Chapter 16: Raging Hormones: The Endocrine System Summary

The endocrine system

The endocrine glands are located in various hormone-secreting tissues throughout the body. They are about ten in number.

Endocrine glands do not have ducts that convey the hormones they secrete. The secreted endocrine hormones are released into extracellular spaces surrounding the glands from where they move into blood capillaries and then into the bloodstream.

Hormones can either be steroidal (cholesterol derivatives) or nonsteroidal(protein derivatives). Steroid hormones include estrogen, progesterone, testosterone, and cortisol.

Nonsteroidal hormones are divided into:

• Modified amino acid derivatives such as norepinephrine, epinephrine, and melatonin.
• Peptide-based hormones including oxytocin, antidiuretic hormone, and melanocyte-stimulating hormone.
• Glycoprotein based hormones which include luteinizing hormone, follicle-stimulating hormone, and chorionic gonadotropin.
• Protein-based hormones such as insulin, parathyroid, and prolactin hormone.

Functions of hormones

1. They regulate the internal body environment controlling their volume and chemical composition.
2. They stimulate response to environmental changes to allow the body to adapt.
3. Hormones promote development and growth by promoting important steps in reproduction.

Glands of the endocrine system

The major endocrine glands include adrenal, pituitary, parathyroid, thyroid, pineal, thymus, gonads, and islets of Langerhans.

The hypothalamus links the nervous system and the endocrine system. The hypothalamus generates neurohormones that regulate the pituitary gland, facilitates fluid and food intake, and controls body heat and weight.

The pituitary gland: It is located below the hypothalamus. The anterior lobe of the pituitary gland maintains and regulates other endocrine glands. Tropic hormones act on other endocrine glands. The anterior lobe produces polypeptide hormones.

• The hormones produced by the anterior lobe of the pituitary hormone include:
  • Follicle-stimulating hormone: It stimulates the Graafian follicle in the ovary to mature into an ovum which then secretes estrogen hormone.
  • Luteinizing hormone: It activates the formation of the corpus luteum on the ovary surface after releasing an ovum.
  •  Prolactin hormone: It promotes the production of milk in the mammary glands.
  • Thyroid-stimulating hormone: It regulates the secretion of triiodothyronine and thyroxin hormones.
• The pituitary’s posterior lobe releases and stores hypothalamus secretions. The hormones secreted by the posterior lobe include:
  • Oxytocin: promotes uterine wall smooth muscle contraction during childbirth and milk production in lactating mothers.
  • Antidiuretic hormone (vasopressin): stimulates smooth muscle contraction in blood vessels, increases water reabsorption in the kidney, and reduces urine production.

Adrenal glands

• The adrenal glands are located at the top of the kidneys. Each kidney contains a medulla and a cortex. The cortex contains various zones(layers) that release different hormones:
  • Zona glomerulosa secretes aldosterone.
  • Zona fasciculata produces cortisone.
  • Zona reticularis produces low amounts of gonadotropin.
• The hormones secreted by the cortex include:
  • Aldosterone: controls retained body electrolytes. It reduces urine output and enhances water conservation.
  • Cortisone (cortisol): it is an insulin antagonist increasing glucose production and maintains normal sugar levels.
  • Estrogen and androgens: These are sex hormones. estrogen generates feminine characteristics. Androgens accelerate maleness.

Thyroid gland

The thyroid gland is the largest gland containing two lobes joined together with a flesh called the isthmus. It is located in front of the neck and below the trachea.

An iodine pump is used to transport iodides into the bloodstream to be used in the synthesis of triiodothyronine and thyroxin which control the rate of metabolism in the body.

Parathyroid gland

The parathyroid gland contains four glands with pea-like size located posterior to the thyroid gland and it produces the parathyroid hormone.

Parathyroid hormone controls the calcium balance in the blood and bones as well as controlling calcium in excreted urine.

The pancreas

The pancreas is considered to be an endocrine and an exocrine gland. This is because it releases some hormones through ducts and others are secreted into the bloodstream directly.

The endocrine glands in the pancreas are cell clusters called islets of Langerhans.

• Types of cells found in islets include:
  • A Cells (alpha cells)  produce glucagon hormone that increases blood sugar.
  • B cells (beta cells) produce insulin hormone that reduces blood sugar levels, increases lipid synthesis, and promotes the synthesis of proteins.
  • D cells (delta cells) produce somatostatin hormone that inhibits glucagon and insulin secretion.
  • F cells (pp cells) produce a polypeptide that controls pancreatic digestive enzymes release.

Revision

endocrine system organs
all glands!!: pituitary, thyroid, thymus, adrenal gland, pancreas, ovary, testis

what does the endocrine system do?
uses chemical signals to regulate the body and keep homeostasis

how does the endocrine system work?

