The endocrine system is a collection of glands. These glands secrete a variety of hormones, which travel to specific target organs via the bloodstream. Hormones have specific functions such as regulating growth, metabolism, temperature and reproductive development. Like the nervous system, the endocrine system acts as a signaling pathway, although hormones are slower acting than nerve impulses. Endocrine signals can last from a few hours to a few weeks. The main control center for the glands in the endocrine system is the hypothalamus in the brain. The field of medicine concerned with the endocrine system is known as endocrinology.
Glands of the Endocrine System
Endocrine glands tend to be vascular and do not have ducts. Ducts are instead found in exocrine glands, which produce hormonal signals outside of the body. The hormones of endocrine glands are stored in vacuoles or granules, ready to be released.
Endocrine glands are found throughout the body and have a variety of different roles. The key endocrine glands and organs are listed below:
- Pineal gland
- Pituitary gland
- Thyroid gland
- Parathyroid gland
- Adrenal glands
- Gastrointestinal tract
The hypothalamus is an almond-sized structure in the limbic system of the brain, and the endocrine system’s control center. Its borders are the following:
- Anteriorly: anterior commissure, lamina terminalis, and optic chiasm
- Posteroinferiorly: the posterior perforated substance
- Inferiorly: the infundibular stalk
- Superiorly: the hypothalamic sulcus and the base of the third ventricle
Anteroposteriorly, the hypothalamus can be divided into three regions: chiasmatic, tuberal and the region of the mammillary bodies. The chiasmatic region lies immediately above the optic chiasm (hence its name) and is related with the circadian rhythm and the variations of the endocrine secretion throughout the day. The tuberal zone contains the tuber cinereum. This mass of grey matter is located between the mammillary bodies and the optic chiasma. The infundibulum projects from the tuber cinereum, becoming continuous with the posterior lobe of the pituitary gland. A structure called the median eminence is separated from the base of the infundibulum by a tuberoinfundibular sulcus. And finally the region of the mammillary bodies, which are hemispheral and pea sized structures situated anteriorly to the posterior perforated substance. Their role is to control memory and emotional expression.
Mediolaterally, the hypothalamus can be divided again into three zones: periventricular, intermediate and, lateral. The regions and zones contain and border several hypothalamic nuclei, each one being responsible for particular functions.
The hypothalamus controls the endocrine system via several pathways. These include direct projections to the posterior pituitary (neurohypophysis), and indirect control over the anterior pituitary (adenohypophysis) via projections to the median eminence and via the autonomic nervous system. The hypothalamus carries out its control by producing releasing or inhibiting hormones, known as neurohormones. Releasing hormones stimulate the production of hormones in the pituitary gland, whilst inhibiting hormones inhibit it.
The neurohormones produced by the hypothalamus to manipulate hormone production by the pituitary gland include:
- Anti-diuretic hormone (ADH): This increases water absorption in the kidneys.
- Corticotropin-releasing hormone (CRH): This stimulates the release of corticosteroids by the adrenal glands, regulating metabolism and immune response.
- Gonadotropin-releasing hormone (GnRH): GnRH stimulates the production of follicle stimulating hormone (FSH) and luteinizing hormone (LH), which combine to maintain ovary and testes functioning.
- Growth hormone-releasing hormone (GHRH) or growth hormone-inhibiting hormone (GHIH): GHRH prompts the release of growth hormone (GH), whilst GHIH has the opposite effect. In children, GH is essential to maintaining a healthy body composition. In adults, it ensures healthy bone and muscle mass and is involved in fat distribution.
- Oxytocin: This is involved in the release of breast milk, orgasm, and smooth muscle contraction. It also regulates body temperature by helping to redistribute heat, and sleep cycles as increasing levels of oxytocin are thought to help induce sleep.
- Prolactin-releasing hormone (PRH) or prolactin-inhibiting hormone (PIH): PRH stimulates the production of breast milk, whilst PIH inhibits it. This can also be seen in males too, although it is a sign of significant health issues.
- Thyrotropin releasing hormone (TRH): TRH triggers the release of thyroid stimulating hormone (TSH), causing the release of thyroid hormones which regulate metabolism, energy, growth, and development.
The pituitary gland (hypophysis cerebri) is a pea-sized, ovoid shaped structure attached via the infundibulum to the tuber cinereum of hypothalamus. It is located within the pituitary fossa (sella turcica) of the sphenoid bone. The diaphragma sellae of the dura mater only partially encloses the gland within the fossa because it contains an aperture for the infundibulum. A venous sinus separates the gland from floor of the fossa.
