10.2 - The Endocrine System

What is the endocrine system?

• A system of glands located throughout the body

• Produces and secretes hormones

How are hormones released by endocrine glands?

• Directly into the bloodstream

• No ducts are used

How are hormones transported in the body?

• Carried through the blood

• Reach target organs and tissues

What is the main function of the endocrine system?

• Regulate body processes using hormones

• Help maintain homeostasis across the body

What is the hypothalamus?

• A region of the brain

• Part of both the nervous system and endocrine system

• Controls the endocrine system

What is the main role of the hypothalamus in hormone control?

• Releases hormones that control the pituitary gland

• The pituitary gland then controls other endocrine glands

What are neurohormones?

• Hormones produced by nerve cells in the hypothalamus

• Travel along neurons

• Diffuse into the blood

• Travel to the pituitary gland to trigger hormone release

How is the hypothalamus controlled?

• By other parts of the nervous system

• Through feedback signals from the body

What feedback system regulates the hypothalamus?

• Negative feedback

• Hormone levels in the blood signal the hypothalamus to increase or decrease hormone release

Why is the hypothalamus important for homeostasis?

• Links the nervous and endocrine systems

• Helps coordinate long-term and short-term body regulation

• Maintains stable internal conditions through feedback control

What is the pituitary gland?

• Known as the “master gland”

• Controls other endocrine glands

• Located in the brain, connected to the hypothalamus

What are the two lobes of the pituitary gland?

• Anterior pituitary

• Posterior pituitary

What does the anterior pituitary do?

• Produces and secretes several important hormones

• Releases hormones into the bloodstream to regulate other glands

What does the posterior pituitary do?

• Stores and releases hormones made by the hypothalamus

• Mainly releases ADH and oxytocin

How is the hypothalamus connected to the anterior pituitary?

• Through a portal blood vessel system (hypothalamic-pituitary portal vein)

• Allows direct hormone communication between them

What hormones does the hypothalamus produce for the anterior pituitary?

• Releasing hormones (stimulate hormone release)

• Inhibiting hormones (block hormone release)

What happens after hormones reach the anterior pituitary?

• Endocrine cells in the anterior pituitary release their own hormones

• These hormones enter the bloodstream

• Travel to target organs and glands

Why is the pituitary gland called the “master gland”?

• Because it controls many other endocrine glands

• It is directed by the hypothalamus but coordinates major body hormone systems

what is the anterior pituitary gland? (secretes?)

• Secretes 8 major hormones

• Produces its own hormones

• Controlled by hypothalamus

What are the 8 anterior pituitary hormones?

• Prolactin (PRL)

• Growth hormone (GH)

• Thyroid-stimulating hormone (TSH)

• Adrenocorticotropic hormone (ACTH)

• Follicle-stimulating hormone (FSH)

• Luteinizing hormone (LH)

• Melanocyte-stimulating hormone (MSH)

• Endorphins

Prolactin (PRL)

• Milk production + mammary gland development

• Helps breast development during pregnancy

• Suckling → more prolactin released

What type of feedback is suckling → prolactin release?

Positive feedback

Growth Hormone (GH)

• Bone + muscle growth (especially in children)

• Protein synthesis ↑ (amino acids uptake ↑)

• Fat breakdown ↑

• Regulates glucose use in body

• too little GH → dwarfism

• too much GH → gigantism

Thyroid-Stimulating Hormone (TSH)

• Stimulates adrenal cortex

• Releases stress hormones (cortisol)

• “Adreno = stress response”

Follicle-Stimulating Hormone (FSH)

• Females: egg development + estrogen production

• Males: sperm production

Luteinizing Hormone (LH)

• Females: regulates menstrual cycle + ovulation

• Males: testosterone production

Melanocyte-Stimulating Hormone (MSH)

• Increases skin pigmentation (melanin)

• Higher in pregnancy

Endorphins

• Reduce pain perception

• “Natural painkillers”

ADH (Antidiuretic Hormone)

• Made in hypothalamus, stored in posterior pituitary

• Increases water reabsorption in kidneys

• Increases blood volume + pressure

• Released when:

  • Dehydration

  • High salt intake

• Alcohol/caffeine ↓ ADH → more urine

When is ADH released?

