How the Endocrine System Works is not another standard introduction to endocrinology, but an innovative and fun way to learn about the importance of the key glands in the human body and the hormones they control. It is explained in 9 easy-to-understand lectures, with additional material on the treatment and management of endocrine disorders. How the Endocrine System Works: âąIs designed for those in need of a concise introduction to this fascinating area of medicine âąHas been rigorously updated to reflect today's endocrinology teaching âąIncludes more focus on the treatment and management of endocrine disorders âąFeatures more on evidence-based medicine, obesity, epidemiology, and biostatistics âąIncludes summaries of key research which affects diagnostic criteria âąIncludes brand new case-based review questions at the end of each chapter âąFeatures full-color diagrams throughout How the Endocrine System Works is the perfect introduction for all medical students, as well as for students of bioscience, and other healthcare disciplines.
Endocrinology is the study of endocrine glands and their secretions. It can be a difficult topic to master because of all the mechanisms and feedback loops to understand. Yet, one way to understand the endocrine system is to break it down into smaller parts, and that is what I have attempted to accomplish in this book. It may help to visualize each endocrine system as part of a much larger group; envision a football team with all the players (quarterback, running backs, center, guards, receivers, punter, etc.). Each of these players must perform his job properly for the team to win. If even one player is out of sync, the play may be botched, even if the other players perform flawlessly. (In endocrinology, this happens frequently, as it does in other human disorders.) The quarterback is in charge of the team, calls the plays, and provides leadership. Hopefully your endocrine system has a good quarterback, but, as in football, some turn in lackluster performances, as do some supporting players.
Football teams often communicate plays with audible signals. Complex organisms also require detailed communication for proper function; they have developed hormones to send messages or commands from one part of the organism to another. Simple, one-celled organisms did not have a great need for detailed endocrine systems. But as organisms became more complex, large intercellular communication mechanisms became necessary for homeostasis.
The word âhormoneâ is derived from the Greek word meaning âarouse to activity.â To many lay people, the word âhormoneâ conjures up images of estrogen or thyroid replacement therapy. In fact, there are many types of hormones, with new ones discovered every day; some have greater significance than others.
The endocrine system sends signals to the body by secreting hormones (e.g., insulin, growth hormone, thyroxine) directly into the circulation. In contrast, the exocrine glands secrete their substances into a duct system (e.g., sweat glands, exocrine pancreas).
The endocrine system is composed of many different glands throughout the body. The endocrine glands may be divided into two categories. The first, or âclassicalâ glands' functions are primarily endocrine in nature. The second, or ânonclassicalâ glands' primary functions are something else, but they also secrete important substances.
The âclassicalâ endocrine glands include the anterior pituitary, thyroid, parathyroids, adrenal cortex and medulla, gonads (testes and ovaries), and the endocrine pancreas. The primary function of these glands is to manufacture specific hormones. Some nonclassical endocrine organs and their hormones include the heart (atrial natriuretic peptide), kidney (calcitriol, renin), lymphocytes (cytokines, interleukins), GI tract (gastrin, secretin, vasoactive intestinal peptide), and many others. Many of the âclassicalâ hormones are under the control of the hypothalamus and pituitary, which may be thought of as extensions of the nervous system. Indeed, the nervous and endocrine systems may function together quite closely (neuroendocrinology).
FUNCTION OF HORMONES
So why are hormones so important, anyway? The first thing that an organism must have in order to survive is energy. Food must be converted into energy, excess energy needs to be converted to storage, and stored energy must be mobilized when necessary to meet the organism's needs. In the chapter on glucose metabolism, we will learn the effects of insulin and glucagon on the body's metabolism and the many disorders where things go awry. Thyroid hormones are important in the regulation of the body's metabolism. Glycogen and lipids are necessary to provide long-term energy needs when food is not available.
The organism must maintain its internal environment. This is not as easy as it sounds. Many hormones play a role here. Hormones such as antidiuretic hormone, aldosterone, and atrial natriuretic peptide are important in water and sodium balance. Calcium is necessary for many bodily functions, and its metabolism is regulated by parathyroid hormone and vitamin D. Several hormones, including thyroid hormones, growth hormone, and sex steroids, control growth and development. These all need to work together as a team for the body to exist in an orderly environment.