The pituitary gland has two main parts: neurohypophysis and adenohypophysis. The neurohypophysis is an actual downgrowth of the diencephalon directly connected to the hypothalamus. Both parts include the infundibulum. The neurohypophysis incorporates the stem of the infundibulum, which is a continuation of the median eminence of the tuber cinereum. It also contains the posterior (neural) lobe. The adenohypophysis can be separated into the pars intermedia (the boundary between the two pituitary lobes) and the pars anterior (anterior lobe), both forming a part of the adenohypophysis. The adenohypophysis also contains the pars tuberalis, a vascularized sheath surrounding the stem of the infundibulum.
The main neurosecretory pathway through the neurohypophysis originates from the supraoptic and paraventricular nuclei of the hypothalamus and terminates near the sinusoids of the posterior lobe. As a result, hormones are released directly in the circulation. Another group of neurons that end in the median eminence and infundibular stem release the inhibitory and releasing hormones within the hypophyseal portal system, ultimately controlling the secretory activity of the adenohypophysis.
The pituitary gland stores some of the hormones that the hypothalamus produces, before releasing them into the blood. Out of the two lobes, the anterior lobe is larger, making up 75% of the gland. It also has a larger role in the release of hormones, although the posterior lobe still does some work.
The anterior lobe secretes a total of 7 different hormones into the bloodstream, which are as follows:
- Human-growth hormone (hGH): hGH stimulates tissue growth and protein synthesis for tissue repair.
- Thyroid-stimulating hormone (TSH): TSH causes hormone production by the thyroid gland.
- Follicle-stimulating hormone (FSH): This causes estrogen production in females, as well as the development of oocytes (immature egg cells). FSH also stimulates sperm production in the testes.
- Luteinizing hormone (LH): LH stimulates estrogen and progesterone production in females, and testosterone production in males.
- Prolactin (PRL): This stimulates milk production in the mammary glands.
- Adrenocorticotropic hormone (ACTH): This is involved in the body’s stress response and causes the production of cortisol in the adrenal cortex.
- Melanocyte-stimulating hormone (MSH): MSH can cause darkening of the skin. It may also be involved in brain activity but its exact role in this is still unknown. The pars intermedia manufactures MSH during fetal development.
Meanwhile, the posterior lobe of the pituitary gland is only involved in the release of two hormones; oxytocin and antidiuretic hormone (ADH). Oxytocin is involved in childbirth, milk production, and orgasm. ADH is important in reducing water loss by decreasing urination and sweating, therefore increasing blood pressure.
Along with the hypothalamus and pituitary gland, the pineal gland (epiphysis cerebri) is found in the brain. It is a small organ located in a depression between the superior colliculi, inferiorly to the splenium of the corpus callosum. The gland is enclosed within the lower layer of tela choroidea of the third ventricle.
The pineal gland has a base that is directed anteriorly and is divided into a superior and inferior laminae by the pineal stalk, which also serves as a point of attachment to the roof of the third ventricle. The laminae contain the posterior and habenular commissures, respectively. The gland parenchyma is highly vascularized and divided into lobules by several septa, which also carry blood vessels and sympathetic nerves. These adrenergic sympathetic axons originate from the tentorium cerebelli and enter the gland as the nervus conarii. The pineal gland parenchyma consists mainly of pinealocytes. The pineal stalk consists mostly of glia.
The pineal gland has a more specific function, being involved only in the secretion of the hormone melatonin. It is released from bulbous expansions of the cell bodies of pinealocytes. This hormone is involved in both sexual development and the sleep-wake cycle. In terms of reproductive development, melatonin blocks the secretion of gonadotropins (FSH and LH) from the pituitary glands.
Melatonin also regulates the sleep-wake cycle by reacting to the amount of light hitting the retina. The retina relays this information to the hypothalamus, which in turn sends information to the pineal gland. The pineal gland secretes melatonin depending on the amount of light hitting the retina. The less light there is, the more melatonin is produced, inducing sleep.
The thyroid and parathyroid glands are endocrine glands at the base of the neck. The thyroid gland is the largest gland of the endocrine system. It is located in the anterior portion of the neck at the level of the C5-T1 vertebrae, deep to the sternothyroid and sternohyoid muscles.
It consists of two lobes, right and left, which ascend upwards to the thyroid cartilage, joined together by an isthmus. The lobes are anterolaterally in relation to the larynx and trachea, while the isthmus is anterior to the second and third tracheal rings. In some individuals, a conical pyramidal lobe ascends from the isthmus towards the hyoid bone.
The thyroid gland is enclosed inside a fibrous capsule, which is attached to the cricoid cartilage and tracheal rings by dense connective tissue. The fibrous capsule itself is enclosed in a loose sheath of fascia. The gland is highly vascularised. The arteries supplying it are the superior and inferior thyroid arteries, which lie between the fibrous capsule and the sheath of fascia. Venous drainage of the gland is via the superior, middle and inferior pairs of thyroid veins, which form the thyroid plexus of veins. Innervation is from the cervical sympathetic ganglia, as well as parasympathetic fibers from the vagus nerves.