• Dehydration

• Salt-heavy meal

• Low blood volume

• Stress (can increase ADH)

Oxytocin

• Milk ejection (let-down reflex)

• Uterine contractions during birth

• Positive feedback (suckling → more oxytocin)

Milk hormones:

• Prolactin = makes milk

• Oxytocin = releases milk

Posterior Pituitary

• Stores and releases:

  • ADH

  • Oxytocin

• Does NOT make hormones (hypothalamus does)

Thyroid Gland

• Endocrine gland in the throat

• Controlled by hypothalamus + anterior pituitary (via TSH)

• Produces thyroid hormones

Thyroxine (T4)

• Main thyroid hormone

• Contains 4 iodine atoms

• Controls metabolism and energy use

Why is iodine important?

• Required to make thyroxine (T4)

• Must come from diet (e.g., iodized salt)

What happens if iodine is low?

• Not enough T4 can be produced

• Pituitary keeps releasing TSH

• Thyroid keeps getting stimulated

Why does negative feedback fail in iodine deficiency?

• TSH continues increasing

• But T4 still cannot be made

• So feedback loop cannot restore balance

Goiter

• Enlarged/swollen thyroid gland

• Caused by constant overstimulation (high TSH)

• Appears as swelling in the neck

What is T4?

• Main thyroid hormone

• Contains 4 iodine atoms

• Lipid-soluble (non-polar)

• Enters cells easily

What happens to T4 in the cell?

• Loses one iodine atom

• Converted into T3

What is T3?

• More active thyroid hormone

• Enters nucleus and binds receptor

• Changes gene expression

What do thyroid hormones control?

• Growth

• Development

• Maturation

• Metabolism

How do thyroid hormones work inside cells?

• T3 enters nucleus

• Binds to receptor on DNA

• Turns genes on/off

• Changes protein production

What is calcitonin?

Protein hormone made by thyroid gland cells

What does calcitonin do?

• Lowers blood calcium levels

• Inhibits release of calcium from bones

Q: Why do we need calcium released from bones?

• Calcium needed in blood for:

  • Muscle contraction (including heart)

  • Nerve signalling

  • Blood clotting

• Bones act as calcium storage

Main thyroid hormones

• T4 (thyroxine)

• T3 (active form)

• Calcitonin

What are the two main thyroid hormones?

• Thyroxine (T4)

• Triiodothyronine (T3)

How can thyroid hormones affect body weight?

• They regulate metabolic rate

• Higher thyroid hormone levels = faster metabolism

• Lower thyroid hormone levels = slower metabolism

What is hyperthyroidism?

• Condition where too much thyroxine (T4) is produced

• Metabolic rate becomes abnormally high

What happens in hyperthyroidism?

• Sugars and nutrients are broken down faster

• More energy is used by the body

• Little excess glucose is stored

• Weight gain is less likely

What happens to glucose during metabolism?

• About 60% of energy released becomes heat

• About 40% is converted to ATP for cellular work

What are symptoms of hyperthyroidism?

• Weight loss

• Rapid heart rate

• Nervousness/anxiety

• Feeling warm more often

Why do people with hyperthyroidism often lose weight?

• Their metabolic rate is very high

• Nutrients are used quickly for energy

• Less energy is stored as fat or glycogen

What is hypothyroidism?

• Condition where too little thyroxine (T4) is produced

• Metabolic rate becomes abnormally low

What happens in hypothyroidism?

• Sugars and nutrients are broken down more slowly

• Less energy is used by the body

• Excess blood sugar is stored as glycogen

What happens when glycogen stores are full?

• Excess glucose is converted to fat

• Leads to weight gain

What are symptoms of hypothyroidism?

• Muscle weakness

• Cold intolerance (feeling cold easily)

• Dry skin and hair

• Weight gain

Why do people with hypothyroidism often gain weight?

• Their metabolic rate is low

• Nutrients are not used as quickly

• More glucose is stored as glycogen and fat

What are the parathyroid glands?

• Four pea-sized glands

• Located behind the thyroid gland

• Regulate calcium and phosphate levels in the blood

What is Parathyroid Hormone (PTH)?

• Hormone secreted by the parathyroid glands

• Controls blood calcium and phosphate levels

When is PTH released?

• When blood calcium levels drop

• Triggered by a negative feedback mechanism

What does PTH do?

