endocrine system

Paracrine

defined as chemical messengers secreted into the interstitial fluid that specifically affect nearby cells.

T3 and T4

chemically classified as monoamines but are unique because they are hydrophobic (lipid-soluble).

Hormone

: A chemical messenger released by endocrine glands, tissues, or cells that travels through the bloodstream to affect distant target cells, tissues, or organs.

Neurotransmitter

A chemical messenger released by neurons (nerve cells) that travels across a synaptic cleft (tiny gap between cells) to affect a nearby cell.

Hormone bloodstream

Distant, slower longer lasting

Neurotransmitter synapse

Nearby, faster shorter lasting

Endocrine Organs

Primarily make hormones

Hypothalamus, pituitary gland, pineal gland, thyroid gland, parathyroid glands ,thymus, adrenal glands, pancreas, gonads (ovaries and testes)

Hypothalamus

produces hormones that regulate the pituitary gland (major hormone-control gland).

Pituitary gland

produces/releases hormones that control growth, reproduction, metabolism, and other glands

Pineal gland

produces melatonin (hormone that regulates sleep/circadian rhythm).

Thyroid gland

produces T₃ and T₄ (thyroid hormones that regulate metabolism) and calcitonin (lowers blood calcium).

Parathyroid glands

produce PTH/parathyroid hormone (raises blood calcium)

Thymus

produces hormones involved in T-cell maturation (development of immune cells).

Adrenal glands

produce: Epinephrine/norepinephrine (fight-or-flight hormones) Aldosterone (regulates sodium, water, and blood pressure) Cortisol (helps regulate stress, metabolism, and immune responses) Sex steroids (reproductive hormones)

Pancreas

produces: Insulin (lowers blood glucose) Glucagon (raises blood glucose) Somatostatin (inhibits/reduces hormone and digestive secretions)

Gonads (reproductive organs):

Ovaries — produce estrogen, progesterone, and inhibin (reproductive hormones).

Testes — produce testosterone, other androgens (male sex hormones), estrogen, and inhibin.

Organs with Secondary Endocrine Function

Skin, liver, kidneys, heart, stomach and small intestine, adipose tissue, bone osseous tissue, skeletal muscle, placenta

Skin

helps produce vitamin D (important for calcium and bone health).

Liver

produces IGF-1 (helps mediate growth hormone effects), angiotensinogen (helps regulate blood pressure), and hepcidin (regulates iron)

Kidneys

produce renin (helps regulate blood pressure), erythropoietin/EPO (stimulates red blood cell production), and calcitriol (active vitamin D; increases calcium absorption).

Heart

produces natriuretic peptides (hormones that reduce blood volume and blood pressure).

Stomach and small intestines

produce digestive hormones such as gastrin (stimulates stomach acid), CCK (helps digestion), ghrelin (stimulates appetite), and PYY (reduces appetite).

Adipose tissue

produces leptin (signals the brain to reduce appetite).

Bone tissue

produces osteocalcin (helps regulate metabolism and pancreatic function)

Skeletal muscle

releases myokines (signaling hormones released by muscle, especially during exercise

Placenta

produces estrogen and progesterone (hormones that support pregnancy).

Target cell

a cell that has a specific receptor (protein that recognizes and binds a hormone) for a particular hormone. Because it has the correct receptor, the cell can respond to that hormone. can have receptors for multiple different hormones, so one cell can respond to several hormones. often have many receptors for a hormone, making them sensitive to that hormone.

Monoamines (biogenic amines)

• hormones made from amino acids (building blocks of proteins).

Examples: Epinephrine, norepinephrine, dopamine Exception: T₃ and T₄ (thyroid hormones) are monoamines but are lipid-soluble.

Peptides and glycoproteins

water-soluble hormones made from amino acids. Examples: Insulin, growth hormone (GH), oxytocin (OT), ADH Remember: Peptides = amino-acid chains

Hypothalamus also releases

Oxytocin and ADH

Somastatin

Plasma membrane

Hydrophilic and protein cannot pass

Thyroid gland

Is intracellular

Target cells

Have a receptor for a chemical

Liver cells

Have exocrine (bile in ducts)and endocrine (release hormones and albumin in blood=

Neuroendocrine

Characteristics of both systems neuron secrete hormones

Corticosteroids and thyroid hormone

are hydrophobic (lipid-soluble), so they do not dissolve well in the water-based blood plasma.

Therefore, they must bind to transport proteins (blood proteins that carry hormones) to travel through the bloodstream.

• Why do they need transport proteins?

