Ch. 17 - Endocrine System

The endocrine system uses ____ while the nervous system uses ____.

1. hormones

2. neurotransmitters

4 Principal Mechanisms of Communication between Cells

1. Gap junctions

   • Pores in cell membrane allow signaling molecules, nutrients, and electrolytes to move from cell to cell

2. Neurotransmitters

   • Released from neurons to travel across synaptic cleft to second cell

3. Paracrines

   • Secreted into tissue fluids to affect nearby cells

   • local

4. Hormones

   • Chemical messengers that travel in the bloodstream to other tissues and organs

Endocrine System

glands, tissues, and cells that secrete hormones

• slower and more broad than the nervous system

  • The two systems can regulate each other

Endocrinology

the study of this system and the diagnosis and treatment of its disorders

Endocrine Glands

Organs that are traditional sources of hormones

Hormones

chemical messengers that are transported by the bloodstream and stimulate responses in cells of another tissue or organ

Exocrine Glands

• Have ducts

  • carry secretion to an epithelial surface or the mucosa of the digestive tract: “external secretions”

• Extracellular effects

  • leaves the body

  • ex) food digestion

Endocrine Glands

• No ducts

  • Have dense capillary networks which allow easy uptake of hormones into bloodstream

• Internal secretions

  • sends into bloodstream

• have intracellular effects such as altering target cell metabolism

What type of cell defies the classification of exocrine versus endocrine?

Liver cells

• releases hormones, releases bile into ducts, releases albumin and blood-clotting factors into blood (not hormones)

chemicals that function as both hormones and neurotransmitters:

• Norepinephrine

• Dopamine

• Antidiuretic hormone

Norepinephrine and glucagon both cause…

hydrolysis of glycogen in the liver

Target Organs/Cells

organs or cells that have receptors for a hormone and can respond to it

• Some target cells ave enzymes that convert a circulating hormone to its more active form

Hypothalamus

Regulates primitive functions from water balance and thermoregulation to sex drive and childbirth

• Shaped like a flattened funnel

• Forms floor and walls of third ventricle of brain

• Many of its functions carried out by pituitary gland

Pituitary Gland

• suspended from hypothalamus by the infundibulum

• Housed in sella turcica of sphenoid bone

• Composed of two structures with separate functions

  • Adenohypophysis (anterior pituitary)

  • Neurohypophysis (posterior pituitary)

Adenohypophysis (anterior lobe)

Makes up the anterior 3/4 of the pituitary

• larger, has lots of blood vessels

• connected to hypothalamus by blood capillaries

• Hypothalamus sends hypothalamic-releasing-and-inhibiting hormones to anterior pituitary

  • causes anterior pituitary to also make and release a hormone

Neurohypophysis (posterior lobe)

Makes up the posterior 1/4 of the pituitary

• made of nerve tissue, not a true gland

• receives signals from the hypothalamus which sends hormones down the axons to be stored in the posterior pituitary

• we do NOT make hormones in posterior pituitary, only store and release them

8 Hormones Produced in the Hypothalamus

• 6 regulate the anterior pituitary

• 2 are stored and released into capillaries in the posterior pituitary

Gonadotropin-Releasing Hormone (GnRH)

releasing hormone that promotes the secretion of TSH, PRL, ACTH, FSH, LH, and GH in the anterior pituitary

Hormones in the Anterior Pituitary (6):

1. Follicle-stimulating hormone (FSH)

2. Luteinizing hormone (LH)

3. Thyroid-stimulating hormone (TSH)

4. Adrenocorticotropic hormone (ACTH)

5. Prolactin (PRL)

6. Growth hormone (GH)

What are the 2 gonadotropin hormones that target gonads?

1. Follicle-stimulating hormone (FSH)

2. Luteinizing hormone (LH)

Follicle-stimulating hormone (FSH)

• Stimulates secretion of ovarian sex hormones

• Stimulates development of ovarian follicles

• Stimulates sperm production

Luteinizing hormone (LH)

• Stimulates ovulation

• Stimulates corpus luteum to secrete progesterone

  • corpus luteum is a yellow scar tissue mass left over after ovulation

• Stimulates testes to secrete testosterone

Thyroid-stimulating hormone (TSH)

Stimulates secretion of thyroid hormone

Adrenocorticotropic hormone (ACTH)

hormone goes to the adrenal cortex and stimulates it to release glucocorticoids

• increases glucose and mineral concentrations

Prolactin (PRL)

After birth, stimulates mammary glands to synthesize milk

Growth hormone (GH)

Stimulates mitosis and cellular differentiation

• growth hormone causes fat, muscle, and bone differentiation

• during sleep, GH is elevated, allowing us to better heal from daily injuries

• GH has widespread effects on the body tissues

• Bone growth, thickening, and remodeling are influenced, especially during childhood and adolescence

• GH promotes the breakdown of adipose

How Growth Hormone Works

Induces liver to produce growth stimulants.

