The endocrine system consists of the glands shown here as well as clusters of hormone-secreting cells in various organs, including the brain, heart, and small intestine.
The glands secrete chemicals (called hormones) that influence almost every cell and organ in the body.
Endocrine glands are ductless glands: they secrete their hormones directly into the bloodstream.
The cells of many different organs are exposed to a particular hormone; however, only cells having receptors for that hormone (called target cells) will respond.
The actions of the endocrine and nervous systems complement one another to ensure that the body maintains homeostasis.
Hormones can be classified as steroid or nonsteroid: steroid hormones are synthesized from cholesterol; nonsteroid (protein-based) hormones are synthesized from amino acids.
Once a hormone reaches target cell, it binds with a receptor to trigger changes within the cell.
Steroid hormones pass easily through a cell’s membrane; once inside the cell, they bind to receptors in the nucleus.
Protein-based hormones can’t penetrate the cell wall; they bind to receptors on the cell surface. The binding activates a second messenger system: a cascade of processes that results in the production of a second messenger. The second messenger activates specific enzymes. The enzymes influence cellular reactions, producing the cell’s response to the hormone.
The pituitary gland influences more body processes than any other endocrine gland.
The pea-sized pituitary gland sits underneath the hypothalamus. It lies in the sella turcica, a cavity within the sphenoid bone.
A stalk called the infundibulum connects the hypothalamus and pituitary.
The pituitary gland is two distinct glands: the adenohypophysis (anterior pituitary) and the neurohypophysis (posterior pituitary).
The anterior pituitary is the larger of the two pituitary glands. It consists of glandular tissue and secretes a number of important hormones under the direction of the hypothalamus.
Neurons within the hypothalamus synthesize releasing hormones (which stimulate the anterior pituitary to secrete its hormones) as well as inhibiting hormones (which suppress hormone secretion by the anterior pituitary).
Neurons of the hypothalamus release their hormones into a system of blood vessels called the hypophyseal portal system. The blood travels straight to the anterior pituitary, where the hormones from the hypothalamus act on target cells in the anterior pituitary, causing them to release certain hormones into the general circulation.
Each hormone acts on the anterior pituitary to release, or suppress, a particular hormone. (For example, thyrotropin-releasing hormone stimulates the anterior pituitary to release thyrotropin [also called thyroid-stimulating hormone]; in turn, this hormone stimulates the thyroid to secrete thyroid hormone [TH].)
Thyroid-stimulating hormone stimulates the thyroid gland to secrete thyroid hormone.
Prolactin stimulates milk production in the mammary glands in females. In males, it may make the testes more sensitive to luteinizing hormone.
Adrenocorticotropic hormone stimulates the adrenal cortex to secrete corticosteroids.
Follicle-stimulating hormone, a gonadotropin, stimulates the production of eggs in the ovaries of females and sperm in the testes of males.
Luteinizing hormone, a gonadotropin, stimulates ovulation and estrogen and progesterone synthesis in females and the secretion of testosterone by the testes in males.
Growth hormone, or somatotropin, acts on the entire body to promote protein synthesis, lipid and carbohydrate metabolism, and bone and skeletal muscle growth.
The posterior pituitary is made of neural tissue (in contrast with the anterior pituitary, which is made of glandular tissue). Instead of synthesizing hormones, the posterior pituitary stores the hormones antidiuretic hormone (ADH) and oxytocin (OT), which are synthesized by the hypothalamus.
Oxytocin stimulates contraction of the uterus during childbirth and triggers the release of milk from the breasts during lactation.
ADH (or vasopressin) acts on the kidneys to reduce urine volume and prevent dehydration.
Nerve fibers that form the posterior pituitary originate in the hypothalamus.
The hypothalamic neurons synthesize hormones, which they send down to the posterior pituitary to be stored until stimulated by the nervous system to release them.
Secretion of pituitary hormones occurs in phases or pulses.
Hormone secretion is controlled by the central nervous system and by the target organs through negative feedback.
Example: Cold stimulates hypothalamus to release thyrotropin-releasing hormone (TRH). TRH stimulates anterior pituitary to release TSH. TSH stimulates thyroid to release thyroid hormones. Thyroid hormones stimulate metabolism to increase warmth and inhibit the release of TSH by the pituitary.
The pineal gland is located on the roof of the brain’s third ventricle.
The pineal gland produces melatonin, a hormone that rises at night (when sunlight is absent) and falls during the day. High melatonin levels trigger sleepiness, making it a key factor in the sleep/wake cycle.
The pineal gland may also regulate the timing of puberty.
The thymus lies in the mediastinum just beneath the sternum.
In children, the thymus gland is large. It begins to shrink in puberty; by old age, it consists of mostly fat and fibrous tissue.
