Human Anatomy
The Endocrine System
Endocrine System
This system’s ductless glands secrete hormones directly into the bloodstream and control body functions that includes growth, metabolism, reproduction, and sleep.
Endocrine Glands
The endocrine system is composed of a group of glands that produce hormones. Hormones travel through the bloodstream and act as chemical messengers. They control and coordinate a wide range of processes throughout the body, including growth, development, metabolism, sleep, sexual function, reproduction, and mood.
A gland is an organ that produces and releases substances that perform a specific function in the body. Endocrine glands lack ducts and release hormones directly into the bloodstream. Since they are secreted in this way, hormones reach every part of the body, but each specific hormone only affects the tissues and cells that have receptors for it.
Endocrine glands and their hormones form part of the endocrine system, which regulates a wide range of functions in the body. There is a second type of gland in the human body, called exocrine glands. These have ducts and do not release hormones, so they are not part of the endocrine system.
Hormones, some of which are referred to as “factors,” can be categorized based on their chemical structure: amino acid derivatives are small molecules that are structurally similar to amino acids; peptides are chains of amino acids and represent the largest class of hormones; and lipid derivatives are either derived from eicosanoids or cholesterol.
The main control center for the endocrine system is the hypothalamus. Every major gland of the endocrine system produces one or more specific hormones or factors. The hypothalamus produces releasing and inhibiting hormones that affect the pituitary gland. These include vasopressin, corticotropin-releasing factor, gonadotropin-releasing hormone, growth hormone-releasing hormone, growth hormone-inhibiting hormone, oxytocin, prolactin-releasing hormone, prolactin-inhibiting hormone, and thyrotropin-releasing hormone.
The pituitary gland (also called the hypophysis) produces stimulating hormones that affect other endocrine glands, including growth hormone, thyroid-stimulating growth hormone, follicle-stimulating hormone, luteinizing hormone, prolactin, adrenocorticotropic hormone, and melanocyte-stimulating hormone. It also stores and releases oxytocin and vasopressin.
The pea-shaped pineal gland, found in the brain, regulates he sleep-wake cycle and produces the hormone melatonin.
The thyroid gland is located in the throat and regulates metabolism through the hormones thyroxine, triiodothyronine, and calcitonin.
The parathyroid glands regulate the levels of calcium in the blood through parathyroid hormone.
The islets of Langerhans, found in the pancreas, regulate growth and the blood’s levels of glucose. They produce glucagon, insulin, and somatostatin.
The gastric mucosa, the lining of the gastrointestinal tract, also performs an endocrine function, secreting hormones. The gut is sometimes called the enteric endocrine system, and some consider it to be the largest endocrine organ in the body. Gastric mucosa regulates digestion and triggers the sense of satiety. It produces gastrin, secretin, gherkin, cholecystokinin, motilin, and gastric inhibitory polypeptide.
Found atop the kidneys, the adrenal glands regulate blood pressure, electrolyte balance, and stress responses. They produce cortisol, corticosterone, aldosterone, epinephrine, norepinephrine, and dopamine.
The gonads (another name for the testes and ovaries) regulate sexual development, behavior, and characteristics as well as gametogenesis, which is the process by which cells divide and differentiate, forming gametes. The testes produce testosterone, and the ovaries produce estrogen and progesterone.
The Human Sex Glands: A gonad is a sex or reproductive gland. They produce gametes (i.e., sex cells) and sex hormones. In females, the reproductive cells are called ova (eggs cells), while male reproductive cells are called sperm. In males, a number of accessory sex glands contribute to the composition of sperm and seminal fluid, including the ampullae, seminal vesicles (vesicular glands), prostate, and bulbourethral glands.
Endocrinology: This is the study of the endocrine system, including specific hormonal secretions and diseases. Endocrinology is also concerned with the physiological, psychological, and behavioral activities triggered by hormones, including metabolism, growth, development, tissue function, sleep, respiration, mood, movement, reproduction, and sensory perception.
Adrenal glands: produce the hormones epinephrine, norepinephrine, cortisol and aldosterone.
Thyroid: produces iodine-containing hormones, which have metabolic, cardiovascular, and developmental effects.
Hypothalamus: connects the endocrine and nervous systems.
Pituitary gland: “master gland” that produces hormones that affect other endocrine glands and also produces growth hormones.
Pancreas: involved in blood-sugar control, metabolism, and digestion.
