The endocrine system is made up of glands that produce and secretes hormones

The endocrine system is made up of glands that produce and secretes hormones, chemical substances produces in the body that regulate the activity of cells or organs. These hormones regulate the body’s growth, metabolism either physical or chemical processes of the body also sexual development and function. The hormones are then being released into the bloodstream to the affected organs.
Hormones are such chemical messengers to transfer information from one cells to another and coordinate the functions of parts of the body. The major glands are the hypothalamus, pituitary, thyroid, parathyroid, adrenals, pancreas, pineal, also reproductive organs. This system are controlled by both positive and negative feedback mechanism.

Figure 1: Organs and tissues of the endocrine system. Adapted from https://slideplayer.com/slide/7550787/Pancreas is divided into two portion which is exocrine and endocrine pancreas. The exocrine pancreas (99% of volume) consists of clusters of gland cells (pancreatic acini) also with attached gland and duct cells that’s secrete large quantities of alkaline and enzyme to lumen of digestive tract. While endocrine pancreas is a small group of scattered cells also known as pancreatic islet. Pancreatic islet is divided into 4 cells which is Alpha cell which secrete glucagon, Beta cells secretes insulin, Delta cells secretes somatostatin and F cells produce pancreatic polypeptide hormone. (Martini et al, 2006).

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Figure 2: The chemical structure of insulin. Adapted from https://www.lookchem.com/cas-772/77282-71-4.html
Insulin is a polypeptide consisting of about 50 amino acids (Figure 2). The main function is to lower raised blood nutrients level especially glucose but also amino acids and fatty acids. When blood glucose is increase, Beta cells will secrete insulin and increase rate of glucose transport into target cells, increase rate of glucose utilization and ATP generation, convert excess glucose to glycogen in liver and skeletal muscles, enhance absorption of amino acid and protein also increase the triglycerides synthesis in adipose tissue. Thus, this mechanism will make the blood glucose concentration declines and back to normal.
Glucagon is produce by Alpha cells and involved in increasing blood glucose level by converting of glycogen to glucose (glycogenolysis) in liver, increased breakdown of fats to fatty acids in adipose tissue and increased synthesis and release of glucose in liver (Martini et al, 2006).

Figure 3: Chemical structure of glucagon. Adapted from https://pubchem.ncbi.nlm.nih.gov/compound/194323#section=2D-Structure

Figure 4: The function of insulin and glucagon in maintaining blood glucose. Adapted from https://courses.lumenlearning.com/wm-biology2/chapter/hormonal-regulation-of-metabolism/
Somatostatin (GHRIH) produce by hypothalamus to inhibit the secretion of both insulin and glucagon to inhibit the secretion of Growth Hormone (GH).

Figure 5: Chemical structure of somatostasin. Adapted from https://pubchem.ncbi.nlm.nih.gov/compound/somatostatin
The renal release the calcitriol (steroid hormone), erythropoietin (peptide hormone) and renin (enzyme). Calcitriol is secreted by kidneys by response to the presence of parathyroid hormone (PTH). Function of calcitriol is the stimulation of calcium and phosphate ion absorption along the digestive tract. Effects of PTH and calcium absorption result in stimulation of calcitriol thus effects also in stimulation of calcium release from bone and suppressing PTH production (Martini et al, 2006).

Figure 6: The chemical structure of calcitriol. Adapted from https://pubchem.ncbi.nlm.nih.gov/compound/calcitriol#section=2D-StructureErythropoietin (EPO) hormone released by kidneys in response to low oxygen levels in kidneys tissues. EPO stimulates the production of red blood cells (RBC) by bone marrow and it is a negative feedback mechanism. Which is oxygenated tissue controls EPO production to produce more RBC thus increase the blood volume (Martini et al, 2006).

Figure 7: The structure of erythropoietin. Adapted from http://www.assignmentpoint.com/science/biology/mechanism-and-function-of-erythropoietin-epo.html
Renin exclusively originates from the juxtaglomerular cells in the afferent arterioles of the kidneys. In blood stream, Renin will converts angiotensinogen, produce by liver, to angiotensin 1. An enzyme known as angiotensin-converting enzyme (ACE) found in the lungs will metabolizes angiotensin 1 to angiotensin 2. Angiotensin 2 cause blood vessels to constrict, stimulate thirst and blood pressure will increase (Figure 8).

