physio endocrine glands & hormones

chemical messenger

any molecule that carries a signal from one cell to another

ligands

messengers that bind to receptors either on the plasma membrane or inside the cell to activate pathways or trigger cellular changes

hormones

chemical messengers that act as ligands binding to receptors and travel through the bloodstream to reach their target

neurohormones

neurons releasing chemical messengers directly into the blood

considered true hormones because they travel through the bloodstream

synaptic signaling

involves neurotransmitters released from axon terminals into the synaptic cleft

these bind to receptors on the postsynaptic cell

the presynaptic neuron releases the neurotransmitter, and the postsynaptic cell receives it

neurotransmitter

chemical messenger that must be released by a neuron, travel a short distance through interstitial fluid, and bind to another cell

endocrine system

maintains homeostasis through coordinated organ function

primary endocrine organs

have hormone secretion as their primary function

pure endocrine organs

glands made of epithelial tissue that only secrete hormones 

pituitary, pineal, thyroid, parathyroid, adrenal glands

other organs with large endocrine components

primarily secrete hormones but have other functions

pancreas, thymus, testes, ovaries, hypothalamus

secondary endocrine organs

secrete hormones but have other primary functions

heart, stomach, liver, kidney, small intestine, skin

the intestines require large quantities of hormones

humoral stimulus

change in blood chemical/nutrition levels (not another hormone)

ex: sodium, potassium, calcium, or glucose changes

when blood calcium falls, parathyroid glands secrete parathyroid hormone, activating osteoclasts to release calcium

neural stimulus

occurs when an action potential triggers hormone secretion

the adrenal medulla, innervated by sympathetic fibers, secretes epinephrine and norepinephrine during “fight or flight” responses

hormonal stimulus

occurs when one hormone triggers another gland to secrete a different hormone

the anterior pituitary secretes hormones that stimulate target glands to release their hormones

pituitary

posterior pituitary (neurohypophysis) and the anterior pituitary (adenohypophysis)

hangs from the hypothalamus via the infundibulum

neurohypopysis (posterior pituitary)

nervous tissue functioning as a brain extension

doesn’t make hormones, hypothalamic neurons in supraoptic and paraventricular nuclei synthesize oxytocin and vasopressin (ADH)

these neurons cell bodies are in the hypothalamus with axons extending through the infundibular stalk to the posterior pituitary, where hormones are stored and released into capillaries

oxytocin

plays roles in mood, reproduction, childbirth, and social bonding

released during labor (cervix/uterus stretching) and breastfeeding (nipple stimulation), it aids birth, maternal bonding, and milk letdown

neural stimuli trigger oxytocin release, its produced in the hypothalamus, and stored in the posterior pituitary

creates positive feedback loop during childbirth: fetal pressure against the cervix causes uterine contractions, releasing oxytocin, which intensifies contractions, producing more oxytocin until birth occurs

vasopressin (ADH)

“water retention hormone”

peptide hormone opposes urine production, helping retain water and controlling blood osmolarity

increases blood pressure through vasoconstriction

produced when dehydrated (elevated blood plasma osmolarity) - humoral stimulus, contrasting with oxytocin’s neural stimulus

adenohypophysis (anterior pituitary)

glandular tissue

portal system

the hypothalamus and adenohypophysis connect and link two capillary beds with a portal vessel

the hypothalamic neurons release hormones, not neurotransmitters in this system

these releasing and inhibiting hormones enter the first capillary bed, travel via portal vein, and act on the anterior pituitary

secrete tropic hormones that control other endocrine organ hormone secretion, having stimulatory or inhibitory effects

cortisol

during stress, the hypothalamic releases corticotrophin-releasing hormone (CRH), stimulating anterior pituitary ACTH production

ACTH travels to the adrenal cortex, stimulating cortisol production

thyroid hormone

the hypothalamus releases thyrotropin-releasing hormone (TRH), stimulating anterior pituitary TSH production

