Chapter 9: The Endocrine System
Function: coordinates functions of body with nervous system
Hormones: chemical substances that are secreted by endocrine cells into ECF
 Mediator molecule released in one part of the body but regulates activity in other parts of the body
 Regulate metabolic activity of other cells
 Affects target cells/organs
Endocrine vs. Nervous
 Endocrine: release hormones as chemical signal, work more slowly, can target any cell in the body
Control of Hormone Release
 Negative feedback mechanisms: chief means of regulating blood levels of nearly all hormones
o Stimulus triggers hormone secretion and rising levels inhibit further release
o Maintaining homeostasis! (levels of hormones within a narrow range)
 Positive feedback can also control (i.e. oxytocin)
 Stimuli that activate endocrine organs:
o Hormonal: stimulated by other hormones
i.e. hypothalamic hormones stimulate anterior pituitary
o Humoral: stimulated by changing blood levels of certain ions and nutrients
i.e. blood calcium levels stimulate release of parathyroid hormone
o Neural: stimulated by nerve fibers
i.e. sympathetic nervous system stimulates adrenal medulla
Endocrine & Exocrine Glands
 Glandular tissue is epithelial tissue!
 Endocrine gland: secrete hormones directly into internal environment
o No ducts
o Secrete hormones
 Exocrine glands: secrete substances into external environment
o Have ducts
o Secrete substances such as sweat, oil, wax, enzymes, etc.
Hormone Receptors
 Receptor: protein on cell membrane of target cell
o i.e. insulin (secreted by pancreas) – liver has insulin-specific receptors that receive insulin and tell liver to take
sugar in and transfer to glycogen for storage
 Two types of hormones:
o Circulating: deposited into fluid/blood, circulates through body
o Local: acts on neighboring cells or itself (paracrine – neighboring) or autocrine (targets itself)
Water-soluble vs. Lipid-soluble
 Chemical classes determine how easily the hormone gets into and moves through the blood stream and the cell membrane
 Lipid-soluble: nonpolar
o Hard to transport because fluids are water-based; need to be bound to transport protein (“chaperone”) that allows
it to move around
o Once it gets to target cell, can move across membrane without protein channel
o i.e. steroid hormones
 Water-soluble: polar
o No help needed for transport
o Cannot go through membrane of target cell – needs a protein receptor; binds to receptor and activates second
messenger system
o i.e. amines, peptide/protein, glycoproteins, eicosanoids
Major Endocrine Organs
 Hypothalamus: major link between nervous and endocrine system!
 Pituitary Gland
o Hangs from inferior surface of hypothalamus
o Some hormones released are tropic hormones: stimulate other endocrine glands to release hormones
o Two lobes:
1. Anterior pituitary (glandular tissue): “master endocrine gland” – controls so many others!
2. Posterior pituitary (nervous tissue): stores 2 hormones for hypothalamus and secretes them
Hormones of the Anterior Pituitary
o Hormones secreted with non-endocrine targets
Growth hormone (GH): general metabolic hormone, growth of skeletal muscles and long bones of the body,
promotes healing and increases ATP
Pituitary dwarfism: hyposecretion during childhood
Gigantism: hypersecretion during childhood
Acromegaly: hypersecretion after growth plates closed
Prolactin (PRL): targets mammary glands in females to stimulate and maintain milk production after childbirth
Tropic hormones
Adrenocorticotropic hormone (ACTH): regulates endocrine activity of the adrenal cortex
Thyroid-stimulating hormone (TSH): influences growth & activity of thyroid, stimulates release of thyroid
hormones (aka thyrotropic hormone or TH)
Gonadotropic hormones (hyposecretion causes sterility)
Follicle-stimulating hormone (FSH): stimulates follicle development in ovaries and sperm development by
testes
Luteinizing hormone (LH): triggers ovulation of egg from ovary and production of progesterone and
estrogen; in men, stimulates testosterone production by testes
Hormones of the Posterior Pituitary
o Posterior pituitary does not make hormones – it stores them for the hypothalamus
o Hypothalamus makes two hormones that are transported via neurosecretory cells to be stored in the posterior
pituitary
Oxytocin: released in significant amounts only during childbirth and in nursing women; contractions of uterus
& milk ejection
Pitocin is synthetic form: used for inducing labor and to control hemorrhage and increase uterine muscle
tone
Antidiuretic hormone (ADH): inhibits or prevents urine production and causes kidneys to reabsorb more
water from the forming urine
Thyroid Gland
o Located at the base of the throat, inferior to the Adam’s apple
Thyroid hormone - two active iodine-containing hormones thyroxine (T4) & triiodothyronine (T3)
targets all cells
controls rate at which glucose is converted to energy (increases basal metabolic rate)
Maintains body temperature
important for normal tissue growth and development; accelerates body growth
Goiter: enlarged thyroid gland resulting in diet deficient in iodine
Cretinism (hypothyroidism)
Graves’ disease (hyperthyroidism)
Calcitonin: inhibits osteoclasts and decreases calcium level in the blood; causes calcium to be deposited in the
bone
Parathyroid Glands
o Glandular tissue found on posterior surface of thyroid gland (“piggybacks” the thyroid)
Parathyroid hormone (PTH): regulates ions (calcium, magnesium, phosphate); antagonist to calcitonin
Most important regulator of blood calcium levels!