This causes:

• Increase in protein synthesis

  • boosts transcription and translation, increases amino acid uptake, suppresses breakdown of proteins, helps build and repair tissue

• Increase in lipid metabolism

  • stimulates adipocytes to break down fats (spares proteins)

• Increase in carbohydrate metabolism

  • has a glucose-sparing effect as mobilizing fatty acids reduces dependence of most cells on glucose, freeing more glucose for the brain

  • stimulates glucose secretion from the liver

• Electrolyte balance

  • Promotes Na+, K+, and Cl- retention in the kidneys

  • enhances Ca absorption in intestines

  • makes electrolytes available to growing tissues

Insulin-like Growth Factors (IGF-I) and Somatomedins (IGF-II)

Growth stimulants

• Stimulate target cells in diverse tissues

• IGF-I prolongs the action of GH

• causes a greater effect of growth hormone

Growth Hormone highs and lows:

• Secretion high during first 2 hours of sleep

• Can peak in response to vigorous exercise

• Also activated by ghrelin, which is released by the empty stomach

• GH levels decline gradually with age

  • Lack of protein synthesis contributes to aging of tissues and wrinkling of the skin

  • Average 6 ng/mL during adolescence, 1.5 ng/mg in old age

  • fat percentage is higher as we age because growth hormone declines

Hormone Half-Life

the time required for 50% of the hormone to be cleared from the blood

• GH half-life: 6 to 20 minutes

• IGF-I half-life: about 20 hours

• protein based hormones have a very short half-life

• lipid based hormones have a long half-life

Hormones in the Posterior Pituitary (2):

1. Antidiuretic hormone

2. Oxytocin

ADH (antidiuretic hormone)

• Increases water retention, thereby reducing urine volume and preventing dehydration

• Also called vasopressin because it can cause vasoconstriction

• ADH raises blood pressure by making us absorb more water from our urine and by making our blood vessels vasoconstrict

Oxytocin (OT)

• triggers smooth muscle contraction

• Surge of oxytocin is released during sexual arousal and orgasm

• Promotes feelings of sexual satisfaction and emotional bonding between partners

• Stimulates labor contractions during childbirth

• Stimulates flow of milk during lactation

  • oxytocin releases milk

• May promote emotional bonding between lactating mother and infant

Control of Pituitary Secretion

Regulated by hypothalamus, other brain areas, and feedback from target organs

• rate of secretion is not constant

Hypothalamic and Cerebral Control of Pituitary Secretion

• Brain monitors conditions and influences anterior pituitary accordingly

  • In times of stress, hypothalamus triggers release of ACTH

  • During pregnancy, hypothalamus triggers prolactin secretion

• Posterior pituitary is controlled by neuroendocrine reflexs

  • Hypothalamic osmoreceptors trigger release of ADH when they detect a rise in blood osmolarity

  • Infant suckling triggers hypothalamic response to release oxytocin

Negative Feedback in Endocrine System

an increase in the target organ’s hormone level inhibits the release of hypothalamic and/or pituitary hormones

• ex) the more hormones the thyroid glands makes, the less stimulation they will receive to make hormones

• most feedback loops in endocrine system are negative

Positive Feedback in Endocrine System

occurs in the endocrine system but not as much as negative feedback

• example of positive feedback in endocrine system is oxytocin inducing contractions

Pineal Gland

Attached to the roof of the third ventricle beneath the posterior end of corpus callosum

• After age 7, it undergoes involution (shrinkage)

  • Down 75% by end of puberty

• Synchronizes physiological function with the 24-hour circadian rhythms

  • Makes melatonin from serotonin during the night

    • Fluctuates seasonally

• Pineal gland may influence timing of puberty in humans

Seasonal Affective Disorder (SAD)

occurs in winter or northern climates

• Symptoms: depression, sleepiness, irritability, and carbohydrate craving

• 2 to 3 hours of exposure to bright light each day reduces the melatonin levels and the symptoms (phototherapy)

Thymus

Plays a role in three systems: endocrine, lymphatic, and immune system

• located on the heart

• Goes through involution (shrinkage) after puberty

• site where T cells are made, which are important in immune defense

• Secretes hormones thymopoietin, thymosin, and thymulin

  • the 3 hormones are from the thymus and help us make T cells/lymphocytes

Thyroid Gland

Largest gland that is purely endocrine

• Made up of 2 lobes and an isthmus below the larynx

• Dark reddish brown color bc of rich blood supply

• Secretes thyroxine (T4) and triiodothyronine (T3) in response to TSH

  • increases metabolic rate, O2 consumption, heat production, appetite, growth hormone secretion, alertness, reflex speed