Increase solubility → allow lipid-soluble hormones to travel through watery blood.

Protect hormones → slow their breakdown.

Prevent kidney filtration → keep hormones from being removed too quickly in urine.

Increase half-life (time a hormone remains in the blood) → bound hormones generally stay in the bloodstream longer.

Corticosteroids

Travel attached to albumin and globulins (blood transport proteins).

Cortisol primarily binds to transcortin.

Aldosterone is an exception because much more of it remains free (unbound).

Thyroid hormone synthesis

Thyroid hormones T₃ and T₄ are made from tyrosine (an amino acid) and iodine inside the thyroid follicles.

Iodide uptake — Thyroid follicular cells take iodide (I⁻) from the blood and convert it into an active form of iodine.

Thyroglobulin production — The cells make thyroglobulin (Tg; a protein that stores thyroid hormone building blocks) and release it into the colloid (fluid-filled space inside the thyroid follicle).

Iodination — Iodine attaches to tyrosine molecules on thyroglobulin.

Coupling — The iodinated tyrosines combine to form:

T₃ = 3 iodine atoms

T₄ = 4 iodine atoms

Endocytosis — TSH (thyroid-stimulating hormone) causes the follicular cells to take the iodinated thyroglobulin back into the cell.

Cleavage — Lysosomal enzymes (enzymes that break down proteins) separate T₃ and T₄ from thyroglobulin.

Release — T₃ and T₄ are released into the bloodstream, with T₄ being released in greater amounts.

Regulation of Thyroid Hormone Secretion

This is controlled by the hypothalamic-pituitary-thyroid (HPT) axis using negative feedback (increased hormone levels shut down further hormone production):

Hypothalamus → TRH → Anterior pituitary → TSH → Thyroid → T₃/T₄

1. Hypothalamus: Releases TRH (thyrotropin-releasing hormone).

2. Anterior pituitary: TRH stimulates it to release TSH (thyroid-stimulating hormone).

3. Thyroid gland: TSH stimulates thyroid follicular cells to make and release T₃ and T₄.

4. Negative feedback: When T₃/T₄ levels become high, they inhibit the hypothalamus and anterior pituitary, decreasing TRH and TSH.

This prevents excessive thyroid hormone production.

T3 and T4

Hydrophobic amino acid protein

Hormonal stimuli

One hormone causes another endocrine gland to release a hormone

Ex:hypothalamus releases GHRH then Ant Pituitary then GH

Somatostatin the inhibits Ant Pituitary then decreases growth hormone

Humoral stimulus

Something in blood changes and the endocrine gland detects it. Then hormone is released.Ex:blood glucose increase then pancreatic beta cells then insulin

Stimulus was change in blood glucose level

Neural stimulus

Nerve signal then goes to endocrine cell then hormone like a response to fight and flight

Ex:adrenal medulla (strew or exercise)

Sympathetic neuron then adrenal medulla then epinephrine and norepinephrine

Corticosteroids and thyroid hormones

Hydrophobic and they must bien to transport proteins to travel thru bloodstream

Monoamides peptides

Hydrophilic with the exceptions of albumin, thyretin and thyroxine binding globulins they are proteins but hydrophobic and carrier proteins

Transcortin

Transports cortisol (hydrophobic)

Plasma membrane receptor

Hydrophilic

 They cannot cross the cell membrane, so they must bind to receptors on the outside of the cell.

Binding activates second messengers (internal signaling molecules such as cAMP), which quickly change cell activity.

Plasma membrane receptors examples

Insulin, glucagon, epinephrine

Cytoplasmic Receptors

Certain hydrophobic hormones

Binding activates second messengers (internal signaling molecules such as cAMP), which quickly change cell activity.

Ex:aldosterone

Nuclear response

n the nucleus (control center containing DNA).

Used by: Hydrophobic hormones, including steroids and thyroid hormones.

Why? These hormones can cross the cell membrane and reach receptors inside the cell.

How they work: The hormone binds to the receptor and directly affects gene transcription (using DNA to make mRNA), leading to new protein production.

Examples: Estrogen and T₃ (thyroid hormone).

Nuclear response

Examples: Estrogen and T₃ (thyroid hormone).

Cytoplasmic

Aldosterone

Signal amplification

A tiny hormone signal produces a huge cellular response

Despite vey low plasma concentration hormones extraordinarily potent

Phosprodiesterase

An enzyme that breaks down cAMP and stops the signaling cascade

Synergistic Effect

Work Together

Two or more hormones work together to produce an effect that is greater than either hormone could produce alone.

