endocrine system review

Endocrine system

The glands, tissues, and cells that secrete hormones.

Endocrinology

The study of the endocrine system and the diagnosis and treatment of its disorders.

Four avenues of cell-to-cell communication

Gap junctions, neurotransmitters, paracrines, and hormones.

Gap junctions

Channels joining cells that allow nutrients, electrolytes, and signaling molecules to pass through the cytoplasm.

Neurotransmitters

Chemical messengers released by neurons that diffuse across a synaptic cleft and bind to receptors.

Paracrines

Chemical messengers secreted by one cell that diffuse to nearby cells in the same tissue; also called local hormones.

Hormones

Chemical messengers transported by the bloodstream that stimulate physiological responses in cells of another tissue or organ.

Endocrine glands

Ductless glands that release their secretions into the bloodstream.

Exocrine glands

Glands that usually secrete their products through ducts onto an epithelial surface.

Fenestrated capillaries

Highly permeable capillaries with patches of large pores; common in endocrine glands.

Endocrine vs nervous system speed

The nervous system responds in milliseconds; hormones may take seconds to days to act.

Hormonal effect duration

Hormonal effects may last for days or longer after a stimulus ends.

Neuroendocrine cells

Cells that act like neurons but release their secretions into the bloodstream like endocrine cells.

Pituitary gland

A gland suspended from the hypothalamus and connected to it by the infundibulum.

Hypothalamus

A brain region that forms the floor and walls of the third ventricle and regulates pituitary activity.

Infundibulum

The stalk connecting the pituitary gland to the hypothalamus.

Sella turcica

A depression in the sphenoid bone that houses the pituitary gland.

Anterior pituitary

The adenohypophysis; about three-quarters of the pituitary and connected to the hypothalamus by the hypophyseal portal system.

Posterior pituitary

The neurohypophysis; nervous tissue that stores and releases hormones made in the hypothalamus.

Hypophyseal portal system

Primary capillaries in the hypothalamus, portal venules, and secondary capillaries in the anterior pituitary.

Releasing hormones

Hypothalamic hormones that stimulate anterior pituitary cells to secrete hormones.

Inhibiting hormones

Hypothalamic hormones that suppress pituitary hormone secretion.

Oxytocin source

Synthesized mainly by neurons in the paraventricular nuclei of the hypothalamus.

ADH source

Synthesized mainly by neurons in the supraoptic nuclei of the hypothalamus.

FSH

Follicle-stimulating hormone; stimulates ovarian follicle development and ovarian sex hormone secretion and stimulates sperm production in males.

LH

Luteinizing hormone; stimulates ovulation and progesterone secretion in females and testosterone secretion in males.

Gonadotropins

FSH and LH.

TSH

Thyroid-stimulating hormone; stimulates thyroid growth and thyroid hormone secretion.

ACTH

Adrenocorticotropic hormone; stimulates the adrenal cortex to release glucocorticoids, especially cortisol.

Prolactin

A hormone that stimulates mammary glands to synthesize milk after childbirth.

GH

Growth hormone or somatotropin; promotes tissue growth, mitosis, and cellular differentiation.

ADH function

Increases kidney water retention, reduces urine volume, and helps prevent dehydration.

ADH other name

Arginine vasopressin (AVP) in neuroscience literature.

Oxytocin functions

Sexual arousal and orgasm, emotional bonding, labor contractions, and milk ejection.

Negative feedback inhibition

A hormone inhibits further secretion by binding to the pituitary or hypothalamus.

TRH-TSH-TH pathway

Hypothalamus releases TRH, anterior pituitary releases TSH, and thyroid releases thyroid hormone.

Positive feedback example

Oxytocin during labor, continuing until the infant is born.

Osmoreceptors

Hypothalamic neurons that detect increased blood osmolarity and trigger ADH release.

Neuroendocrine reflex

Release of hormones in response to nerve signals.

IGF

Insulin-like growth factors produced largely in response to GH and involved in stimulating target cells.

GH direct targets

Especially cartilage, bone, muscle, and fat.

Protein-sparing effect of GH

GH mobilizes fat for fuel, reducing the need to break down protein.

Glucose-sparing effect of GH

GH mobilizes fatty acids and reduces dependence on glucose so cells do not compete with the brain.

GH and bone growth

IGF-I accelerates bone growth at epiphyseal plates and stimulates chondrocyte and osteogenic cell multiplication.

GH secretion increases with

Deep sleep, vigorous exercise, trauma, hypoglycemia, and to a lesser extent high-protein meals.

High-carbohydrate meal effect on GH

Tends to suppress GH secretion.

Pineal gland

An endocrine gland attached to the roof of the third ventricle.