The thymus secretes thymosin and thymopoietin, which have a role in the development of the immune system. Because it secretes hormones, the thymus is a member of the endocrine system; the actions of the hormones make the thymus part of the immune system.
The thyroid is the largest endocrine gland; it consists of two large lobes connected by a narrow band of tissue called the isthmus.
The thyroid gland resides in the neck; it wraps around the anterior and lateral portions of the trachea.
Thyroid tissue is made of tiny sacs called thyroid follicles. Each follicle is filled with a thick fluid called thyroid colloid. The cells lining the sacs secrete the two main thyroid hormones: T3 (triiodothyronine) and T4 (thyroxine). (Unlike other glands, the thyroid gland can store the hormones for later use.)
Cells between the thyroid follicles (parafollicular cells) secrete the hormone calcitonin in response to increasing blood calcium levels; calcitonin triggers the deposition of calcium in bone and, thus, promotes bone formation.
Parathyroid glands lie on the posterior surface of the thyroid; they secrete parathyroid hormone (PTH) in response to low blood levels of calcium.
Most people have four parathyroid glands, but the number of glands, and their locations, can vary.
PTH is the main hormone used to maintain normal blood levels of calcium. (Normal nerve and muscle function, blood clotting, cell membrane permeability, and the function of certain enzymes all depend on adequate levels of calcium.)
PTH inhibits new bone formation and stimulates the breakdown of old bone, causing calcium (and phosphate) to move out of bone and into the blood. PTH encourages the kidneys to reabsorb calcium. PTH also prompts the kidneys to activate vitamin D, which is important for intestinal absorption of calcium.
Calcitonin (secreted by the thyroid) has antagonistic effects on parathyroid hormone (PTH). The interaction of these two hormones helps the body achieve calcium homeostasis.
Each adrenal gland is two distinct glands. The inner portion (adrenal medulla) consists of modified neurons and functions as part of the sympathetic nervous system. The outer portion (adrenal cortex) is glandular tissue and secretes steroid hormones (corticosteroids).
The adrenal medulla contains modified neurons (chromaffin cells) that act as part of the sympathetic nervous system. These cells secrete the catecholamines epinephrine and norepinephrine in response to stimulation.
Catecholamines prepare the body for physical activity by increasing heart rate and blood pressure, stimulating circulation to the muscles, and dilating the bronchioles; they also boost glucose levels (a source of fuel) by breaking down glycogen into glucose (glycogenolysis) and converting fatty acids and amino acids into glucose (gluconeogenesis).
The adrenal cortex consists of three layers of glandular tissue:
Zona glomerulosa (the outermost layer) secretes mineralocorticoids.
Zona fasciculata (the middle layer) secretes glucocorticoids.
Zona reticularis (the innermost layer) secretes sex steroids.
Aldosterone acts on the kidneys to promote Na+ retention and K+ excretion; in turn, it also causes water retention.
Cortisol (the principal glucocorticoid) helps the body adapt to stress and repair damaged tissues by stimulating the breakdown of fat and protein, converting fat and protein to glucose, and the release of fatty acids and glucose into the blood. Cortisol has an anti-inflammatory effect, suppresses the immune system if secreted over a long term, and is essential for maintaining a normal blood pressure.
Sex steroids include a weak form of androgen (that is converted to the more potent androgen testosterone) and small amounts of estrogen.
The pancreas contains both endocrine and exocrine tissues: the majority of the pancreas acts as an exocrine gland, but a small percentage serves an important endocrine function.
Exocrine cells (acini) secrete digestive enzymes into ducts that drain into the small intestine.
Interspersed with the exocrine cells are clusters of endocrine cells called pancreatic islets or the islets of Langerhans. The pancreatic islets contain alpha cells, beta cells, and delta cells.
Alpha cells secrete the hormone glucagon between meals, when blood glucose levels decline. Glucagon stimulates liver cells to convert glycogen into glucose and to convert fatty acids and amino acids into glucose (gluconeogenesis). The resulting glucose is released into the bloodstream, causing blood glucose levels to increase.
Beta cells secrete the hormone insulin. After eating, the levels of glucose and amino acids in the blood increase. Insulin stimulates cells to absorb these nutrients, causing blood glucose levels to decline.
Delta cells secrete somatostatin, a hormone that works within the pancreas to regulate the other endocrine cells. (It inhibits the release of glucagon, insulin, and growth hormone.)
Sex hormones stimulate the production of eggs (in females) and sperm (in males).
Estrogen helps promote the development of female characteristics (such as breast development) and contributes to the development of the reproductive system.
After ovulation, the corpus luteum (the tissue left behind after a rupture of a follicle during ovulation) secretes progesterone. Progesterone, in combination with estrogen, helps maintain the uterine lining during pregnancy.
Testosterone triggers the development of male sexual characteristics; it also sustains sperm production.
