Endocrine System is one of the most important systems in our body. Without that system, our body will simply malfunction. Imagine what happens when your body’s blood pressure goes haywire. What will happen if the electrolyte balance of the body is lost? What will happen if homeostasis breaks? What will happen if the nerves in your body fail to perform? You will cease to exist!
Yes, this makes the endocrine system extremely important just like every other system. The problem is that the endocrine system is needed by every other system to function correctly. Thus, in this article, we are going to learn about endocrine system facts and find out what it means and what it does. Ready?
Endocrine System Facts: 1-7 | Two Systems – Endocrine & Exocrine Systems
1. There are two systems which exclusively deals with glands. They are the endocrine system and the exocrine system.
2. The exocrine glands’ secretions are mostly saliva, enzymes, sweat, etc. The secretions transport through ducts. Examples of exocrine glands are salivary glands, gastric glands, etc.
3. The endocrine glands secrete hormones. They don’t contain ducts (hence called ductless glands) and release hormones directly into the bloodstream.
4. There are two types of endocrine glands. They are known as exclusive endocrine glands and partial endocrine glands.
5. Exclusive endocrine glands – the glands which secrete only hormones are called ‘exclusive endocrine glands.’ They have no other function. Examples: Parathyroid gland, pineal gland, adrenal gland, pituitary gland, and thymus.
6. Partial endocrine glands – these organs secrete hormones, and they perform other functions as well. Examples: Gonads (ovary in females, testis in males), pancreas, duodenal epithelium, gastric epithelium, kidneys, and placenta in pregnant women.
Note: Hypothalamus performs neural functions and secretes hormones, and medical professionals treat it as a neuroendocrine gland.
7. There are eleven organs and glands which secrete hormones and are a part of the endocrine They are:
- Pituitary gland
- Thyroid gland
- Parathyroid glands
- Pineal gland
- Adrenal glands
- Hormones from the epithelium of the gastrointestinal tract, kidneys, and
Let us learn about each of the glands/organs in details.
Endocrine System Facts: 8-15 | Hypothalamus & Hormones It Releases
8. Hypothalamus is the basal part of the diencephalon. Diencephalon is a part of the forebrain. It extends downwards as the pituitary stalk.
9. It links nervous and endocrine systems. Though the pituitary gland is the master gland, the hypothalamus controls the pituitary gland.
11. These cells secrete two types of hormones. The first type is releasing hormones (stimulates the hormonal secretion of the pituitary gland) and the other category is inhibiting hormones (inhibits the hormonal secretion of the pituitary gland).
12. The anterior pituitary connects to the hypothalamus through the hypothalamic-hypophyseal portal blood vessel. This blood vessel carries the hormones which regulate the anterior pituitary. Anterior pituitary connects to the hypothalamus through epithelial tissue.
13. Hypothalamus directly controls the posterior pituitary via a nerve bundle called hypothalamic-hypophyseal. This axis directly sends nerve signals to the posterior pituitary. The posterior pituitary connects to the hypothalamus through neural tissue.
14. Functions of the hypothalamus are maintaining blood pressure, homeostasis1, body temperature, the fluid and electrolyte balance of the body (equilibrium of water and minerals like sodium, potassium in the body). It also influences emotional responses as it is a part of the limbic system.
15. The hormones released by the hypothalamus are:
|Name of the hormone||Functions|
|Growth Hormone-Releasing Hormone (GHRH)||Stimulates the secretion of Growth Hormone (GH)|
|Thyrotropin Releasing Hormone (TRH)||Stimulates the secretion of Thyroid Stimulating Hormone (TSH)|
|Gonadotropin Releasing Hormone (GnRH)||Stimulates the secretion of Follicle Stimulating Hormone (FSH)|
|Corticotrophin Releasing Hormone (CRH)||Stimulates the secretion of Adrenocorticotrophic Hormone (ACTH)|
|Prolactin Releasing Hormone (PRH)||Stimulates the secretion of Prolactin|
|Luteinizing Hormone Releasing Hormone (LHRH)||Stimulates the secretion of Luteinizing Hormone (LH)|
|MSH Releasing Hormone||Stimulates the secretion of Melanocyte Stimulating Hormone (MSH)|
|Growth Hormone Inhibiting Hormone (GHIH) or Somatostatin||Inhibits the secretion of Growth Hormone, Prolactin and Thyroid Stimulating Hormone (TSH)|
|Prolactin Inhibiting Hormone (PIH)||Inhibits the secretion of Prolactin|
|MSH Inhibiting Hormone||Inhibits the secretion of MSH|
Endocrine System Facts: 16-74 | Pituitary Gland
16. The pituitary gland is located in sella turcica (a cavity of sphenoid bone which is present at the base of the brain).
17. The word pituitary means to “grow ” Hypophysis is its another name. This gland is derived from two embryonic structures – brain and pharyngeal epithelium.
18. It weighs mere 0.5 grams and is just one centimeter in diameter. It has a length of 1 centimeter and is 0.5 centimeters thick. Pituitary gland increases in its size in pregnant women.
19. Infundibulum2 connects the pituitary to the hypothalamic region of the brain.
20. Pituitary Gland consists of two parts. The first one is the anterior part which is glandular. It is known as Adenohypophysis.
21. The other one is the posterior neural part which is called Neurohypophysis.
Endocrine System Facts: Adenohypophysis – A Part of Pituitary
22. Adenohypophysis is derived embryologically from the dorsal3 pouch of the pharyngeal epithelium4. The pouch is known as Rathke’s pouch. Neurohypophysis extends as the continuation of the hypothalamus.