Figure 8: The release of erythropoietin and renin, and an overview of the renin-angiotensin system. Adapted from http://www.austincc.edu/apreview/PhysText/Renal.htmlAdrenal glands composed of two parts which is cortex (outer) and medulla (inner). The adrenal cortex produce three groups of steroid hormones from cholesterol which is glucocorticoids, mineralocorticoids and androgen (sex hormone). While adrenal medulla stimulated by its extensive sympathetic nerve supply to produce adrenaline and norepinephrine hormone.
Secretions of glucocorticoids is controlled through a negative feedback mechanism involving the hypothalamus and anterior pituitary. Glucocorticoids stimulated by ACTH from anterior pituitary and by stress (Figure 10). Glucocorticoids takes place in gluconeogenesis, hyperglycemia, lipolysis, breakdown of protein, also anti-inflammatory action and wound healing (Rose et al, 2006).

Figure 9: The chemical structure of glucocorticoid. Adapted from https://en.wikipedia.org/wiki/Glucocorticoid
Figure 10: Negative feedback regulation of glucocorticoid secretion. Adapted from Rose et al, 2006).

Aldosterone is the main mineralocorticoid. Production of aldosterone associates with blood potassium level. When the blood potassium level is high, more aldosterone is secreted (Bijlani et al, 1997). The function are associates with the maintenance of water and electrolyte balance in body. Means that negative feedback mechanism will stimulates the reabsorption of sodium (Na2+) by renal tubules (for water retention) and excretion of potassium (K+) in urine. Aldosterone also involved in regulation of blood volume and blood pressure too (Renin- angiotensin-aldosterone system) (Figure 12).

Figure 11: The chemical structure of mineralocorticoids (aldosterone). Adapted from https://pubchem.ncbi.nlm.nih.gov/compound/aldosterone

Figure 12: Negative feedback regulation of aldosterone secretion. Adapted from Rose et al, 2006).

Androgens of adrenal origin can be converted into estrogen by adipose tissue. In females, androgens is important in anabolic function contributes to bone density and muscle mass (Bijlani et al 1997).

Figure 13: The chemical structure of androgen. Adapted from https://en.wikipedia.org/wiki/Androgen
Adrenaline and some noradrenaline are released into blood from medulla during stimulation of sympathetic nervous system (Figure 15). Noradrenaline (postganglionic neurotransmitter of the sympathetic division of the autonomic nervous system. They potentiate the fight or flight respond by increasing heart rate, blood pressure, dilating pupils, also increase metabolic rate.

Figure 14: The chemical structure of norepinephrine and epinephrine. Adapted from http://reset.me/story/norepinephrine-a-key-player-in-stress-depression-and-adhd/

Figure 15: The sympathetic outflow, the main structures supplies and the effects of stimulation. Adapted from https://www.thinglink.com/scene/608646811262910466
List of references
Anne Waugh and Allison Grant, Ross and Wilson, Anatomy and Physiology, Tenth Edition, 2006. Pp 211-234. Retrieved November 28.

Annu Rev Physiol, 2011. US National Library of Medicine National Institunes of Health. Renin Release: Sites, Mechanisms, and Controls. Retrieved November 28 from https://www.ncbi.nlm.nih.gov/pubmed/20936939Austin Community College, 2008. Functions of Urinary System. Renin-Angiotensin Mechanism. Retrieved November 28 from http://www.austincc.edu/apreview/PhysText/Renal.html#topDavid McCulloch, 2014. How insulin works. Retrieved November 28 from https://wa.kaiserpermanente.org/healthAndWellness/index.jhtml?item=%2Fcommon%2FhealthAndWellness%2Fconditions%2Fdiabetes%2FinsulinProcess.htmlDeane Alban, March 9, 2016. Norepinephrine: A Key Player in Stress, Depression, and ADHD. Retrieved November 28 from http://reset.me/story/norepinephrine-a-key-player-in-stress-depression-and-adhd/Frederic H. Martini, Wiiliam C. Ober, Claire W. Garrison, Ralph T. Hutchings, 2006. Fundamentals of Anatomy and Physiology. Seventh Editon. Pp 621-623. Retrieved November 28
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Sriram. “Steroids”. Medicinal Chemistry. Pearson Education India, pp 437. Retrieved November 29 from https://en.wikipedia.org/wiki/Androgen Stephen Kemp and Melissa Conrad Stoppler, 2018. WebMD. Anatomy of the Endocrine System. Retrieved November 28 from https://www.emedicinehealth.com/anatomy_of_the_endocrine_system/article_em.htm#what_is_the_endocrine_system