Ovaries: produce ova, the female reproductive cells.
Testes: produce and store sperm and produce testosterone.
Thymus: produces T cells, which are critical to the adaptive immune system.
The position and location of the thymus: This pink lobular gland is found behind the sternum and has an asymmetrical flat shape. It facilitates the maturation of T cells, which are vital to the immune system. Its two lobes merge in the middle and are surrounded by a capsule that contains blood vessels that extend into its inner layers.
The Hypothalamus and Pituitary Gland
Together, the hypothalamus and pituitary gland control the endocrine system. The almond-sized hypothalamus links the endocrine and nervous systems. It produces releasing factors and inhibiting factors, which stop and start the production of the hormones that the hypothalamus produces and helps regulate the function of other endocrine glands.
The hypothalamus is located below the thalamus and above the pituitary gland and brain stem. It is the endocrine system’s control center, regulating body temperature, hunger, thirst, moods, sleep, and sex drive, It also controls the pituitary gland through various pathways and controls the release of hormones from other glands.
The pituitary gland, also called the hypophysis, is a small gland that is connected to the hypothalamus by a slender stalk called the infundibulum. Located behind the sphenoid bone, it has two main parts: the adenohypophysis (the anterior lobe) and the neurohypophysis ( the posterior lobe).
The hypothalamus is connected to the pituitary gland through direct projections to the neurohypophysis and indirect control of the adenohypophysis. The pituitary gland also stores some of the hormones produced by the hypothalamus before releasing them into the bloodstream.
The hypothalamus produces two types of neurohormones: hypothalamic releasing factors (also called releasing hormones and releasing factors) stimulate the production of hormones in the pituitary gland, and inhibiting factors (also called inhibiting hormones) inhibit this production.
The main releasing factors include corticotropin-releasing factor, gonadotropin-releasing hormone, thyrotropin-releasing hormone, and growth hormone-releasing hormone.
Corticotropin-releasing factor, also called corticotropin-releasing hormone (CRH), stimulates the release of corticosteroids by the adrenal glands, regulating metabolism and immune responses.
Gonadotropin-releasing hormone (GnRH) stimulates the production of follicle-stimulating hormone (FSH) and luteinizing hormone, which maintain the ovaries’ and testes’ function. In turn, follicle-stimulating hormone triggers estrogen production in females and sperm production in males. Luteinizing hormone stimulates estrogen and progesterone production in females and testosterone production in males.
Thyrotropin-releasing hormone (TRH) stimulates the release of thyroid-stimulating hormone (TSH) from the thyrotropes, which are endocrine cells in the adenohypophysis. Thyroid-stimulating hormone (TSH), in turn, stimulates the release of hormones from thyroglobulin and stimulates the growth of follicular cells in the thyroid.
Growth hormone-releasing hormone (GHRH) stimulates the release of growth hormone from the adenohypophysis, while growth hormone-inhibiting hormone (GHIH) inhibits its release. Growth hormone (GH), in turn, promotes the growth of bone and muscles as well as the kidneys, liver, and adipose tissue. It also controls how the body uses glucose.
Other neurohormones include prolactin, adrenocorticotropic hormone, and melanocyte-stimulating hormone. Prolactin promotes the development of glandular tissue in the female breast during pregnancy and stimulates milk production. Adrenocorticotropic hormone (ACTH) is involved in the body’s stress responses and triggers the production of cortisol in the adrenal cortices. Melanocytes-stimulating hormone (MSH) regulates the production of the pigment melanin by melanocytes in the skin.
Neurohormones produced by the hypothalamus and stored in the neurohypophysis before being released to the rest of the body include vasopressin and oxytocin. Vasopressin, also called antidiuretic hormone (ADH), promotes the reabsorption of water in the kidneys. Closely related to vasopressin is oxytocin, which can trigger and strengthen labor contractions and signal the release of milk for breastfeeding. It also regulates testosterone production, orgasm, and sleep.
The hypothalamus position is located at the base of the brain, below the thalamus and above the pituitary gland.
Hypothalamus nuclei: The hypothalamus contains a number of small nuclei, which have a variety of functions, including providing a link to the endocrine system via the pituitary gland and regulating blood pressure, heart rate, sweating, memory, sleep, and circadian rhythms.
The pituitary gland is a small gland in the brain that secretes hormones and affects organs. It sits in the sella turcica (or “Turkish saddle”), which is a bony hollow at the base of the skull, behind the bridge of the nose.