23. Adenohypophysis is divided into 3 zones – pars intermedia, pars distalis, and pars tuberalis.
24. Pars tuberalis: This part is thickest near the anterior portion of the pituitary stalk and is usually absent near the posterior end. Groups of squamous cells5 surround pars tuberalis which may give rise to tumors.
25. Pars intermedia: In humans, the pars intermedia is rudimentary (totally absent in whales, birds, etc.). Their cells usually merge with the cells of pars distalis. Cells of pars intermedia are either basophils6 or chromophobes7. In humans, basophil cells of pars intermedia invade into pars nervosa.
26. Pars distalis: It forms 75% of the adenohypophysis. Only three types of cells are seen in this area – acidophils8 (40%), basophils (10%) and chromophobes (50%).
Endocrine System Facts: Hormones Secreted by Adenohypophysis
27. Adenohypophysis secretes six hormones which are:
- Growth Hormone
- Thyroid Stimulating Hormone
- Adreno Cortico Trophic Hormone
- Follicle Stimulating Hormone
- Luteinizing Hormone
Endocrine System Facts: Growth Hormone Secreted by Adenohypophysis
28. Growth Hormone is also known as a Somatotropic Hormone (STH). It is a peptide9 hormone.
29. Secretion of Growth Hormone depends on hypothalamus’ secretion of Growth Hormone-Releasing Hormone and Growth Hormone-Inhibiting Hormone.
30. It promotes the growth of all the metabolic processes and tissues of the body. Growth hormone influences the metabolism of carbohydrates, lipids, and proteins.
31. It stimulates the processes like chondrogenesis (formation of cartilage) and osteogenesis (formation of bones).
33. Growth Hormone decreases the use of glucose by cells and increases the release of fatty acids from adipose tissue10 (so that cells the which can use fatty acids for their functioning instead of glucose will use fatty acids or fats). By doing this, it reserves the glucose for tissues which are entirely dependent on glucose, such as the brain.
Endocrine System Facts: Thyroid Stimulating Hormone Secreted by Adenohypophysis
34. Thyroid Stimulating Hormone is also known as Thyrotropin. This hormone is made up of glycoproteins11. It is a tropic hormone12.
35. TSH stimulates the thyroid to secrete two hormones – thyroxine (T4) and Tri-iodothyronine (T3).
36. Secretion of TSH works on a negative feedback mechanism between the circulating level of thyroxine, and the secretion of TRH by the hypothalamus.
37. When thyroxine is less in blood, the hypothalamus releases TRH. TRH reaches the pituitary TRH stimulates pituitary glands to release TSH. The thyroid gland secretes thyroxine once TSH stimulates it.
38. In the same way, when thyroxine levels are high in the bloodstream, the hypothalamus doesn’t release TRH. In addition to this, there is evidence which shows that nerve secretions (neurotransmitters) in some areas of the hypothalamus also influence the secretion of TSH.
39. TSH controls the development and maintenance of the thyroid gland.
40. It also promotes the accumulation of iodine, conversion of this accumulated iodine into thyroid hormones and releasing the thyroid hormones into the blood.
41. Oversecretion of TSH increases the size of the thyroid gland which is generally prevented by Thyroxine because of a negative feedback mechanism.
Endocrine System Facts: Adreno Cortico Trophic Hormone (ACTH) Secreted by Adenohypophysis
42. Adreno Cortico Trophic Hormone is a polypeptide or protein hormone. It is a tropic hormone (stimulates another endocrine gland).
43. The adrenal cortex13 is stimulated by ACTH to release the hormones – mineralocorticoids and glucocorticoids.
44. ACTH stimulates the synthesis of melanin14 in melanocytes15. It also stimulates the adipose tissue to release fatty acids.
45. It also induces the secretion of insulin (a hormone).
46. Just like TSH, secretion of ACTH works on a negative feedback mechanism and nerve secretions (neurotransmitters) of some areas of the hypothalamus.
Endocrine System Facts: Follicle Stimulating Hormone (FSH) Secreted by Adenohypophysis
48. Follicle Stimulating Hormone is a glycoprotein hormone. It is a gonadotropin hormone (stimulates gonads).
49. It encourages the functions of gonads of both males and females.
50. In females, the target organ for FSH is It promotes the growth of Graafian follicles16 which increases the weight of the ovary. It also stimulates the secretion of estrogen.
51. Target organs are testes in the case of males. It increases the rate of spermatogenesis17 by stimulating the germinal epithelium18 of the seminiferous tubules19.
Endocrine System Facts: Luteinizing Hormone (LH) Secreted by Adenohypophysis
52. Luteinizing Hormone – It is also known as Interstitial Cells20 Stimulating Hormone (ICSH).
53. It is a glycoprotein hormone. In females, LH (along with FSH) promotes ripening of ovarian follicles21 and induces ovulation.
54. It also maintains corpus luteum22 and stimulates the synthesis and secretion of ovarian hormones.
55. In males, this hormone targets the Leydig cells or interstitial cells of testes. It also stimulates the production and secretion of testosterone.
Note: Production of both LH and FSH starts from pre-puberty age. They are not produced during childhood.
Endocrine System Facts: Prolactin Secreted by Adenohypophysis
56. Prolactin is also known as Luteotropin, Luteotrophic Hormone, Lactogenic Hormone, Mammotropin, etc.
57. It is a protein or peptide hormone.
58. Did you know Prolactin is popular by the name “Jack of all trades?” Wondering why such a name? It is so called because it performs near about 80 hormonal functions!
59. It stimulates the formation and secretion of milk in mammals. Along with estrogen, it stimulates the growth of mammary glands and readies them for milk secretion.
60. It induces corpus luteum to secrete progesterone. It suppresses the release of LH and ovulation. Along with these functions, it has a role to play in growth, integument23, and osmoregulation24.
