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 The endocrine system consists of discrete individual glands and tissues that produce and secrete chemical messengers called hormones. Hormones enter the bloodstream for dispersal throughout the body. They exert effects only on specific cells, called target cells, that possess receptors for them. For simplicity, the endocrine glands and tissues that are common to both males and females, are shown only on the female figure. Your goals for learning are: To identify the well-established endocrine glands and tissues of the body. To review some well-understood hormones that are produced by each gland or tissue. To identify some of the major target tissues for each hormone. To understand a simplified function for each hormone that is reviewed. Here is what you need to know: The difference between endocrine and exocrine glands; and the composition of a mixed gland. The difference between endocrine glands and endocrine tissues. The basic categories of organic molecules. The components and function of a negative feedback system. Scientific study in the area of endocrinology continues to reveal new hormones, new functions for known hormones, and insight into the complex interactions between hormones. In this topic, we will review some of the well established endocrine glands and tissues, some of the hormones they produce, their target tissues, and a simple function for each. Let's start with the pituitary gland. The pituitary gland has two major lobes: -The anterior lobe is glandular tissue and produces six well understood hormones. -The posterior lobe is neural tissue that releases two hormones produced by neurons in the hypothalamus. We will study the pituitary gland in detail and review the posterior pituitary hormones later in the video. Let's review the anterior pituitary hormones. The peptide hormones of the anterior pituitary include: thyroid-stimulating hormone also called thyrotropin; follicle-stimulating hormone and luteinizing hormone, also called gonadotropins; adrenocorticotropic hormone also called corticotropin; growth hormone and prolactin. Let's review the target tissues and functions of these hormones. TSH stimulates secretion of thyroid hormones by the thyroid gland. In females, FSH stimulates maturation of the ovarian follicle and production of estrogen. LH triggers ovulation, and stimulates production of estrogen and progesterone. In males, FSH stimulates sperm production and LH stimulates production of testosterone. ACTH stimulates the secretion of glucocorticoids and other hormones of the adrenal cortex. In addition to their individual effects these four tropic hormones - thyrotropin, the gonadotropins, and corticotropin help maintain size of their target glands. Growth hormone stimulates growth and energy metabolism. Most of the growth effects of growth hormone require intermediary proteins called somatomedins or insulin-like growth factors. They are produced by many target tissues but released into the bloodstream 82 00:03:46,523 00:03 :48,102 primarily by the liver. Both insulin-like growth factors and growth hormone are required for growth of cartilage, hence full adult height of the skeleton. Metabolic effects of growth hormone involve mobilizing fat stores and thereby sparing glucose for use by glucose-dependent tissues. PRL stimulates lactation in females. Although present in males, its action is uncertain. Next, let's take a look at the pineal gland. The pineal gland produces the amine hormone melatonin. Although the functions of melatonin in humans are not completely understood, studies suggest that melatonin helps coordinate body activities with light-dark cycles. Next, let's discuss the thyroid gland. The thyroid gland produces two very different hormones. The follicular cells produce thyroid hormone, an amine, and the parafollicular or C cells produce calcitonin, a peptide. We will review TH first and study it in detail later in the video. The large follicles in the gland are filled with a protein colloid called thyroglobulin. They are unique in the endocrine system and provide for storage of large amounts of thyroglobulin from which thyroid hormone is made. Let's briefly review the targets and functions of TH. Thyroid hormone regulates many metabolic functions. Thyroid hormone is essential for growth. Thyroid hormone is essential for development of the nervous system. Thyroid hormone is essential for nervous system function in adults. Thyroid hormone amplifies the activity of the sympathetic nervous system. Now let's review the targets and functions of calcitonin. Calcitonin inhibits the activity of osteoclasts, thereby preventing bone resorption and release of calcium. This lowers blood calcium concentration. It protects the skeleton from excess resorption, but is not required for calcium homeostasis, the daily regulation of blood calcium. Next, let's look at the parathyroid glands. The parathyroid glands produce the peptide parathyroid hormone. Chief cells are the source of parathyroid hormone; the function of oxyphil cells is not known. The chief cells form clusters and cords, a common arrangement of secretory cells in endocrine glands. Let's look at the chief cells to review the target organs and functions of PTH. Declining blood calcium levels cause the parathyroid glands to secrete parathyroid hormone. It is the primary regulator of blood calcium levels. Number One - parathyroid hormone acts directly on the kidney to increase reabsorption of calcium. Number Two - parathyroid hormone increases resorption of bone. This increases blood calcium levels. Number Three - parathyroid hormone promotes the final conversion of vitamin D to its active form, the steroid hormone calcitriol, that increases uptake of calcium from the intestine. Ultimately calcium balance depends on absorption of calcium from the intestine. Note that the rising blood calcium levels act as a negative feedback signal to decrease parathyroid hormone secretion. Blood calcium levels control parathyroid hormone levels. Now, let’s look at the thymus. The thymus is large at birth, and increases in size