The pituitary gland release hormones from both the adenohypophysis (its anterior lobe) and the neurohypophysis (its posterior lobe). The parts of the body that are affected by the hormones released by the adenohypophysis: Ovaries=Gonadotropin, skin=MSH, testes=Gonadotropin, Mammary glands=Prolactin, Bones & tissues=GH, Adrenal cortex=ACTH and Thyroid=TSH. The parts of the body that are affected by the neurohypophysis’s hormones: Kidneys=ADH, Uterus=Oxytocin, and Mammary glands=Oxytocin.
The Pancreas and Thyroid
The pancreas is a large, gourd-shaped gland located behind the stomach that has both an endocrine and exocrine function. The thyroid is located in the front of the throat, adjacent to the larynx and trachea. It uses iodine from food to make two hormones - triiodothyronine (T3) and thyroxine (T4) - that help regulate body temperature, metabolism, and heart rate.
The pancreas is an elongated, tapered organ located across the back of the abdomen, behind the stomach. The pancreas’s hormonal (or exocrine) function is managed by irregularly shaped islets of Langerhans, which constitute 1 of 2 percent of the cell mass in a healthy adult.
There are five types of cells in the islets of Langerhans. The first of these is alpha cells, which secrete glucagon and constitute about 30 percent of the islets. Glucagon’s primary role is to prevent blood glucose levels from dropping too low. It does this in three ways. Firstly, it facilitates the conversion of glycogen stored in the liver into glucose, which is a process called glycogenolysis. Secondly, it stimulates the production of glucose from amino acid molecules, which is a process called gluconeogenesis. Lastly, it controls the amount of glucose consumed by the liver, so that more glucose is available to be secreted into the bloodstream.
Beta cells secrete insulin and amylin, and they make up about 60 percent of the islets. Insulin stimulates the cell’s absorption of glucose and decreases blood glucose levels. It also stimulates lipogenesis and decreases lipolysis, which facilitates the production and storage of lipids and glycogen. Insulin also increases the transmission of amino acid into cells. Amylin, meanwhile, inhibits the secretion of glucagon, delays gastric emptying, and acts as a satiety agent.
The remaining 10 percent of the islets consist of delta cells, epsilon cells, and PP cells. Delta cells secrete somatostain, which blocks the secretion of insulin and glucagon. Somatostatin also inhibits the secretion of several gastrointestinal hormones, including gastrin, secretion, cholecystokinin, and vasoactive intestinal peptide. Epsilon cells secrete ghrelin, which functions as an orexigenic hormone, which means it stimulates the appetite. It also stimulates fat deposits and the release of growth hormone. PP cells secrete pancreatic polypeptide, which functions as a satiety hormone. It also suppresses the pancreas’s digestive functions and slows gastric emptying.
The thyroid is an endocrine gland located in front of the throat, below the prominence of the thyroid cartilage. It consists of two large lateral lobes that are connected by an isthmus, which is a thin band of tissue. The thyroid’s lobes lie on either side of the larynx and upper trachea.
Thyrocytes, also called follicular cells and epithelial cells, are cells in the thyroid that synthesize thyroglobulin, which is a protein stored in the hormones thyroxine (T4) and triiodothyronine (T3). Triiodothyronine is the metabolically active form of thyroxine, and it regulates body temperature, metabolism, and heart rate. Parafollicular cells in the thyroid secrete a different hormone, called calcitonin.
Pancreas: This is a dual-purpose organ. As an exocrine gland, it secrets digestive enzymes into the duodenum. It also contains the endocrine islets of Langerhans, which secrets hormones, including insulin and glucagon.
The position of the pancreas: The gourd-shaped pancreas lies just about in the center of the mid-torso.
Thyroid: This important butterfly-shaped gland sits low on the front of the neck, just below the Adam’s apple. There are two lobes side by side connected by an isthmus. If your thyroid is healthy, you should not be able to feel it.
Balancing Blood Sugar: Glucose supplies cells with energy, and the concentration of glucose in the blood is called the blood sugar level. When levels go up after eating, insulin from the pancreas moves glucose from the blood to the cells, and the blood sugar levels go down. If the production of insulin is insufficient, glucose can overload the bloodstream, which is called hyperglycemia. Too little glucose in the blood is called hypoglycemia. Both conditions can be symptoms of diabetes.