Synergist example

FSH + testosterone → increased sperm production

Permissive effect

 One Prepares for Another

One hormone allows or enhances the effect of another hormone.

The first hormone often increases the number of receptors (proteins that bind hormones) for the second hormone.

permissive example

Estrogen → increases progesterone receptors in the uterus, allowing progesterone to have a stronger effect.

Antagonist effect

Opposite Effects

One hormone opposes or reverses the effect of another hormone.

Example: Insulin ↓ blood glucose, while glucagon ↑ blood glucose.

Up-regulation

More Receptors → More Sensitive

The target cell increases the number of hormone receptors.

More receptors means the cell can bind more hormone molecules.

This makes the cell more sensitive and produces a stronger response.

Example: Estrogen increases progesteronne receptors in the uterus, making the uterus more responsive to progesterone.

dOwn regulation

Fewer Receptors → Less Sensitive

The target cell decreases the number of hormone receptors.

Fewer receptors means fewer hormone molecules can bind.

This makes the cell less sensitive and produces a weaker response.

Usually occurs when the cell is exposed to high hormone levels for a long time, helping prevent overstimulation.

Specificity

Type of receptor selectively binds. Receptor for one hormone will not bind to other hormones

Saturation

All the receptor molecules are occupied by hormone molecules and a cell can have multiple receptors k the more receptors bound to hormone can dictate degree of response.

The target cell decreases the number of hormone receptors.

Fewer receptors means fewer hormone molecules can bind.

This makes the cell less sensitive and produces a weaker response.

Usually occurs when the cell is exposed to high hormone levels for a long time, helping prevent overstimulation.

Iodine is required to make thyroid hormones (T₃ and T₄).

If a person does not get enough dietary iodine, the thyroid cannot produce enough thyroid hormone.

Low thyroid hormone means there is less negative feedback (the normal system that reduces hormone production) on the hypothalamus and pituitary.

The pituitary responds by releasing more TSH (thyroid-stimulating hormone) to try to stimulate the thyroid.

Constantly high TSH overstimulates the thyroid, causing the thyroid cells to grow and the gland to enlarge.

Goiter

Low iodine → ↓ T₃/T₄ → ↓ negative feedback → ↑ TSH → thyroid enlargement →

Hyposecretion

A hormone is released in too small an amount

Hypersecretion

A hormones is released in too large an amount

Pituitary dwarfism

GH hyposecretion in childhood

Too little GH → abnormally short stature.

Goiter

Not enough iodine → ↓ thyroid hormone → ↑ TSH → thyroid enlargement.

Gigantism

GH hypersecretion in childhood

Too much GH before growth plates close which excessive height

Acromegaly

GH hypersecretion in adulthood

Too much GH after growth plates close and enlargement of bones and soft tissues like hands feet and face

Hyperthyroidism

Thyroid hormone hypersecretion

Graves’ disease is a common cause abnormal antibodies stimulate the thyroid like TSH. Weight loss, heat intolerance, rapid heartbeat, and bulging eyes

Anterior pituitary

Adenohypophysis

Made of endocrine gland tissue produces and secretes ists own hormone

Controlled by hypothalamus thru hypotheseal portal system

Posterio pituitary

Neurohypophysis

Made of nervous tissue

Does not make its own hormones

Stores and release ADN and OT which are actually produced by neurons in hypothalamus

Connected to hypothalamus by. The hypothalamo’hypophyseal tract

ADH

Increases water retention by kidneys and can increase BP

CRH secretion would not raise the blood concentration

Thyroxine

Not a hormone

Thyroxine binding globulin

Thyroid gland consists of

spherical thyroid follicles that contain iodine-containing colloid

FSH

Named for the effect on gamete production

Which of these has more exocrine than endocrine tissue?

Pancreas

When blood glucose levels increase, a hormone called insulin is released from endocrine cells in the pancreas. The role of insulin is to return blood glucose levels to normal. Which part of this negative feedback loop is the stimulus?

Increasing blood glucose levels

What is delivered over the hypothalamic-hypophyseal portal system?

Releasing and inhibiting hormones

Which hormone promotes gluconeogenesis as a way to increase blood glucose levels and has potent anti-inflammatory effect?

Cortisol

Atrial natriuretic peptide (ANP) is secreted by__ and promotes_.

Heart;sodium and water loss

Aldosterone directly regulates ___and belongs to the group of____ hormones

extracellular sodium and potassium ion levels; mineralocorticoid

High levels of testosterone inhibit the release of:

• gonadotropin-releasing hormone (GnRH).