Thyroid Follicles

Sacs that make up most of the thyroid

• Contain protein-rich colloid

Follicular Cells

simple cuboidal epithelial cells that line follicles

Parafollicular (C or clear) Cells

cells that secrete calcitonin with rising blood calcium

• stimulates osteoblast activity and bone formation in children

Parathyroid Glands

4 glands partially embedded in the back of the thyroid gland

• secretes parathyroid hormone (PTH)

• cutting out the parathyroid gland would cause us to lose calcium homeostasis

Parathyroid Hormone (PTH)

increases blood Ca²⁺ levels

• promotes synthesis of calcitriol

• increases absorption of Ca

• decreases urinary excretion

• increases bone resorption

Adrenal Medulla

• is the small inner portion of the gland

• acts as an endocrine gland and a ganglion of the sympathetic nervous system

• made of modified sympathetic postganglionic neurons called chromaffin cells

  • When stimulated, they release catecholamines (epinephrine and norepinephrine) and some dopamine directly into the bloodstream

Effects of Catecholamines as Hormones

• Increase alertness and prepare body for physical activity

  • Mobilize high-energy fuels, lactate, fatty acids, and glucose

  • Glycogenolysis and gluconeogenesis boost glucose levels

  • Epinephrine inhibits insulin secretion and so has a glucose-sparing effect

• Increases blood pressure, heart rate, blood flow to muscles, pulmonary airflow, and metabolic rate

• Decreases digestion and urine production

Adrenal Cortex

Adrenal cortex surrounds the medulla and secretes several corticosteroids (hormones) from 3 layers of glandular tissue

• zona glomerulosa

• zona fasciculata

• zona reticularis

Zona Glomerulosa

thin, outer layer of cortex

• Cells are arranged in rounded clusters

• Secretes mineralocorticoids

  • these regulate the body’s electrolyte balance

Zona Fasciculata

thick, middle layer of cortex

• Cells arranged in fascicles separated by capillaries

• Secretes glucocorticoids and androgens (testosterone)

Zona Reticularis

narrow, inner layer of cortex

• Cells make a branching network

• Secretes glucocorticoids and sex steroids

Mineralocorticoids

Steroid hormones that regulate electrolyte balance

• secreted in the zona glomerulosa

• aldosterone stimulates Na+ retention and K+ excretion

  • Na+ pulls more water into the blood

  • means less urine being produced

  • plays a role in the renin-angiotensin-aldosterone

    • regulates blood pressure and fluid balance

Glucocorticoids

• Secreted by the zona fasciculata and zona reticularis in response to ACTH

• Regulates metabolism of glucose and other fuels

• Cortisol and corticosterone stimulate fat and protein breakdown, gluconeogenesis, and release of fatty acids and glucose into the blood

• Helps body adapt to stress and repair tissues

• Anti-inflammatory effect with short-term use

• immune system suppression with long-term use

Sex Steroids

Secreted by the zona fasciculata and zona reticularis

• includes androgens and estradiol

Androgens

sets libido (sex drive) throughout life

• large role in prenatal male development

  • includes Dehydroepiandrosterone (DHEA) which other tissues convert to testosterone

Estradiol

produced in the ovaries, but some is released from adrenal glands

• this becomes important after menopause for sustaining adult bone mass

• a type of estrogen hormone

• is most abundant of female hormones

Interdependence of the Adrenal Cortex and the Adrenal Medulla

The medulla and cortex of the adrenal gland depend on each other

• Medulla shrinks/breaks down without the stimulation of cortisol

• Some chromaffin cells in the medulla extend into the cortex

  • They stimulate the cortex to secrete corticosteroids when stress activates the sympathetic nervous system

Pancreatic Islets

sections of the pancreas made up of three different kinds of cells:

• Beta cells

• Alpha cells

• Delta cells

Glucagon

• secreted by the alpha cells of the pancreas

• Released between meals when blood glucose concentration is lower

• Raises blood sugar levels

  • does this in the liver

  • stimulates gluconeogenesis, glycogenolysis, and the release of glucose into the blood

  • In adipose tissue, stimulates fat catabolism and release of free fatty acids

• Glucagon is also released when amino acid levels in blood are rising to promotes amino acid absorption and provide cells with raw material for gluconeogenesis

Insulin

• secreted by beta cells in the pancreas

• Secreted during and after meal when glucose and amino acid blood levels are rising

• Stimulates cells to absorb these nutrients and store or metabolize them

  • lowers blood glucose levels

  • Promotes synthesis of glycogen, fat, and protein

  • Suppresses use of already-stored fuels

  • Brain, liver, kidneys, and RBCs absorb glucose without insulin, but other tissues require insulin