1. brain issues a command (most of the time)
2. an obedient gland produces a chemical signal, hormone, that circulates in the blood
3. the hormone finds its target cell
4. the target cell changes

hormones
chemicals released from one type of cell that travel near and far in the body to affect changes in other types of cells

glands
tissues that produce hormones controlled (directly or indirectly) by the central nervous system. they release hormones into the bloodstream

fight or flight
threat: attack, harmful event
brain: processes the signals, releases ACTH
ACTH: (adrenocorticotropic hormone) pituitary gland secretes this hormone
cortisol and adrenaline are released
lead to physical effects

cns controls…
the pns and endocrine function

the pns (muscle function)
stimulates muscles to contract

endocrine system (muscle function)
signal muscle cells to grow
trigger release of glucose to power contractions
cool the body when muscles overheat

controlling the body map
sensory -> cns -> gland -> hormone -> target

comparison of signals from pns to those from endocrine system
pns: short lived, fast signal, targets cells that are near
endocrine: slow signal that is long lived and can target cells from far away

different types of hormones based on where they act
endocrine: far
paracrine: regional
autocrine: very close (itself or neighbors)

hormone based
protein or fat

protein based hormones
amines: small size, derived from single amino acids, adrenaline, t3, t4;
polypeptides: mid size, 100 amino acids or fewer, insulin and ADH;
glycoproteins: large size, more than 100 amino acids and decorated with sugars, FSH, LH <– sex hormones;
water soluble; bind to receptors target cell’s surface;
cannot cross the phospholipid bilayer; always needs to have a second messenger because it will only bind to the receptor on the cell’s surface, binding changes the formation of the receptor, receptor is activated to make enzymes; activates about 10 secondary messengers

lipid (fat) based hormones
steroids: derived from cholesterol- testosterone, estrogen, progesterone, aldosterone, cortisol; lipids and are not water soluble and need to travel through the blood bound to a carrier protein; can slip right into the cell through the plasma membrane; receptors for steroid hormones are in the cytoplasm (metabolism effect) or nucleus (gene expression)

proteins and steroid hormons both…
have specific receptors associated with target cells; can induce target cells to change gene expression, metabolism and protein secretion

second messenger for protein hormones
produced by adenylyl cyclase from ATP= cAMP, turns substrates into products

hypothalamus
part of the brain, master controller of the endocrine system, made of nervous tissue

pituitary glands
connected to the brain, right next to hypothalamus, made of cells that secrete hormones
made of two parts: posterior and anterior

regulatory cascade
one signal activates a gland, which produces another signal which activates something else

posterior pituitary
stores hypothalamus hormones for later release, extension of the hypothalamus

anterior pituitary
makes its own hormones, discrete gland

hormones made by the hypothalamus
ADH (produced when there’s low blood volume due to dehydration; acts on the kidney and smooth muscles of blood vessels to do vasoconstriction), oxytocin (released when someone goes through childbirth, contracts uterus, releases milk; bonding hormone), both are stored in posterior pituitary

hormones produced by anterior pituitary
TSH (thyroid-stimulating hormone); ACTH (acts on adrenal glands – on top of kidneys- and tells them to produce other proteins); growth hormone (acts directly on tissue, promotes growth of bone and muscle); FSH and LH (sex hormones, glycoproteins); prolactin (tells mammary glands to produce milk)

map of endocrine system feedback reg
hypothalamus–> releasing hormones–> anterior pituitary–> tropic hormones–> target glands–> hormones which do negative feedback to stop the body from producing hormones

pancreas
functions to regulate blood sugar levels: not too high, but not too low, secretes glucagon and insulin

insulin
tells cells in body to take sugar inside cells (cellular uptake of glucose), tells liver cells to store by converting glucose into glycogen (storage carb), tells fat cells to convert glucose into fat

glucagon
tells liver cells to convert glycogen into glucose, tells fat cells to break down and release glucose into blood stream

alpha cells
secrete glucagon

beta cells
secrete insulin

thyroid
makes two hormones: thyroxine and calcitonin;
hypothalamus signals to anterior pituitary to release thyroid-stimulating hormone (TSH), thyroid is stimulated to produce thyroxine– requires iodine
if no iodine: thyroxine shuts down the anterior pituitary and the hypothalamus (negative feedback). goiter (enlarged thyroid): lack of negative feedback due to having iodine deficiencies

thyroxine hormone
made in thyroid; increases metabolic rate and promotes growth;

calcitonin hormone
made in thyroid; inhibits the release of calcium from the bones, activates osteoblasts

glands important to regulation of calcium in blood
parathyroid, thyroid

parathyroid
produces parathyroid hormone which is essential for life; produces PTH when low calcium in the blood, stimulates osteoclasts which release the calcium to increase the calcium level in the blood; NOT activated by a signal from the brain