Note: ACTH and MSH are formed when a large molecule called pro-opiomelanocortin gets cleaved. During this process endorphins and encephalins are also formed from the same molecule. Endorphins and Encephalins are usually considered as body’s “natural opiates.” These hormones help in relieving the pain.
Note: Pars Intermedia (part of Adenohypophysis) secretes MSH or Melanocyte Stimulating Hormone. It is a peptide hormone. Its role is limited compared to other hormones. It causes hyperpigmentation25 in women during pregnancy. Cushing’s disease and Addison’s disease may also cause hyperpigmentation. In other vertebrates, it is quite useful. For example, some of the amphibians and reptiles change their color and pigmentation to avoid predators.
Endocrine System Facts: Neurohypophysis – A Part of Pituitary
61. Neurohypophysis is divided further into pars nervosa and infundibulum.
62. Neurohypophysis secretes only two hormones which are:
Endocrine System Facts: Vasopressin Released by Neurohypophysis
63. Vasopressin is also known as AntiDiuretic Hormone (ADH). It is a peptide hormone.
64. It promotes the reabsorption of water and electrolytes by Distal Convoluted Tubules (DCT) of nephrons.
65. In simple words, it reduces the loss of water in the form of urine (loss of urine from the body is known as Diuresis).
66. Vasopressin is also called as AntiDiuretic Hormone as it minimizes the loss of water. When this hormone is released in large amounts, it constricts the blood vessels and increases blood pressure.
Endocrine System Facts: Oxytocin Released by Neurohypophysis
67. Oxytocin consists of a series of amino acid residues. It is a neuropeptide26.
68. The word oxytocin means rapid birth.
69. Oxytocin is known for stimulating the smooth muscles of the uterus and causing contractions, thereby helping in the delivery of the baby.
70. It also induces secretion of milk.
71. Myoepithelial cells get stimulated by the oxytocin. These cells surround the alveoli27 and ducts of mammary glands.
72. Contraction of cells leads to expulsion of milk from alveoli to the ducts of mammary glands. Milk comes out from the ducts.
73. Many observations show that oxytocin might have a role in reabsorption of sodium by the kidneys. Such observations might explain why vasopressin and oxytocin are produced in equal quantities in both the sexes (genders).
74. Oxytocin also affects the non-pregnant uterus to facilitate the transport of sperms. Just in case you didn’t know, oxytocin is called love hormone.
Did You Know? Vasopressin and Oxytocin are made up of nine amino acids. 7 of them are the same, and only two differ and yet their functions are entirely different.
Endocrine System Facts: 75-95 | Thyroid Gland
75. The thyroid gland is our body’s largest endocrine gland. It has a shape of a butterfly.
76. The word thyroid originates from the Greek word “Thyros” meaning shield. It weighs nearly 20 to 30 grams in an adult.
77. It consists of two lobes which are present on either side of the larynx or voice box.
78. These two lobes are joined together with the help of a glandular tissue28 called Isthmus of the thyroid.
79. The lobes are yet again divided into numerous lobules. Lobules consist of follicles29 which are known as acini (plural of acinus).
80. Glandular cuboidal30 or squamous epithelial31 cells lines each acinus.
81. Colloid (a gelatinous glycoprotein mixture) is present in the cavities or lumen of these acini. This colloid contains thyroglobulin – a large glycoprotein complex.
82. The hormones remain bound to the thyroglobulin molecules until they get secreted. When the hormones are secreted, the colloid is completely ingested by the thyroid cells.
83. Once the hormones are secreted, the peptide bonds get hydrolyzed, resulting in the discharge of free Tri-iodothyronine (T3) and Tetra-iodothyronine (T4) or Thyroxine into the capillaries.
84. Iodine is necessary for the regular production of Thyroxine. Thyroxine consists of nearly 65% of iodine. Thyroxine helps in maintaining the homeostasis of iodine.
85. Thyroxine controls BMR, e., Basal Metabolic Rate32 and heat production by the body. It promotes growth by stimulating synthesis of proteins.
86. Thyroxine maintains blood Pressure (BP). It also reduces the serum cholesterol, and normal levels of thyroxine are necessary for proper functioning of gonads and efficient operation of muscles.
87. It plays a significant role in developing the nervous system especially at the time of birth and in the first year.
89. As the metabolism increases, the rates of secretion of other hormones also increase. For example, when thyroxine increases the metabolism of glucose, then the body requires more amount of insulin to act on glucose.
90. Thyroxine increases the absorption of glucose from the small intestine.
91. Thyroxine is essential for the normal functioning of sex organs. Lack of thyroid hormones causes loss of libido in men and women. Additionally, lower levels of thyroxine (than what is normal) in women cause frequent and excessive menstrual bleeding.
92. Thyroid gland’s C cells or parafollicular cells secrete thyrocalcitonin (TCT). This hormone is also known as Calcitonin.
93. Thyrocalcitonin is a polypeptide hormone. It controls the blood calcium and phosphate levels. It suppresses the effect of the parathyroid gland and reduces the level of calcium in the blood.
94. TCT suppresses bone reabsorption, and in the kidneys, it decreases the reabsorption of sodium, phosphorus, potassium, magnesium, calcium, etc.
Note: The action of calcitonin is weak and is only for the short term. Parathyroid glands generally overpower calcitonin.
95. TCT hormone decreases the secretion of Hydrochloric acid in the stomach. It also decreases the secretion of glucagon (a hormone) and insulin as well.
Endocrine System Facts: 96-104 | Parathyroid glands
96. There are four parathyroid glands present in the human body. Each pair is present in each lobe of the thyroid They are attached on the posterior side of the thyroid gland.
97. Parathyroid glands have two types of cells – chief cells and oxyphil Chief cells secrete parathyroid hormone or parathormone (PTH). Functions of Oxyphil cells are not known.