The thyroid gland is important in regulating metabolism. It produces 2 important metabolic hormones, thyroxine (T4) and triiodothyronine (T3). T4 contains 4 iodine atoms, whilst T3 contains 3 iodine atoms. T3 and T4 both affect the body’s metabolism by influencing protein production of every cell in the body. This protein production in turn affects tissue growth, temperature, energy use, and heart rate. The thyroid gland also produces calcitonin, which is an antagonist to parathyroid hormone.
Location and Characteristics
The parathyroid glands (usually 4 in total) are small, flattened, and oval structures located on the posterior surface of each lobe of the thyroid gland. They normally lie between the fibrous capsule of the thyroid gland and its external fascial sheath.
The glands are separated into two superior and two inferior ones. The location of the superior parathyroid glands is quite constant, at the level of the inferior border of the cricoid cartilage, 1 cm superior to the entry point of the inferior thyroid arteries into the thyroid gland. The inferior parathyroid glands are usually situated near the inferior poles of the thyroid gland but have a more varied location.
Arteries supplying the parathyroid glands branch from the inferior thyroid arteries. Venous drainage is via parathyroid veins which subsequently drain into the thyroid venous plexus. Innervation is from the parasympathetic fibers from the vagus nerves, similar to the thyroid gland.
The parathyroid glands maintain calcium levels in the blood by producing parathyroid hormone. Together with calcitonin, these two hormones maintain the level of calcium ions in the blood, which is important in bone health, as well as muscle and nervous system function.
Enteric Endocrine System
The gastrointestinal tract itself can produce hormones and is known as the enteric endocrine system. Hormone secreting cells are dispersed throughout the lining of the stomach and small intestine. These cells do not produce hormones continuously, instead they do so in response to the environment inside the stomach and intestine, reacting to the amount of food moving through.
The pancreas is particularly important in the enteric endocrine system, as it releases the hormones insulin and glucagon, which regulate blood sugar levels. The pancreas is an accessory digestive gland. It crosses the bodies of the L1 and L2 vertebrae transversely. The pancreas is situated anteriorly to the stomach and between the duodenum on the right and the spleen on the left. Its anterior margin is in contact with the transverse mesocolon.
This gland has four parts: a head, neck, body, and tail.
- The head is attached to the descending and horizontal parts of the duodenum, embracing it in a C-shaped fashion. The uncinate process is an inferior projection from the head, which extends posterior to the superior mesenteric artery (SMA).
- The short neck of the pancreas is covered by peritoneum and is located adjacent to the pylorus of the stomach. The hepatic portal vein is formed posterior to it, by the joining of the splenic vein and the superior mesenteric vein (SMV).
- The body of the pancreas continues transversely from the neck, passing anteriorly to the aorta and L2 vertebra and posterior to the omental bursa. The anterior surface is covered by peritoneum and also forms part of the stomach bed.
- The tail is situated anterior to the left kidney and it is an intraperitoneal structure. It is closely related to the hilum of the spleen and the left colic flexure.
Running from the tail to the head, through the parenchyma, is the main pancreatic duct. It joins the common bile duct, just outside the duodenum, forming the short hepatopancreatic ampulla (ampulla of Vater). This structure opens into the descending part of the duodenum. The hepatopancreatic sphincter (sphincter of Oddi) prevents the reflux of duodenal content into the ampulla. The main pancreatic duct also contains a sphincter that controls the flow inside it.
The blood supply is via pancreatic arteries, which branch off several vessels located nearby. Venous drainage is via pancreatic veins and most of these empty in the splenic vein. Innervation of the pancreas is from the vagus and abdominopelvic splanchnic nerves.
There are six key gastrointestinal hormones:
- Gastrin: This is stimulated by the presence of peptides and amino acids in the gastric lumen, and is important in the secretion of gastric acid.
- Secretin: This is produced in response to acidic pH levels, and causes the production of water and bicarbonate from the pancreas and bile duct to help increase pH again.
- Ghrelin: Ghrelin stimulates appetite and feeding.
- Motilin: Motilin is involved in movement and contractions of the gastrointestinal tract.
- Cholecystokinin: This stimulates the secretion of pancreatic enzymes and emptying of the gallbladder in response to an increase in fatty acids and amino acids in the small intestine.
- Gastric inhibitory polypeptide: This prevents gastric movement and secretions, and causes the release of insulin in response to an increase in glucose and fat in the small intestine.
The adrenal (suprarenal) glands are two triangular shaped glands found on top of the kidneys. They have a yellowish appearance and are located between the superomedial aspects of the kidneys and the diaphragm. The glands are surrounded by renal fascia, which also provide an attachment point to the crura of the diaphragm. A septum separates the glands from the kidneys. The two glands are not identical. The right one is more pyramidal and apical, while the left one is more crescent-shaped. They also have slightly different positions and relations. Veins and lymphatic vessels enter and leave each gland via the hilum.