Increases blood calcium levels by promoting resorption of calcium from bone matrix into blood (increases
activity of osteoclasts)
Also acts on kidneys – slows loss of ions from blood to urine
Adrenal Glands
o Two bean-shaped glands that curve over the top of the kidneys
o Functionally two organs in one:
1. Adrenal cortex (glandular)
Corticosteroids (mineralocorticoids, glucocorticoids, sex hormones)
2. Adrenal medulla (neural)
When stimulated by sympathetic nervous system, releases catecholamines
epinephrine (adrenaline)
norepinephrine (noradrenaline)
o Stimulated in times of stress (“fight or flight”)
o Allows body to better deal with short-term stressors
Hormones of the Adrenal Cortex
o Mineralocorticoids: regulate mineral (salt) content of the blood (Na+/K+ ions) – target kidney tubules to regulate
electrolyte and water balance in body
Aldosterone
o Glucocorticoids: promote normal cell metabolism and help body to resist long-term stressors (increase blood
glucose levels); anti-inflammatory properties
Cortisone
Cortisol
PTSD – stress hormones produced after traumatic event
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Sex hormones
Androgens (male - testosterone)
Estrogens (female)
o Disorders
Addison’s Disease (hyposecretion of adrenal cortex hormones)
Cushing’s Syndrome (hypersecretion of cortisol)
Congenital Adrenal Hyperplasia (CAH): genetic disorder (don’t make cortisol)
Pancreatic Islets
o Pancreas is a mixed gland
Pancreatic islets (endocrine – also called the islets of Langerhans)
Pancreas also produces enzymes for the digestive system (exocrine)
Insulin – released by beta cells in response to high levels of glucose (acts on all body cells) – decreases blood
glucose levels
Diabetes mellitus (hyposecretion)
Glucagon – antagonist of insulin; released by alpha cells; helps regulate blood glucose levels by increasing
them (primarily targets liver, stimulating it to break down stored glycogen)
Pineal Gland
o Small, cone-shaped gland that hangs from roof of third ventricle of brain (diencephalon - epithalamus)
Melatonin – “sleep trigger” that plays an important role in establishing the body’s day-night cycle; sets
“biological clock” and rhythms
o Light can affect gland – Seasonal Affective Disorder (SAD) & Jet Lag
Thymus
o Located in upper thorax, posterior to sternum (lymphatic organ)
Thymosin – promotes normal development of T lymphocytes (immunity); may slow aging process
Gonads
o Produce sex hormones identical to those produced by adrenal cortex
o Release of hormones stimulated by anterior pituitary gonadotropins
Hormones of the ovaries
Estrogens: development of sex characteristics in women (growth and maturation of reproductive organs) and
the appearance of secondary sex characteristics at puberty
Progesterone: acts with estrogen to bring about menstrual cycle, prepares mammary tissue for lactation,
maintains pregnancy
Hormones of the testes
Androgens (testosterone): promotes growth and maturation of the reproductive system organs at puberty;
causes secondary sex characteristics to appear; necessary for production of male gametes
o Hyposecretion of these hormones causes sterility
Other Tissues/Organs
o Hormone-producing cells are found in fatty tissue and in walls of small intestine, stomach, kidneys, and heart
(Table 9.2, page 331)
o Placenta: organ formed temporarily in uterus of pregnant women that acts as a respiratory, excretory, and
nutrition-delivery system for fetus
Produces several hormones that help to maintain the pregnancy and prepare for delivery of the baby
Human chorionic gonadotropin (hCG): stimulates the ovaries to continue producing estrogen and
progesterone so that the lining of the uterus is not sloughed off in menses
Human placental lactogen (hPL) – works with estrogen and progesterone in preparing the breasts for
lactation
Relaxin: causes mother’s pelvic ligaments and the pubic symphysis to relax and become more flexible, which
eases birth passage