Melatonin

A monoamine synthesized by the pineal gland at night from serotonin; associated with circadian rhythms.

Thymus

A bilobed gland in the mediastinum involved in endocrine, lymphatic, and immune functions.

Thymus function

Site of T-cell maturation.

Thymic hormones

Thymopoietin, thymosin, and thymulin.

Thyroid gland location

Adjacent to the trachea immediately below the larynx.

Thyroid isthmus

A narrow bridge of tissue usually joining the two thyroid lobes.

Thyroid follicles

Sacs filled with protein-rich colloid and lined by simple cuboidal follicular cells.

Follicular cells

Cells of thyroid follicles that secrete T4 and T3.

T4

Thyroxine or tetraiodothyronine; the thyroid secretes mostly T4.

T3

Triiodothyronine; the more potent thyroid hormone at target cells.

Primary effect of thyroid hormone

Increases the body's metabolic rate.

Calorigenic effect

An increase in heat production caused by thyroid hormone.

C cells / parafollicular cells

Thyroid cells located between follicles that secrete calcitonin.

Calcitonin

Hormone released in response to rising blood calcium; stimulates osteoblast activity and calcium deposition.

Parathyroid glands

Usually four small glands partially embedded in the posterior surface of the thyroid.

PTH

Parathyroid hormone; secreted in response to low blood calcium.

Adrenal glands

Suprarenal glands that sit on the superior surface of each kidney.

Adrenal medulla

Inner portion of the adrenal gland that acts as an endocrine gland and sympathetic ganglion.

Chromaffin cells

Neuroendocrine cells in the adrenal medulla that release catecholamines.

Adrenal medulla catecholamines

Mostly epinephrine, with norepinephrine and a trace of dopamine.

Epinephrine effects

Increases alertness, mobilizes energy fuels, raises glucose, heart rate, blood pressure, pulmonary airflow, and metabolic rate.

Adrenal cortex

Outer region of the adrenal gland that produces corticosteroids.

Corticosteroid categories

Mineralocorticoids, glucocorticoids, and sex steroids.

Zona glomerulosa

Outer adrenal cortex layer that secretes mineralocorticoids.

Zona fasciculata

Middle adrenal cortex layer that secretes glucocorticoids and androgens.

Zona reticularis

Inner adrenal cortex layer that secretes glucocorticoids and androgens.

Aldosterone

The most significant mineralocorticoid; causes kidneys to retain sodium and excrete potassium.

Aldosterone effect on blood pressure

Water follows retained sodium, helping maintain blood volume and pressure.

Cortisol

The most potent glucocorticoid; promotes fat and protein catabolism, gluconeogenesis, and anti-inflammatory effects.

Excess glucocorticoids

Can suppress the immune system.

Adrenal androgens

Sex steroids secreted by the adrenal cortex; DHEA is the major androgen.

Pancreatic islets

Endocrine cell clusters in the pancreas that regulate glycemia.

Alpha cells

Pancreatic islet cells that secrete glucagon.

Beta cells

Pancreatic islet cells that secrete insulin.

Delta cells

Pancreatic islet cells that secrete somatostatin.

Glucagon

A hormone released when blood glucose is falling; raises blood glucose through liver and fat actions.

Insulin

A hormone released during and after meals that promotes nutrient uptake, metabolism, and storage.

Insulin targets

Primarily the liver, skeletal muscle, and adipose tissue.

Insulin effects

Promotes synthesis of glycogen, fat, and protein and antagonizes glucagon.

Somatostatin from pancreatic delta cells

Released with insulin and inhibits stomach acid secretion.

Hyperglycemic hormone

A hormone that raises blood glucose.

Hypoglycemic hormone

A hormone that lowers blood glucose; insulin.

Gonads

Ovaries and testes; organs with both endocrine and exocrine functions.

Ovarian hormones

Chiefly estradiol, progesterone, and inhibin.

Theca cells

Ovarian cells that synthesize androstenedione.

Granulosa cells

Ovarian cells that convert androstenedione to estradiol and smaller amounts of estriol and estrone.

Corpus luteum

The remains of an ovarian follicle after ovulation; secretes progesterone.

Inhibin

A hormone that suppresses FSH secretion.

Sertoli cells

Sustentacular cells in seminiferous tubules that secrete inhibin.

Leydig cells

Interstitial cells between seminiferous tubules that produce gonadal steroids.

Testosterone functions

Promotes male reproductive development, male physique development, and sex drive.

Skin endocrine role

Epidermal keratinocytes use UV radiation to convert a cholesterol-like steroid into cholecalciferol.

Calcitriol

A calcium-regulating hormone produced through conversions involving the liver and kidneys; promotes intestinal calcium absorption.