The Adrenal Glands and Parathyroid Glands
The adrenal glands, located below the diaphragm and atop each kidney, control the body’s stress responses by releasing catecholamine and steroid hormones. The four parathyroid glands are located just behind the thyroid. They control the levels of calcium and phosphate in the blood through the parathyroid hormone.
The adrenal glands, also called the suprarenal glands, are enclosed in adipose (i.e., fatty) capsules that function as a protective layer. Each gland can be divided into two layers: the adrenal cortex, which is the outer layer, and the adrenal medulla, which is the inner layer. Both layers serve endocrine functions and produce different hormones. The adrenal cortices have three anatomic zones that have different structural and anatomic characteristics and are also regulated by different hormones. These three zones are the zone glomerulosa, the zone fasciculata, and the zone reticularis.
The zona glomerulosa is made up of small, rounded cells that secrete a mineralocorticoid hormone called aldosterone. (Mineralocorticoid hormones are a type of steroid hormone.) The secretion of aldosterone follows a diurnal rhythm, with higher levels typically being released during sleep. It controls the uptake of water in the kidney’s distal convoluted tubules and collecting duct, thereby regulating sodium and potassium levels, which helps regulate blood pressure.
The zona fasciculata is the middle zone of each adrenal cortex and lies below the zona glomerulosa. It is the thickest of the zones and is made up of parenchymal cells called spongiocytes, which are arranged in columns separated by venous sinuses. This layer secretes glucocorticoid hormones such as cortisol and corticosterone. Glucocorticoid hormones are another type of steroid hormone. They stimulate the glucose/glucogen turnover in the liver, which helps increase the overall blood glucose level, providing more energy when the body is stressed. Glicocorticoid hormones also lower inflammation by preventing the release of pro-inflammatory substances.
The zona reticularis is the innermost zone off each adrenal cortex. its cells are smaller and more irregular compared to the other zones. Adrenal androgens are produced in the zona reticular; these serve as precursors for testosterone. Androgen precursors, such as dehydroepiandrosterone and androstenedione from cholesterol, are also biosynthesized in this zone. These androgens are released into the bloodstream and transported to the gonads, where there are converted into testosterone or estrogen.
The adrenal medulla lies at the center of each adrenal gland. It consists of chromaffin cells (which make neurohormones), splanchnic nerves and dilated capillaries. Chromaffin cells are neuroendocrine cells that are modified sympathetic ganglia. They convert the amino acid tyrosine into catecholamine hormones like dopamine, epinephrine (adrenaline), and norepinephrine (noradrenaline). Catecholamines are released when the sympathetic nervous system is activated through the thoracic splanchnic nerves that end in the medulla. The physiological effects of these catecholamines depend on the neuroreceptors found at the site where the hormone is acting.
The parathyroid glands are four small endocrine glands in the neck and, anatomically. are closely associated with the thyroid gland. they help regulate calcium homeostasis (i.e., calcium concentrations to and from the bones). The parathyroid glands are composed of chief cells and oxyphil cells. The chief cells are more numerous and produce parathyroid hormone. Scientists have yet to discover the function of oxyphil cells, although is has been determined that they appear at the onset pf puberty.
Parathyroid hormone (PTH), also called parathormone, elevates the calcium levels in the blood through several mechanisms. PTH stimulates osteoblasts to secrete a growth factor that binds to osteoclasts. This increases osteoclast activity, which increase blood calcium levels. PTH also reduces the amount of calcium ions excited through urine. In addition, it stimulates the production of calcitroil in the kidneys, which helps the intestines absorb calcium. PTH levels will remain elevated until blood calcium levels return to normal.
The adrenal medulla is the inner part of each adrenal gland. It controls hormones that initiate the flight-or-fight response, including epinephrine (adrenaline) and norepinephrine (noradrenaline).
Stress responses: The fight-or-flight response is how the body and mind react during an emergency. Stress responses start in the brain and move from the pituitary gland to the adrenal medullas, which may release epinephrine or cortisol, which travels to other parts of the body that might need to react, including the heart, lungs, and eyes.
The heart’s hormone: Atrial natriuretic peptide (ANP) is secreted by the heart’s atria in response to an increase in atria pressure. It triggers the blood vessels in the kidneys to dilate, which increases the body’s output of urine, eliminating salt and water, which helps normalize atrial pressure. ANP and other peptides of this family play an important role in regulating blood pressure.