98. Did you know oxyphil cells are not present in children and other animals? They are seen only in adult human beings. However, their number is always less than the chief cells.
99. Parathormone is a hypercalcemic hormone (increases the level of calcium in the blood and decreases the level of phosphate). It is a peptide or protein hormone.
100. Parathormone controls the phosphate and calcium homeostasis. Secretion of this hormone depends on the level of calcium in the blood.
101. Parathormone stimulates the osteoclasts (bone cells) to dissolve bone matrix. In this way, phosphate and calcium are released into the blood (hence the blood calcium level increases).
102. Parathormone amps up the reabsorption of calcium ions and excretion of phosphate ions by renal tubules of kidneys. It reduces the pH of urine.
103. The hormone promotes activation of vitamin D so that the intestinal mucosal cells33 absorb calcium. On the other hand, the vitamin D’s deficiency greatly lowers the efficiency of parathyroid glands.
104. PTH and TCT play a significant role in maintaining the level of calcium in the body.
Endocrine System Facts: 105-115 | Thymus
105. The thymus gland is partially lymphoid organ and partially endocrine gland.
106. It is present above the aorta and heart and behind the sternum.
107. The thymus is active and bigger in children. When a person reaches puberty, it starts shrinking and converting into mere fat tissue. For other animals, it may stay as is for a lifetime.
108. The thymus has two lobes. It is covered by a fibrous capsule which gets divided into an outer cortex and inner medulla.
109. Thymus secretes four hormones (thymulin, thymosin, thymopoietin, thymichumoral factor).
110. Pre-processing of T-lymphocytes34 is the primary function of the thymus.
111. T-Lymphocytes originate from the bone marrow. Then they migrate to the thymus gland, and there they start increasing in number and variety so that they can deal with various antigens35.
112. Thymus processes the T-lymphocytes in such a way that they don’t attack the antigens present in the body. It simply prevents autoimmune disorders (there may be other reasons for getting autoimmune disorders).
113. This whole process happens before birth and continues for a few months after birth.
114. T-Lymphocytes provide cell-mediated immunity (an immune response which doesn’t require any antibodies36 but involves phagocytes37, T-lymphocytes, etc., in response to antigens).
115. Thymosins also produce antibodies for humoral immunity (immunity related to the fluids of the body).
Endocrine System Facts: 116-120 | Pineal Gland
116. It is present on the dorsal side of the forebrain. It secretes melatonin.
117. Melatonin plays a significant role in regulating diurnal (24 hours) rhythm of our body.
118. Melatonin maintains the sleep-wake cycle, body temperature, etc.
119. It even influences the menstrual cycle, defense capability, metabolism, and pigment It mildly influences sexual drive and reproduction in humans.
120. In lower form animals, it has a greater role to play. It influences sexual and reproductive function, it is photosensitive, and it is called third eye or epiphysis.
Endocrine System Facts: 121-153 | Adrenal Gland
In this section, we will divide the facts into the structure of the Adrenal gland and the hormones released by each section of the gland. Let us continue…
Endocrine System Facts: Structure of Adrenal Gland
121. Adrenal glands are present on the anterior (top) end of the kidneys.
122. As they are present on the anterior end of the kidneys, they are also known as suprarenal glands.
123. They are just 6 grams each and have a length of 25 to 50 millimeters. They have a pyramidal structure.
124. Adrenal gland’s outer region is the cortex, and the inner one is the medulla (these two regions are anatomically, embryologically and functionally different).
125. Histologically38, the cortex contains three zones – zona glomerulosa, zona fasciculata, and zona reticularis.
126. Zona glomerulosa is the thin outer It makes up about 15% of the adrenal cortex. It secretes mineralocorticoids like aldosterone and deoxycorticosterone.
127. Zona fasciculata is the middle and the most extensive It constitutes near about 75% of the adrenal cortex. It secretes glucocorticoids like corticosterone, cortisol (also known as hydrocortisone), minute amounts of adrenal estrogen and androgen.
128. Zona reticularis is the most buried layer of the cortex. It constitutes only 10% of the whole cortex. It secretes adrenal androgen and minute amounts of estrogen and glucocorticoids.
129. The adrenal cortex is loaded with vitamin C and cholesterol, the two compounds which are necessary to synthesize steroid hormones.
130. The adrenal medulla is the central part of the adrenal glands. It consists of ovoid and columnar cells. These cells are present mostly around the blood capillaries.
131. The adrenal medulla secretes adrenaline (epinephrine) and noradrenaline (norepinephrine). These hormones are together known as catecholamines.
132. These cells (ovoid and columnar cells) secrete 80% adrenaline and 20% noradrenaline. The organs in the body get stimulated by both sympathetic nerves and hormones of the adrenal medulla.
Endocrine System Facts | Hormones Secreted by Cortex of Adrenal Glands
Different parts of the Adrenal Cortex release different types of hormones. The facts related to those hormones are mentioned below:
Hormones Released by Zona Glomerulosa of Adrenal Cortex
133. Mineralocorticoids (released by Zona glomerulosa of the cortex of the Adrenal gland) regulate electrolyte and water balance of our body.
134. Mineralocorticoids help in retaining sodium in salivary glands, sweat glands, renal tubules, etc. They promote the excretion of magnesium through urine.
135. The reabsorption of sodium and water is stimulated by Aldosterone, and it expels potassium and phosphorus from the body. Therefore, Aldosterone helps in maintaining osmotic pressure, blood pressure, and electrolytes.
Hormones Released by Zona Fasciculata of Adrenal Cortex
136. Glucocorticoids (released by Zona fasciculata of the cortex of the Adrenal gland) stimulate the life-saving activities like glucogenesis (production of glucose), lipolysis (breakdown of lipids)and proteolysis (breakdown of proteins).