The blood supply to the adrenal glands is via superior, middle, and inferior suprarenal arteries. Venous drainage is via the right and left suprarenal veins, which subsequently drain into the internal jugular vein and left renal vein, respectively. Innervation is from the celiac plexus and abdominopelvic splanchnic nerves.
Structure and Function
The glands are divided into two parts; the adrenal cortex and the adrenal medulla. The adrenal cortex is the outer part of an adrenal gland, and produces hormones vital to life such as glucocorticoids - the horomes hydrocortisone (cortisol), and corticosterone. Hydrocortisone regulates energy production, blood pressure, and heart function. Corticosterone plays a role in immune responses and reduction in inflammation. The adrenal cortex also produces aldosterone, which controls blood pressure.
The adrenal medulla is the inner portion of the gland. It is actually a mass of nervous tissue containing many capillaries and sinusoids. The medulla produces hormones such as adrenaline. The adrenal medulla helps the body deal with stress by producing two hormones, epinephrine and norepinephrine. Epinephrine is more commonly known as adrenaline and is involved in the body’s fight or flight response, increasing heart rate and blood glucose levels, and causing an increase in blood flow to the brain and muscles. Norepinephrine works with adrenaline, by constricting blood vessels and increasing blood pressure during the stress response.
The endocrine organs in the reproductive systems are the ovaries and testes, in females and males respectively. The testes are paired ovoid glands that produce spermatozoa and the male hormones, mainly testosterone. Each testis is suspended in the scrotum by its own spermatic cord, the left one hanging more inferiorly than the right one, mostly due to the length of the spermatic cord. The testes are almost completely covered by the visceral layer of the tunica vaginalis, a closed peritoneal sac. A recess in the tunica vaginalis represents the sinus of the epididymis. The tunica vaginalis also has a parietal layer, which is adjacent to the internal spermatic fascia. A fluid filled cavity is located between the visceral and parietal layers, conveying some degree of mobility for the testes.
The testes have a tough fibrous outer surface called the tunica albuginea. On the internal, posterior aspect of the fibrous there is a ridge called the mediastinum of the testis. Fibrous septa extend from this ridge between lobules formed by seminiferous tubules. Spermatozoa are produced inside these tubes. Straight tubules join the seminiferous tubules to the rete testis, which are canals situated in the mediastinum of the testes.
These glands receive their blood supply from the testicular arteries, originating from the abdominal aorta. The venous drainage is via the pampiniform venous plexus, which surrounds the testicular artery. The plexi of each testis join to form the left and right testicular veins. They drain into the left renal vein and inferior vena cava, respectively. Innervation of the testes is via the testicular plexus, which originates from the renal and aortic plexi.
The ovaries are almond-shaped glands in which oocytes develop and produce the female hormones. Each one is suspended by the mesovarium, a peritoneal fold subdivision of the broad ligament of the uterus. Before puberty, the surface of the ovary is covered by the ovarian mesothelium, also known as surface epithelium, giving it a shiny appearance. This structure consists of a single layer of cuboidal cells. After puberty, the surface of the ovary becomes scarred due to ovulation, which involves ruptures of ovarian follicles and oocyte discharge.
The ovaries lie suspended inside the pelvic cavity on each side of the uterus, close to the lateral wall of the pelvis. Specifically, they are located inside the ovarian fossa. Attached to the superolateral aspect of the ovary is the suspensory ligament of the ovary. This is a peritoneal fold enclosing the ovarian vessels, lymphatics, and nerves, which become continuous with the mesovarium. The ligament of the ovary passes through the mesovarium, keeping the ovary attached to the uterus. It is the remnant of the ovarian gubernaculum of the fetus.
Blood supply to the ovaries is via the ovarian arteries arising from the abdominal aorta. Venous drainage is via the pampiniform venous plexus inside the broad ligament. The veins from the plexus join to form the ovarian veins, which accompany the ovarian arteries. The right ovarian vein drains in the inferior vena cava, while the left one drains in the left renal vein. Innervation is from the ovarian plexus.
Sex hormones are produced in these organs as a result of LH and FSH production by the pituitary gland. The hormones they produce are important in sexual development, reproduction, and regulation of the menstrual cycle.
The two key hormones produced by the ovaries are estrogen and progesterone. Their production is triggered by the release of hormones by the hypothalamus. There are three types of estrogen: estradiol, estrone, and estriol. These combine to ensure healthy sexual development and fertility. Estradiol is important in breast development, fat distribution, and development of the reproductive organs. Progesterone is most important during pregnancy and ovulation, where it ensures that the lining of the uterus is suitable for foetus growth.
In males, testosterone is produced by the testes. Testosterone enhances bone growth, hair growth, and the development of sexual organs during puberty. Testosterone is also important in increasing muscle strength.