until puberty. It gradually atrophies and is replaced by adipose and fibrous tissue in adults. Secretory cells produce a family of peptide hormones that include thymosin and thymopoietin. Let’s review the targets and functions of the thymic hormones. Although the function of thymic hormones is poorly understood, they appear to regulate T cell development, and play a role in the immune response. Next up, the adrenal gland. The paired adrenal glands consist of a large outer region called the adrenal cortex, and a small central region called the adrenal medulla. The cortex secretes three families of steroid hormones: mineralocorticoids, glucocorticoids, and androgens. Note that each is produced by a different group of cells. The medulla secretes amine hormones known as catecholamines. Let's review targets and functions of the mineralocorticoids. Cells of the zona glomerulosa produce hormones called mineralocorticoids because they affect salt balance. Aldosterone is the most important. Among its functions are sodium retention and potassium excretion by the kidneys. Let's review the targets and functions of the glucocorticoids and androgens. Cells of both zona fasciculata and zona reticularis produce a family of hormones called glucocorticoids because they regulate glucose metabolism. This family includes cortisol, cortisone, and corticosterone. Only cortisol is secreted in significant amounts in humans. We will study cortisol in detail later in the video. In addition to regulating energy metabolism, cortisol regulates the immune system and facilitates the stress response. Cells of both zona fasciculata and zona reticularis also produce a family of hormones called androgens. Androgens are male sex hormones that include androstenedione and testosterone. Active secretion at puberty in both sexes causes the early growth spurt and appearance of axillary and pubic hair. Androgens are also responsible for sex drive in females. We will now review targets and functions of the catecholamines. The adrenal medulla is a modified sympathetic ganglion that produces a family of amine hormones called catecholamines. The two main hormones are epinephrine and norepinephrine. Epinephrine is 4 or 5 times more abundant than norepinephrine. We will study it in detail later in the video. The catecholamines are important during the "fight or flight" response when immediate physical action is called for. Now, let’s take a look at the pancreas. The pancreas is a mixed gland that has both endocrine and exocrine functions. The exocrine secretions are digestive juices. Acinar cells produce them and ducts carry them to the small intestine. Let's review the targets and functions of the pancreatic hormones. Cells in the pancreatic islets secrete hormones. The islets are an excellent example of the arrangement of cells in clusters and cords surrounded by capillaries. This common arrangement of endocrine cells provides for good access to the circulatory system. The majority of islet cells are beta cells that secrete the peptide hormone insulin. Smaller numbers of alpha cells secrete the peptide hormone glucagon. Other rare cells produce the peptide somatostatin, but its function in humans is not well understood. Let’s look at the beta cells to briefly review the targets and functions of insulin. Insulin promotes storage of organic fuel molecules by its actions on liver, muscle, and adipose tissue. It increases glucose uptake in all three tissues and promotes glycogen synthesis in liver and muscle, and triglyceride synthesis in liver and adipose tissue. The end result of insulin secretion is decreased blood levels of glucose. Now let's look at the alpha cells to review the targets and functions of glucagon. Glucagon stimulates production and secretion of organic fuel molecules by the liver. It stimulates breakdown of glycogen, and synthesis of glucose and ketones. The end result of glucagon secretion is increased blood levels of glucose. Next, let’s look at the ovaries. Ovaries produce the female sex hormones. They include the steroids estrogen and progesterone, and the peptide hormone inhibin. Estrogen and inhibin are produced first by the granulosa cells, then by the corpus luteum. Just prior to ovulation, small amounts of progesterone are produced by the granulosa and theca cells. The corpus luteum is the major source of progesterone. Let's review the targets and functions of the ovarian hormones. Estrogen and progesterone together regulate ovarian function, cyclic changes in the uterine lining, and breast development during puberty. Estrogen promotes development of the female gamete (the egg), development of other secondary sex characteristics, and maturation of the reproductive organs in females. Progesterone is required for pregnancy. Inhibin suppresses secretion of FSH. Next, let's look at the testes. Testes produce the male sex hormones. They include the steroid testosterone and the peptide inhibin. Testosterone is produced by the interstitial cells, and inhibin is produced by the sustentacular cells. Let's look at the seminiferous tubule to review the targets and functions of the testicular hormones. Testosterone promotes development of the male gamete (the sperm); development, maturation, and maintenance of the male reproductive organs; development of the secondary sex characteristics; and sex drive. As in the female, inhibin suppresses secretion of FSH. Hormones are produced by endocrine glands. They are also produced by endocrine cells located in organs that have non-endocrine function. We refer to these as endocrine tissues. They include the brain, heart, kidney, and gastrointestinal tract. Endocrine cells may be diffusely located, for example in the stomach and intestines, or may be found in clusters like the neurons of the hypothalamus. Let's start with hypothalamus tissue. We have seen that the pituitary gland has two major lobes, and we have reviewed the glandular tissue of the anterior lobe earlier in the video. Here we will review the posterior lobe and the endocrine function of the ventral hypothalamus. Let's briefly review the targets and functions of the