137. Cortisol is a type of glucocorticoid. It maintains renal and cardiovascular functions.
138. Cortisol suppresses the immune functions by producing anti-inflammatory reactions.
139. It plays a role in decreasing the level of red and white blood cells. It stimulates the secretions of the digestive system.
140. Cortisol is generally known as the stress combat hormone.
Hormones Released by Zona Reticularis of Adrenal Cortex
141. Adrenal androgen (released by Zona reticularis of the cortex of Adrenal gland) is vital for the growth of hair in the axial region (underarm), pubic region and on the face during puberty.
Endocrine System Facts | Hormones Secreted by the Medulla of Adrenal Glands
142. The adrenal medulla secretes only two hormones (adrenaline and noradrenaline), and they are often called ‘3F’ hormones – fight, flight and fright hormones; ‘emergency’ hormones; or ‘fight or flight’ hormones.
Adrenaline Released by Adrenal Medulla
143. Adrenaline stimulates the breaking of glycogen (our body stores glucose in the form of glycogen) into glucose. It also encourages the release of fatty acids from adipose tissue.
144. During an emergency, it increases BP and heart rate. It increases the blood flow to striated muscles39, heart and nervous tissue as it is readying the body for either fight or flight.
145. It relaxes smooth muscles and stops digestion and peristalsis. It relaxes bronchi, dilates the pupil, increases sweating, closes sphincters. It increases the body temperature and the production of heat.
146. It causes goosebumps by causing muscle contractions related to hair follicles (this response is vestigial in humans). It increases respiration and keeps the body alert.
147. It also promotes quick coagulation of the blood and generates faster, sharper responses to the external stimuli.
148. In simple words, adrenaline is the stress hormone. It readies your body for either fight, flight or fright.
149. Did You Know? Adrenaline draws out blood from the skin and face, and that is the reason why your face and skin become pale when you are frightened.
150. Did You Know that adrenaline is one of the simplest hormones ever known to humanity? Not only this, it was the first hormone to be isolated in crystalline form in 1901 and was synthesized in 1904.
Noradrenaline Released by Adrenal Medulla
151. Noradrenaline functions both as a chemical transmitter at the sympathetic nerve endings and as a hormone.
152. Noradrenaline supports the adrenaline functions.
153. There are three differences between adrenaline and noradrenaline:
- Adrenaline dilates the blood vessels, and noradrenaline constricts the blood vessels.
- Adrenaline has a greater impact on cardiac stimulation than the noradrenaline.
- Adrenaline has more impact on tissue metabolism (5 to 10 times more) than noradrenaline.
Endocrine System Facts: 154-188 | Pancreas
In this section of Endocrine System facts, we will learn about the pancreas. The section is divided into several sub-sections like the structure of pancreas and different hormones release by different types of cells in the pancreas.
Endocrine System Facts | Structure of Pancreas
154. The pancreas is a composite gland, that is, both exocrine and endocrine gland.
155. It is present below the stomach and looks like a leaf and weighs nearly 85 grams.
156. Pancreas consists of two types of tissues – ‘acini’ and ‘islet of Langerhans’.
157. Acini releases digestive enzymes. Islet of Langerhans secretes insulin, somatostatin, and glucagon.
158. Do You Know from where the name islets of Langerhans came? Paul Langerhans, a German pathologist, physiologist discovered islets of Langerhans in 1869.
159. Islets of Langerhans – Pancreas consists of nearly 1 to 2 million islets of Langerhans. Each islet comprises – 60% cells as beta cells, 25% of cells as alpha cells, and 10% of cells as delta cells.
160. There are other cells (present in minute quantities) like C cell, PP cell. PP cell secretes pancreatic polypeptide hormone whose function is not known till date.
Endocrine System Facts | Hormones Released by Pancreas
161. Alpha cells secrete glucagon. Beta cells secrete insulin, and delta cells secrete somatostatin.
Glucagon Secreted by Alpha Cells
162. Glucagon is a polypeptide hormone. It is a hyperglycaemic hormone (increasing blood glucose level).
163. The Glucagon also acts on the liver and encourages glycogenolysis (conversion of glucose to glycogen) and gluconeogenesis.
164. Glucagon reduces the uptake of glucose by the cells of the body.
165. One of the important functions of glucagon is to provide immediate defense against hypoglycemia.
166. It is an important regulator of amino acid metabolism. Glucagon converts amino acids to glucose thereby increasing the availability of glucose to tissues.
167. Low blood sugar levels stimulate the alpha cells of the islets of Langerhans and high blood sugar levels in blood inhibit them.
Insulin Secreted by Beta cells
168. Insulin is a protein or a peptide hormone. It maintains glucose levels in our body.
169. Insulin hormone was named as insulin in the year 1922. Frederick G. Banting first extracted it in 1922.
170. Insulin is the first protein to be synthesized chemically in the year 1963.
171. Eli Lily and Company first synthesized human insulin in large scale. They started producing it in the 1980’s.
172. If you are wondering what diabetic patients used before “human insulin,” then the answer is here – they injected purified animal insulin. James Bertram Collip for the first time purified animal insulin.
173. It decreases the levels of glucose in the blood by stimulating the body cells to uptake glucose (especially fat cells and muscles). It first acts on the muscular tissue.
174. It favors glycogenesis (formation of glycogen), uptake of fatty acids, lipogenesis (formation of lipids) and decreases gluconeogenesis (glucose production from lactic acid and other non-carbohydrate molecules) in hepatic or liver cells.
175. When hepatic cells can take no more glucose, it is converted into fatty acids and transported to adipose tissue as triglycerides. Insulin’s functions in adipose tissue are similar.