And, of course, reproduction is essential for the continued survival of any organism. Specialized reproductive organs (gonads) produce sex steroids that are necessary for spermatogenesis and ovulation (as well as normal growth and development). Gonads are under the complex control of the hypothalamicâpituitary axis (HPA).
COMPOSITION OF HORMONES
Hormones are made from a variety of different molecules. The vast majority of hormones are of the protein or peptide variety. Proteins are chains of amino acids linked together. Some of these peptide hormones are only a few amino acids in length; most are much larger, with some being over 200 amino acids in length. Even the very small protein vasopressin (a nonapeptide) looks quite complex.
Arginine Vasopressin
Glycoproteins are sort of a hybrid hormone, consisting of a peptide hormone associated with a carbohydrate moiety. The four human glycoprotein hormones include LH (luteinizing hormone), FSH (follicle-stimulating hormone), TSH (thyroid-stimulating hormone or thyrotropin), and human chorionic gonadotropin (ÎČ-hCG). These hormones all share a common alpha subunit (α-SU); the ÎČ subunits differ from one to another.
Instead of being linked together to form proteins, one or two amino acids may be modified to form hormones. The amino acid tyrosine is modified to form the catecholamines (e.g., epinephrine and norepinephrine). While technically two amino acids joined together form a peptide, these amino acids are usually modified in some manner. The catecholamine hormones are very important in the nervous system. The thyroid or iodothyronine hormones (thyroxine, triiodothyronine) are made by joining two modified tyrosine molecules and adding several iodine atoms.
Tyrosine
Cholesterol, a molecule that we associate with atherosclerosis, is in fact essential to life. It is the precursor of steroid hormonesâsuch as cortisol, aldosterone, estradiol, and testosteroneâand sterol hormones, such as calcitriol.
Cholesterol
Another common hormone precursor is the lowly fatty acid (the major storage component of fat), which serves as a precursor of hormones called eicosanoids. The most important eicosanoids, the prostaglandins, are derived from arachidonic acid. Other eicosanoids include thromboxanes, leukotrienes, and prostacyclins. They are important in smooth muscle contraction, hemostasis, inflammatory and immunologic responses, circulation, and respiratory and gastrointestinal systems.
Simple ions such as calcium also have hormone-like effects, and calcium metabolism will be discussed in Lecture 6.
HOW HORMONES WORK
Hormones must have a way to tell the other cells what to do. The end effect of a hormone is usually at the nucleus, resulting in the production of a protein that has some effect on the cell. Some hormones go directly to the nucleus and have an effect there. These types of hormones tend to be ones that can easily traverse the cell membrane; for this to happen, the hormone usually must be ânonpolarâ (non-charged). These include the steroid and iodothyronine hormones.
Hormones Interacting Directly with Nuclear or Cytoplasmic Receptors
The second class of hormones have no direct effect but instead bind to cell surface receptors, which initiates production of one or more second messengers that carry out the action. One messenger may trigger another messenger, which may trigger yet another, and so on. This concept of âmultiple messengersâ is called an amplification cascade and is the reason that some of these hormones are effective at extremely low concentrations (e.g., 10â12 mol/L). An analogy to the game of football would be a running back carrying the ball behind his blockers. Without the blockers, he is likely to be tackled quite quickly, ending the play abruptly; with multiple blockers, his power is âamplifiedâ to the extent that many more yards can be gained than would have been possible alone. These hormones tend to be highly electrically charged, and include polypeptide, glycoprotein, and catecholamine hormones, and therefore cannot easily traverse the cell membrane....
Table des matiĂšres
Normes de citation pour How the Endocrine System Works
APA 6 Citation
Neal, M. (2016). How the Endocrine System Works (2nd ed.). Wiley. Retrieved from https://www.perlego.com/book/994503/how-the-endocrine-system-works-pdf (Original work published 2016)
Chicago Citation
Neal, Matthew. (2016) 2016. How the Endocrine System Works. 2nd ed. Wiley. https://www.perlego.com/book/994503/how-the-endocrine-system-works-pdf.
Harvard Citation
Neal, M. (2016) How the Endocrine System Works. 2nd edn. Wiley. Available at: https://www.perlego.com/book/994503/how-the-endocrine-system-works-pdf (Accessed: 14 October 2022).
MLA 7 Citation
Neal, Matthew. How the Endocrine System Works. 2nd ed. Wiley, 2016. Web. 14 Oct. 2022.