• Stimulates bone cells to release calcium into the blood

• Stimulates release of phosphate into the blood

• Raises blood calcium levels back to normal

Why is calcium important in the body?

• Enzyme activation

• Nerve signal transmission across synapses

• Muscle contraction

• Blood clotting

How does PTH maintain homeostasis?

• Low blood calcium → PTH released

• Calcium released from bones

• Blood calcium rises

• PTH secretion decreases once normal levels are restored

What are the adrenal glands?

• Endocrine glands located on top of each kidney

• Made of two regions:

  • Adrenal medulla (inner)

  • Adrenal cortex (outer)

How are the two parts of the adrenal gland regulated?

• Adrenal medulla → regulated by the nervous system

• Adrenal cortex → regulated by hormones

What hormones does the adrenal medulla produce?

• Epinephrine (adrenaline)

• Norepinephrine (noradrenaline)

How do epinephrine and norepinephrine work?

• Bind to receptors on the plasma membrane of target cells

• Trigger rapid responses throughout the body

When is the adrenal medulla stimulated?

• During stress

• By sympathetic nerves

• Activates the fight-or-flight response

What situations cause the release of epinephrine and norepinephrine?

• Danger

• Fear

• Excitement

• Injury

• Anger

• Anxiety

What is the fight-or-flight response?

• A rapid response that prepares the body to deal with danger or stress

• Helps the body either confront the threat or escape from it

How does epinephrine prepare the body for stress or physical activity?

• Increases heart rate

• Increases blood pressure

• Increases breathing rate

• Raises blood glucose levels

• Directs more blood to muscles

• Increases energy availability

What is the main function of epinephrine?

Prepare the body for immediate action during stressful situations

What is the main function of norepinephrine?

• Supports the fight-or-flight response

• Helps increase alertness and blood pressure

What hormones does the adrenal cortex produce?

• Glucocorticoids (e.g., cortisol)

• Mineralocorticoids (e.g., aldosterone)

• Small amounts of sex hormones

What is cortisol?

• A glucocorticoid hormone

• Helps regulate blood glucose levels

• Helps break down fats when blood glucose is low

How does cortisol help during stress?

• Increases amino acid levels in the blood

• Amino acids are converted into glucose by the liver

• Blood glucose levels rise

• Provides more energy for the body to recover

What is the overall effect of cortisol?

• Increases available energy

• Helps the body cope with and recover from stress

Is cortisol increasing blood sugar a good thing or a bad thing?

• Good in the short term

  • Provides energy during stress

  • Helps survival and recovery

• Can be harmful if prolonged

  • Chronically high blood sugar can contribute to health problems

  • Long-term stress can have negative effects on the body

What is aldosterone?

• A mineralocorticoid hormone

• Involved in osmoregulation

• Helps regulate sodium and water balance

• Helps maintain blood volume and blood pressure

What does aldosterone do?

• Increases sodium reabsorption by the kidneys

• Water follows sodium by osmosis

• Increases blood volume and blood pressure

What is the role of sex hormones produced by the adrenal cortex?

• Produced in small amounts

• Contribute to sexual development and reproductive functions

Cortisol vs. Aldosterone

• Cortisol

  • Regulates blood glucose

  • Breaks down fats

  • Helps respond to stress

• Aldosterone

  • Regulates salt and water balance

  • Maintains blood pressure

  • Important in osmoregulation

What is the pineal gland?

• Small endocrine gland near the center of the brain

• Produces the hormone melatonin

• Helps regulate biological rhythms (sleep–wake cycle)

What is melatonin?

• Hormone that controls sleep patterns

• Secreted mainly during darkness

• Helps regulate the body’s biological clock

How is melatonin controlled by light?

• Light detected by photoreceptors in the eyes

• Signals sent to the pineal gland

• Light → melatonin secretion is inhibited

• Darkness → melatonin secretion increases

When is melatonin released?

• At night or in dark conditions

• Helps signal the body that it is time to sleep

Why does light make it harder to sleep?

• Light stops melatonin production

• Less melatonin = less sleep signal to the brain

• Body stays more alert

What is the function of melatonin?

• Maintains circadian rhythm (biological clock)

• Coordinates sleep–wake cycles

• Adjusts to environmental light patterns

What is jet lag?

• Temporary disruption of the biological clock

• Happens when traveling across time zones

• Melatonin timing is out of sync with local light–dark cycle