176. Insulin stimulates the uptake of proteins and its incorporation into tissues. It promotes protein formation and stops the degradation of proteins.
177. It also suppresses the breakdown of glycogen to glucose and amino acids or fatty acids into glucose.
178. Therefore, insulin is a hypoglycemic hormone (decreasing blood glucose level).
179. Availability of insulin depends on its secretion and inactivation. A proteolytic enzyme (an enzyme which digests or breaks down protein), insulinase (present in the liver) plays a significant role in the inactivation of insulin.
180. High blood sugar levels stimulate the beta cells and the secretion of insulin while low blood sugar levels in blood inhibit the secretion of insulin.
181. Did You Know that Insulin also has a half-life period? Its half-life time in plasma is just 6 minutes. It vanishes from the circulation of blood within 10 to 15 minutes.
182. Did You Know that Humulin (insulin produced by recombinant DNA technology or genetic engineering) is administered intravenous but not as oral? The reason for this is simple. Digestive enzymes digest insulin.
Somatostatin Secreted by Delta cells
183. Somatostatin is also called a somatotropin-release inhibiting factor.
184. As it inhibits somatotropin (growth hormone), it was considered (proved in earlier experiments of Brazeau and his associates where they isolated somatostatin from sheep brain) that it is a hypothalamic hormone.
185. In later experiments and studies, however, it is considered that this hormone is secreted by the delta cells of the islets of Langerhans.
186. Somatostatin decreases the levels of both insulin and glucagon. It decreases the glucose level in blood of healthy individuals.
187. Recent studies indicate that somatostatin can alleviate diabetes symptoms by decreasing the glucagon’s concentration.
188. It also decreases the digestive activities of the alimentary canal. It may be because that somatostatin is giving more time for the digestive tract to absorb nutrients.
Endocrine System Facts: 189-222 | Gonads
189. Males and females have different gonads. Men have testis (plural testes), and women have an ovary.
Endocrine System Facts: Testes – Gonads of Males
190. A pair of the testis is present in the scrotal sac of men. It functions both as a sex organ and an endocrine gland. Each has a size of a walnut.
191. The wall of the testis consists of connective tissue. Inside a testis, there are 250 small compartments called lobules.
192. Each lobule is surrounded by connective tissue.
193. These lobules consist of one to three seminiferous tubules. It is in the seminiferous tubules that the sperms are produced.
194. Interstitial tissue (interstitial cells collectively are known as interstitial tissue) is present among the seminiferous tubules.
195. Interstitial cells are also known by another name – the Leydig cells.
196. Leydig cells produce several types of sex hormones which are collectively known as androgens. The primary hormone is the testosterone. It is a steroid.
197. FSH and LH influence the testosterone to initiate the maturation of reproductive organs of males, appearance of secondary sexual characters, the growth of facial and axillary hair, male sexual behavior, masculine voice, and muscular
198. FSH and LH also stimulate the development of accessory sex characters like the penis, seminal vesicle (here the semen is produced) and prostate gland. Sexual urge and sexual behavior in males are because of the testosterone.
199. Testosterone improves the bone matrix and encourages the process of spermatogenesis.
Endocrine System Facts: Ovaries – Gonads of Females
200. Women have a pair of ovaries in the pelvic region of the abdomen. Each ovary has a size and shape of a large almond.
201. Ovary consists of ovarian follicles and stromal tissues40. It produces ova (plural of ovum or egg).
202. Ovaries secrete estrogen and progesterone.
Estrogen Secreted by Ovaries
203. Estrogen is a collective name given to three hormones – estradiol, estrone, and estriol. Secretion of estrogen starts just before puberty and continues until the female is sexually active. FSH stimulates it.
204. Did You Know that estrogen is secreted not only by many invertebrates but also by nearly 30 families of higher plants? The function of it is not known.
205. It is responsible for the growth, development, and maintenance of the female reproductive tract, secondary sexual organs, and secondary sexual characteristics.
206. It helps in repairing the uterine lining after menstruation mostly preparing for implantation of a zygote.
207. It inhibits the secretion of lactogenic hormone and gonadotropic hormones secreted by the pituitary gland.
208. It also affects fat deposition, hair distribution, protein deposition, and skin balance and electrolyte balance. After menopause, ovaries stop secreting estrogen.
Progesterone Secreted by Ovaries
209. Progesterone is the second primary female sex hormone. It is secreted by corpus luteum – a temporary endocrine tissue in the later-half of the menstrual cycle.
210. Progesterone promotes the secretory changes that happen in the endometrium41 of the uterus (started by estrogen).
211. It helps estrogen in preparing the uterus for implantation.
212. Along with estrogen, progesterone is responsible for the changes of repeating menstrual cycles.
213. Progesterone also initiates and controls the cyclical changes of the menstrual cycle (along with estrogen).
214. It promotes the development of lobules and alveoli of the breasts and makes them secretory and also enlarges the breasts.
215. However, progesterone plays no role in the secretion of the milk.
216. It also influences the development of fallopian tubes42.
217. It affects kidney functioning and maintains electrolyte balance.
218. Progesterone is found in the adrenal cortex, placenta, and testes of few of the mammals.
219. It is believed that the progesterone is present in the testes and adrenal cortex because it plays an intermediate role in the production of other steroid hormones.
220. Uterine contractions decrease during the pregnancy by the influence of Progesterone.
Relaxin Secreted by Corpus Luteum and Placenta
221. Relaxin is a hormone which was first described in 1929.
222. It is produced by corpus luteum and placenta.
223. It is a polypeptide. It relaxes the pelvic ligaments during childbirth, and it is claimed that it softens the cervix43 at the time of delivery.