posterior pituitary’s hormones. The posterior lobe of the pituitary gland is neural tissue that releases hormones produced by neurons in the hypothalamus. The hormones travel down the axons of the hypothalamic neurons and are released from axon terminals into the general circulation. The peptide hormones, oxytocin and vasopressin (also called antidiuretic hormone, ADH) are produced by separate neurons located in the paraventricular and supraoptic nuclei. Next, let’s look at the paraventricular and supraoptic nucleus. In females, oxytocin stimulates uterine contractions during labor and promotes milk release (or letdown) from the breasts. Functions in the male are uncertain. Vasopressin controls water reabsorption in the kidney, and thereby participates in controlling water balance, blood volume, and blood pressure. Next, let's review ventral hypothalamus. Neurons of the ventral hypothalamus produce hormones that enter a special vascular system. From the capillaries in the hypothalamus, they enter the hypophyseal portal vessels that deliver them to capillary beds of the anterior pituitary. Hormones exit easily there. Let's go to the anterior lobe of the pituitary to review the ventral hypothalamic hormones. All but one of the ventral hypothalamic hormones are peptides. They stimulate or inhibit release of the anterior pituitary hormones, and their names reflect their actions. Let's review different ventral hypothalamic hormones. Thyrotropin-releasing hormone stimulates secretion of thyroid stimulating hormone, or thyrotropin. Recall that TSH acts on the thyroid gland to stimulate secretion of thyroid hormone. Gonadotropin-releasing hormone stimulates secretion of follicle-stimulating hormone and luteinizing hormone, together called the gonadotropins in adults of both sexes. Recall that FSH and LH stimulate maturation of the gametes and production of the reproductive hormones. Corticotropin-releasing hormone stimulates secretion of adrenocorticotropic hormone or corticotropin. Recall that ACTH stimulates secretion of glucocorticoids and other hormones of the adrenal cortex. Two hormones regulate growth hormone: growth hormone-releasing hormone stimulates secretion, and growth hormone inhibiting hormone, also called somatostatin, inhibits secretion. Recall that growth hormone stimulates growth and energy metabolism. The major hormone that regulates secretion of prolactin is the catecholamine dopamine, also called prolactin-inhibiting hormone. Dopamine inhibits the secretion of prolactin. Several factors called prolactin releasing factors cause secretion of prolactin. The identity of these factors is uncertain, although thyrotropin-releasing hormone may be one. Prolactin is different from the other anterior pituitary hormones because it is normally inhibited by the hypothalamus. Next, let’s explore the endocrine tissue of the heart. Specialized muscle cells in the atria of the heart secrete the hormone atrial natriuretic peptide. Let's review the target and functions of ANP. Atrial natriuretic peptide causes the kidneys to increase excretion of sodium. It participates in the control of salt balance and thereby contributes to control of blood volume and blood pressure. Now, let’s look at the endocrine tissue of the kidney. The kidney produces the peptide hormone erythropoietin and steroid hormone calcitriol. Let's review the targets and functions of these renal hormones. Erythropoietin stimulates bone marrow to increase production of red blood cells or erythrocytes. Recall that parathyroid hormone promotes the final conversion of vitamin D to its active form, the steroid hormone calcitriol, in the kidney. Calcitriol increases uptake of calcium from the intestine. Next, let’s look at the stomach. The organs of the gastrointestinal tract produce five peptide hormones that regulate the function of the tract. The hormones are produced by specific endocrine cells that are interspersed in the lining of the tract. The stomach produces the peptide gastrin. Gastrin is produced by G cells in the pyloric antrum of the stomach. Remember that although the GI hormones are produced in the lining of the GI tract, they enter the bloodstream to be transported to their target organs. Gastrin stimulates hydrochloric acid secretion in the stomach, and stimulates growth of the gastric mucosa. Next, let’s look at the small intestine. The proximal portion of the small intestine, including duodenum and jejunum, produces four hormones: cholecystokinin, secretin, motilin, and glucose-dependent insulinotropic peptide. Let's review their targets and functions. Here you see the liver, gall bladder, bile duct, pancreatic duct, duodenum, and jejunum. Let's go to the pancreatic duct to review CCK. CCK is produced by I cells. It causes the gallbladder to contract, thereby moving bile into the small intestine, the exocrine pancreas to produce digestive enzymes that move into the small intestine, and growth of the exocrine pancreas and mucosa of the gall bladder. Let's go to the liver to review secretin. Secretin is produced by S cells. It causes both the liver and the exocrine pancreas to produce bicarbonate that moves into the small intestine. Bicarbonate neutralizes the acidic chyme that comes from the stomach. Because of this action, secretin is called "nature's antacid." Secretin also stimulates growth of the exocrine pancreas. Let's go to the pancreas to review GIP. In the presence of glucose, GIP stimulates release of insulin by the endocrine pancreas. Let's go to the small intestine to review motilin. Motilin is released about every 90 minutes during times of fasting. It stimulates production of a migrating peristaltic contraction, called the migrating motility complex, that acts to sweep the contents of the small intestine along toward the terminal ileum. Here is a summary of what we have covered: We have reviewed the well-established endocrine glands and endocrine tissues of the body. We have learned about the well-understood hormones produced by these glands and tissues. We have identified the target tissues for each hormone, and reviewed a simplified function for each hormone.