sex steroids, secreted by the testes, ovaries, placenta, and adrenal cortex, and corticosteroids, secreted only by the adrenal cortex (the outer portion of the adrenal gland). The corticosteroids include cortisol, which regulates glucose balance, and aldosterone, which regulates salt balance. The amino hormones secreted by the adrenal medulla (the inner portion of the adrenal gland), known as catecholamines, include epinephrine (adrenaline) and norepinephrine (noradrenaline). These are derived from the amino acid tyrosine. Another hormone derived from tyrosine is thyroxine, secreted by the thyroid gland. The pineal gland secretes a different amine hormone, melatonin, derived from tryptophan. All hormones may be categorized as lipophilic, which are fat-soluble, or lipophobic (polar), which are water-soluble. The lipophilic hormones include the steroid hormones and thyroxine; all other hormones are water-soluble. This distinction is important in understanding how these hormones regulate their target cells.