Endocrine System Facts: 224-245 | Placenta
224. The placenta is generally called an “incomplete endocrine gland” as it lacks some enzymes for the complete synthesis of hormones.
225. So how does placenta produce hormones? The answer is simple. The maternal liver, fetal liver and adrenal glands (of both mother and fetus) provide the much-needed precursors to the placenta to produce the hormones. It produces some hormones.
Endocrine System Facts | Progesterone Produced by the Placenta
226. The placenta also produces progesterone by converting either cholesterol or pregnenolone44 from the blood of mother to progesterone.
227. Progesterone is extremely important in maintaining pregnancy.
228. It prevents premature expulsion of the fetus from the uterus by reducing the contractility and excitability of the muscles of the uterus.
Endocrine System Facts | Estrogen Produced by the Placenta
229. The placenta produces estrogen with the help of fetal adrenal glands and liver.
230. Estrogen helps in the growth and development of breasts as well as muscles of the uterus during pregnancy.
231. It induces changes in maternal metabolism by altering the protein and carbohydrate synthesis and the functioning of the adrenal and thyroid glands.
Endocrine System Facts | Human Chorionic Gonadotropin Produced by the Placenta
232. One of the essential hormones is Human Chorionic Gonadotropin (HCG), and the placenta produces it.
233. It is a glycoprotein, and its structure and functioning are similar to LH. Aschheim and Zondek demonstrated this in the year 1927.
234. The presence of HCG characterizes urine of pregnant woman. Its levels reach a maximum between 80th and 90th days of pregnancy.
235. HCG maintains the corpus luteum’s activity of secreting progesterone and estrogen continuously during pregnancy.
236. These hormones delay menstruation and promote the growth of the endometrium.
237. Progesterone and estrogen also promote the storage of nutrients in the endometrium.
238. HCG also has an interstitial cell-stimulating effect on the testes. It results in the production of testosterone by fetal testes.
239. This secretion of testosterone is responsible for the growth of male sex organs.
240. At the end of pregnancy, the testosterone secreted is responsible for the descent of testes into the scrotum.
Endocrine System Facts | Human Chorionic Somatomammotropin Produced by the Placenta
241. Human Chorionic Somatomammotropin (HCS) was formerly known as Human Placental Lactogen (HPL). It is similar to the Growth Hormone. The placenta is responsible for producing HCS.
242. It also promotes the development of breasts. It helps HCG in maintaining corpus luteum.
243. It similarly changes carbohydrate and protein metabolism as growth hormone does.
244. It decreases insulin sensitivity and glucose utilization in the mother so that the fetus can have huge amounts of glucose at its disposal.
245. It stimulates the release of fatty acids from the adipose tissue in the mother so that the mother’s body would sustain on that.
Did you know that all the steroid hormones originate from cholesterol?
Endocrine System Facts: 246-267 | Other Hormones Secreted by Other Organs
246. There are other organs which secrete some hormones. They act as partial endocrine glands.
The Hormone Secreted by the Heart
247. In the heart, cardiocytes45 present on the atrial walls secretes Atrial Natriuretic Factor (ANF). The hormone is secreted when there is an increase in blood pressure.
248. ANF causes dilation of blood vessels, and in turn decreases the blood pressure.
Hormones Secreted by the Kidneys
249. Different parts of kidneys secrete renin, erythropoietin, and calcitriol.
250. Juxta Glomerular cells46 (JGA) of kidneys secrete renin. Renin, along with angiotensin and aldosterone forms a hormone system called Renin-angiotensin-aldosterone system or Renin-angiotensin system (RAAS or RAS).
251. RAAS or RAS hormone system increases the blood pressure and increases the reabsorption of sodium and water. It also promotes the expulsion of potassium.
252. JGA cells also secrete erythropoietin and stimulate the formation of RBC (erythropoiesis) in bone marrow.
253. Nephron’s proximal tubules secrete calcitriol which is an active form of Vitamin D3.
254. Vitamin D3 promotes calcium and phosphorus absorption from the intestines and increases bone formation.
Hormones Secreted by the Gastrointestinal Tract
255. There are some specialized cells in the gastrointestinal tract which secretes hormones like gastrin, cholecystokinin (CCK), secretin and gastric inhibitory peptides (GIP). All these hormones are polypeptides.
256. Gastrin acts on the gastric glands and promotes the secretion of HCl (hydrochloric acid) and pepsinogen47.
257. Duodenum secretes cholecystokinin when the food contains acid and fat.
258. Cholecystokinin stimulates the gallbladder to secrete bile into the duodenum. It also stimulates the secretion of pancreatic enzymes.
259. Cholecystokinin also stimulates gastric acid secretion and secretion of duodenal succus entericus48.
260. Cholecystokinin inhibits gastric motility and enhances duodenal motility.
261. The intestinal lining secretes Enterocrinin and Duocrinin. There is some evidence that the secretion of Secretin is under the influence of these two hormones.
262. Secretin is a hormone secreted by the cells of the duodenal wall into the bloodstream when the acidic chyme enters the small intestine.
263. It stimulates the acini cells of the pancreas to secrete bicarbonate ions and water to neutralize the acidity.
264. It also regulates water homeostasis. Secretin promotes the flow of bile from the gallbladder and other intestinal juices from the small intestine.
265. Enterogastrone is secreted when liquids enter the duodenum. It inhibits the secretion of gastric juice.
266. Pancreozymin is a peptide hormone which stimulates the secretion of pancreatic juice.
267. Gastric Inhibitory Peptide inhibits gastric secretion and its motility (the ability of the gastric secretions to move). It is secreted in the small intestine, and it is identical with enterogastrone.
Glossary of Terms
Homeostasis1: Homeostasis is the tendency to maintain an equilibrium maintained in the internal body by living organisms.