• Insufficiency or inaction is cause of diabetes

Somatostatin

• secreted by D or delta (δ) cells

• Partially suppresses secretion of glucagon and insulin

• slows down digestive activity so we can pull more nutrients out of food

Hyperglycemic Hormones

raise blood glucose concentration (includes hormones from other glands)

• Glucagon, growth hormone, epinephrine, norepinephrine, cortisol, and corticosterone

Hypoglycemic Hormones

lower blood glucose

• Insulin

Gonads

Ovaries and testes - sex glands

• both endocrine and exocrine

• Exocrine product: whole cells

  • eggs and sperm (cytogenic glands)

• Endocrine product: gonadal hormones

  • mostly steroids

Ovarian Hormones

Estradiol, progesterone, and inhibin

Testicular Hormones

Testosterone, weaker androgens, estrogen, and inhibin

Ovary

synthesizes a lot of female sex hormones which come from the ovarian follicle

• Theca cells synthesize a hormone, then that hormone is converted to mainly estradiol by the granulosa cells

• After ovulation, the remains of the ovarian follicle becomes the corpus luteum

  • Secretes progesterone for 12 days following ovulation

  • Follicle and corpus luteum secrete inhibin

Functions of Estradiol and Progesterone

• Develops female reproductive system and physique, including adolescent bone growth

• Regulates menstrual cycle, sustains pregnancy

  • progesterone causes uterine lining to thicken

  • uterine glands don't get activated until exposed to progesterone

  • progesterone lowers which causes the uterine lining to shed

• Prepares mammary glands for lactation

Inhibin

suppresses FSH secretion from the anterior pituitary

• released from Nurse cells

• Limits FSH secretion in order to regulate sperm production

Testes

• Microscopic seminiferous tubules produce sperm

• Tubule walls contain sustentacular (Sertoli) cells

  • are nerve cells

• Leydig cells (interstitial cells) lie in clusters between tubules

• testosterone is main hormone in the testes

Functions of Testosterone

Testosterone and other steroids from interstitial cells nestled between the tubules

• Stimulates development of male reproductive system in fetus and adolescent, and sex drive

• Sustains sperm production

Endocrine Functions of Skin

Keratinocytes convert a cholesterol-like steroid into cholecalciferol using UV from sun

• vitamin D acts like a hormone

Endocrine Functions of Liver

• involved in the production of at least five hormones

• Converts cholecalciferol into calcidiol

• Secretes angiotensinogen (a prohormone)

  • Precursor of angiotensin II (a regulator of blood pressure)

• Secretes 15% of erythropoietin (stimulates bone marrow)

  • increases red blood cell production

• Source of IGF-I that controls action of growth hormone

Hepcidin

promotes intestinal absorption of iron

Endocrine Functions of Kidneys

plays a role in production of at least 3 hormones

• converts calcidiol to calcitriol, the active form of vitamin D

  • increases calcium absorption by intestine and inhibits loss in the urine

• secretes renin that converts angiotensinogen to angiotensin l

  • angiotensin ll created by converting enzyme in lungs

    • constricts blood vessels and raises blood pressure

• produces 85% of eyrthropoietin

  • stimulates bone marrow to produce red blood cells

Endocrine Functions of the Heart

• the atrial muscle secretes 2 natriuretic peptides in response to an increase in blood pressure

• theses decrease blood volume and blood pressure by increasing Na+ and H2O output in the kidneys, and the opposite affect for angiotensin ll

• lowers blood pressure

• makes us lose fluid through urination more to lower the BP

Endocrine Functions of the Stomach and Small Intestines

secrete at least 10 hormones secreted by enteroendorine cells

• coordinates digestive motility and glandular secretion

Endocrine Functions of Adipose

adipose tissue secretes leptin

• Slows appetite

Endocrine Functions of Osseous Tissue

Osteocalcin is secreted by osteoblasts

• makes us have more beta cells in the pancreas which means more insulin so we can lower our blood sugar

• Inhibits weight gain and onset of type 2 diabetes

Endocrine Functions of Placenta

Secretes estrogen, progesterone, and others

• Regulates pregnancy, stimulates development of fetus and mammary glands

Type 1 Diabetes

Immune disorder that results in loss of beta cells in the pancreas

• results in blood sugar that is too high

• body does not produce insulin in the body, so blood sugar cannot absorb into adipose tissue, so adipose will have a hard time making body fat

Type 2 Diabetes

Primarily linked to unhealthy lifestyle

• Type II diabetes is more common

  • More body fat decreases insulin sensitivity of other cells

  • Target cells have fewer insulin receptors

Senescence of Endocrine System

Endocrine system

• Degenerates less than any other system

• Only reproductive, growth, and thyroid hormones show major declines

• Other hormones secreted at fairly stable rate

  • Target cell sensitivity may decline

Pituitary gland

• is less sensitive to negative feedback inhibition by adrenal glucocorticoids

• Response to stress is prolonged