PTH (parathyroid hormone)
produced by parathyroid; stimulates osteoclasts

condition: high sugar in blood
insulin released from the pancreas (beta cells), targets the liver and fat cells which take in sugar to store it as glycogen/ fat, all cells take in sugar

condition: low sugar in blood
glucagon released from the pancreas (alpha cells), targets the liver which converts glycogen to glucose and releases it into the blood, fat cells convert into glucose, release into blood

condition: high Ca2+ in blood
calcitonin released by the thyroid, targets osteoblasts which make bone and store Ca2+, inhibits osteoclasts from working/ emitting Ca2+

condition: low Ca2+ in blood
PTH released by parathyroid, targets osteoclasts to degrade bone and release Ca2+

Endocrine System
System of communication in multicellular organisms through chemical signals such as neurotransmitters, local regulators, hormones, and pheromones

Exocrine glands
Secrete chemicals outside your body (sweat)

Endocrine glands
Secrete chemicals inside your body into your bloodstream (endorphins & enkephalins, neurotransmitters, local regulators, & hormones)
Examples: hypothalamus & pituitary gland, thyroid/parathyroid glands, pancreas, testes/ovaries

Key Principles of Signaling Mechanisms

1. Hormones act at low concentrations
2. Target cell/tissues are very specific
3. The same hormone can affect different target cells differently
   **Endocrine gland secretes chemicals into the bloodstream, through which they travel to their target cell

Hypothalamus

• In the brain
• Bridge between the nervous system and the endocrine system
• Receives and sends out nerve impulses & hormonal signals
• Tells the pituitary gland what to do

Antagonistic pairs
Hormones work together in feedback mechanisms

• insulin & glucagon: high blood glucose=insulin to store glucose, low blood glucose=glucagon to release glucose
• oxytocin: caused by contractions, causes more contractions

Peptide (protein) hormones

• Cannot get into lipid bilayer b/c hydrophilic
• Bind to a receptor on the outside of the target cell
• Activate a secondary messenger within the protein (usually cAMP) and that activates enzymes within the target cell to do something

Steroid (lipid) hormones

• Can pass through the lipid bilayer b/c hydrophobic
• Bond to a receptor inside the cytoplasm of the target cell
• Receptor+hormone complex goes into the nucleus, binds to the DNA, begins transcription/translation processes of a gene or genes
• Makes a protein (that wasn’t already present in the cell

Differences between peptide and steroid hormones

• peptide hormones cannot pass through the lipid bilayer, steroid hormones can
• peptide hormones activate enzymes that are already present in the target cells, steroid hormones create proteins within their target cells

Thyroid gland & goiters

• Thyroid gland produces T3 & T4 (made of iodine)
• When you don’t have enough iodine, then your thyroid gland gets really big trying to fulfill the shipment order of T3 & T4, so you get a goiter

Scrotum
Holds the testicles outside the body because they need to be cooler than body temperature

Testis
Where sperm production happens

Epididymis
Where sperm cells are stored

Urethra
Tube where sperm leaves from

Vas deferens
Tube between epididymis and urethra

Prostate gland, cowper’s gland, seminal vesicles
Make semen

Prostate gland
40% of semen

• Provides nutrients

Seminal vesicles
60% of semen

• Alkaline, which neutralizes the vagina’s acidity

Cowper’s gland
Releases lubrication before ejaculation

Hormonal control of male reproductive system

Ovaries
Where eggs are stored (in follicles)

Fallopian tubes
Where eggs are transported between the ovaries and the uterus

Uterus
The womb

Endometrium
Lining inside uterus

Vagina
Yup

Cervix
Opening to uterus

Hormonal control of female reproduction

• Hypothalamus releases GnRH
• Pituitary releases LH, FSH
• FSH gets one egg cell to start meiosis
• Ovaries start releasing estrogen => thickens up lining of the uterus & tells pituitary to release LH
• LH causes ovulation (follicle [egg] turns into corpus luteum
• Corpus luteum (in ovary) releases progesterone => thickens uterus lining, progesterone = negative feedback on hypothalamus, causes GnRH to stop being released
• If egg is fertilized, egg goes into the lining of the uterus and HCG is released, which tells the corpus luteum to keep releasing progesterone (so no more GnRH)
• If egg is not fertilized, the it just leaves and the progesterone is not released, so the uterus lining stops thickening, so no estrogen is released