Infundibulum2: Infundibulum is a hollow stalk which connects the hypothalamus to the neurohypophysis or posterior pituitary gland.
Dorsal3: Dorsal refers to the back of a living organism.
Pharyngeal epithelium4: Epithelium of pharynx. The epithelium is one of the four primary tissues present in living organisms. The other three basic tissues are muscle tissue, connective tissue, and nervous tissue.
Squamous cells5: Cells which are derived from squamous epithelium. Squamous epithelium is a layer of flat shaped cells.
Basophils6: Basophils are a type of leukocytes or white blood cells.
Chromophobes7: Chromophobes are a type of pituitary cells that don’t stain easily.
Acidophils8: In the pituitary, acidophil is used to describe two types of cells – somatotrophs (which produce growth hormone) and mammotrophs (which produce prolactin).
Peptide9: Peptides are chains of amino acids.
Adipose tissue10: It is a loose connective tissue which is made up of adipocytes or fat cells. The loose connective tissue is a type of connective tissue which connects epithelial tissue to the organs.
Glycoproteins11: A group of proteins which have carbohydrate groups connected through the polypeptide chain.
Tropic hormone12: Tropic hormones are the hormones which act on other endocrine glands. Example: thyroid gland and adrenal glands.
Adrenal cortex13: Adrenal cortex is the outer part of the adrenal glands. The inner portion is known as the adrenal medulla.
Melanin14: Melanin is a pigment which gives living organisms their color.
Melanocytes15: Melanocyte is a cell which produces the natural pigment, melanin.
Graafian follicles16: Graafian follicle is a fluid-filled sac present in the ovary. Each Graafian follicle consists of an ovum.
Spermatogenesis17: A dynamic process which produces sperm cells.
Germinal epithelium18: Germinal epithelium is the deepest layer of a testicle.
Seminiferous tubules19: Seminiferous tubules are the exocrine part of the testes. The cells present in the seminiferous tubules produce sperms.
Interstitial Cells20: Cells present in the testes which are responsible for the production of testosterone.
Ovarian follicles21: Ovarian follicles are the fluid-filled sacs in the ovary which contains an immature egg.
Corpus luteum22: A structure which remains for a short time (and then degenerates) in the uterus after the ovum is discharged. It remains active when the woman is pregnant.
Integument23: Integument is a natural covering of a living organism. Example: skin, shell, husk, etc.
Osmoregulation24: Osmoregulation is the maintenance of equilibrium of the osmotic pressure in the liquids present in an organism. Osmoregulation occurs by regulating water and salt concentrations.
Hyperpigmentation25: Darkening of a part of the body because of increased production of melanin.
Neuropeptide26: Peptides which are used by neurons to communicate with each other.
Alveoli27: Sacs present in the mammary gland. Milk is produced and stored in alveoli.
Glandular tissue28: A group of epithelial cells which can produce secretions.
Follicles29: A small sac or cavity which is secretory.
Glandular cuboidal cells30: Cells derived from glandular cuboidal epithelium. The glandular cuboidal epithelium is a layer of cube-shaped cells which can produce secretions.
Squamous epithelium31: A layer of flat shaped cells.
Basal Metabolic Rate32: BMR is the amount of energy used by the endothermic animals per unit time at rest. Endothermic animals are warm-blooded animals. Endotherms can maintain constant body temperature irrespective of the environment.
Intestinal mucosal cells33: Cells present in the innermost layer of the alimentary canal which is known as mucosa. All the layers present in the alimentary canal are (from deepest to outermost) mucosa, submucosa, muscularis, and serosa.
T-lymphocytes34: A type of white blood cells which play an essential role in the immune system and are produced from the stem cells in the bone marrow. Stem cells are the undifferentiated cells which can develop into specialized cells.
Antigens35: Any foreign particle or body or toxin which induces an immune response by the body.
Antibodies36: It is a large protein which is used by the immune system to neutralize the pathogens, antigens, etc. It is also known as an immunoglobulin (Ig).
Phagocytes37: A type of cell which can engulf and absorb bacteria, other small cells, particles, etc.
Histology38: Study of tissues and cells under a microscope.
Striated muscles39: Muscles which are under voluntary control. They are also known as skeletal muscles or voluntary muscles.
Stromal tissue40: It is a type of connective tissue which is present in the stroma (part of an organ which has a connective or structural role to play). The stromal tissue of the ovary is highly vascularized and have more cells than the stromal tissue of other organs. They may even secrete hormones.
Endometrium41: It is a tissue which lines the lumen or cavity of the uterus.
Fallopian tubes42: A pair of tubes which connects ovaries to the uterus. The ovum travels through the fallopian tubes from the ovary to reach the uterus.
Cervix43: It is the narrow passage (similar to a neck) which forms the lower part of the uterus.
Pregnenolone44: Pregnenolone is an endogenous (originating from inside an organism, cell or tissue) steroid (it is an organic compound which is biologically active) and it is a precursor (a compound participating in a chemical reaction for creating another compound) or metabolic intermediate (a molecule that is a precursor to biologically important molecules) in the biosynthesis of most of the steroid hormones.
Cardiocytes45: Muscle cells of the heart. It is also known as myocyte of the heart.
Juxta Glomerular cells46: These cells are specialized smooth cells which produce, store and release renin.
Pepsinogen47: A proenzyme which gets activated into pepsin, an enzyme, by the action of HCl or Hydrochloric acid in the stomach.
Duodenal succus entericus48: Succus entericus is the alkaline secretion released by the glands present in the wall of the duodenum.
Offline Source Referred: Animal Physiology and Biochemistry by H.R. Singh and Neeraj Kumar | Pages 362 to 384.