TSH stimulates thyroid gland thyroid hormone production, regulating metabolism

prolactin

the hypothalamus releases prolactin-releasing hormone (PRH), stimulating anterior pituitary prolactin production

prolactin targets breasts, stimulating milk production in lactating mothers

the hypothalamus can also produce prolactin-inhibiting hormone (dopamine), inhibiting prolactin secretion and preventing milk production

thyroid hormone

the hypothalamus produces TRH, stimulating anterior pituitary TSH production

TSH targets the thyroid gland, triggering thyroid hormone production

cortisol

the hypothalamus produces CRH, stimulating anterior pituitary ACTH production

ACTH travels to the adrenal cortex, triggering cortisol production for stress response

growth hormone

the hypothalamus produces GHRH, stimulating anterior pituitary GH production

GH directly stimulates growth of bones (via epiphyseal plates), muscles, and organs

in the liver, GH triggers IGF-1 production, which circulates and produces similar but distinct growth effects

the hypothalamus can also produce GHIH, inhibiting anterior pituitary growth hormone production

sex hormones

the hypothalamus produces GnRH, triggering anterior pituitary LH and FSH release

these target gonads for sex hormone release

male testes primarily produce testosterone (androgen), whole female ovaries produce estrogen

thyroid gland

primary endocrine gland in the neck composed of glandular tissue

sits laterally below the Adam’s apple, secreting T3 and T4

thyroid hormones

regulate metabolism, growth, and development

permissive hormones, helping other hormones function

hyperthyroidism

makes weight gain difficult

hypothyroidism

makes weight loss difficult

calcitonin

regulates blood Ca2+ levels with parathyroid hormone

decreases blood calcium by inhibiting osteoclasts (preventing bond breakdown) and stimulating osteoblasts (promoting calcium storage)

parathyroid glands

secrete parathyroid hormone (PTH), working with calcitonin to control blood calcium

Calcitonin and PTH oppose each other

While calcitonin lowers blood calcium, PTH increases it by stimulating osteoclast activity, breaking down bone matrix to release calcium

Triggered by humoral stimulus - blood calcium levels

Adrenal glands

Two, sit above the kidney

Each has two regions - the outer adrenal cortex with three hormone secreting layers, and the inner adrenal medulla

Adrenal cortex

Comprising 80% of adrenal gland mass, secretes cholesterol derived steroids called adrenocorticoids

These lipophilic molecules pass through cell membranes easily and are made on demand

Adrenal cortex - mineralocorticoids

(Primarily aldosterone)

Regulate kidney potassium secretion and sodium reabsorption

Like ADH, aldosterone reduces urination

Increases sodium reabsorption with water following passively

Adrenal cortex - glucocorticoids

(Primarily cortisol)

Regulate stress response, blood glucose levels, and substrate metabolism

Cortisol is a largely anti-inflammatory, suppressing the immune system

Adrenal cortex - sex hormones

(Primarily androgens)

Secreted in smaller amounts

Important female testosterone source, contributing significantly to overall androgen levels

Adrenal medulla

Comprising 20% of adrenal glands, it secretes catecholamines - tyrosine-derived amine messengers

It secretes 80% epinephrine (hormone), 20% norepinephrine (neurotransmitter), and 1% dopamine (neurotransmitter) —> neural stimuli

Catecholamines are water-soluble and hydrophilic with plasma membrane receptors

Plays a central role in fight or flight, triggered by the sympathetic nervous system during stress/excitement

When stimulated, it releases catecholamines - primarily epinephrine with some norepinephrine - into the bloodstream

These increase heart rate / contractility, raise respiratory rate, and redirect blood flow toward skeletal muscles for action preparation

Gonads

Primary endocrine glands differing between sexes

Male gonads = tested

Female gonads = ovaries

Testes

Produce gametes/sperm and secrete androgens, producing male secondary sex characteristics (deep voice, muscle mass, protruding Adam’s apple, square jaw)

Testosterone- anabolic steroid promoting growth

Ovaries

Secrete estradiol (an estrogen) and progesterone, producing ova/oocytes (eggs)

Estrogens is a category of several hormones including estradiol

All estrogens are made from androgens

Estrogens can contribute to cancer development, so drugs preventing androgen-to-estrogen conversion (aroma tase inhibitors) treat breast cancer patients by lowering estrogen levels and reducing cancer cell growth

Pancreas (exocrine function)

Secretes substances (bicarbonate and digestive enzymes) directly into the small intestine

Most digestive enzymes are created in the pancreas

Pancreas (endocrine function)

Pancreas bulges contain islets of langerhans made of various cell types secreting different hormones

Islets contain pancreatic endocrine cells

Alpha cells secrete glucagon

Beta cells secrete insulin

Insulin and glucagon work opposingly to control blood glucose levels

Insulin causes glucose removal from blood for glycogen storage in cells, while glucagon causes liver glycogen breakdown, putting glucose into blood to increase plasma glucose levels

Type 1 diabetes

Genetic condition where the pancreas produces little/no insulin

If beta cells don’t produce insulin, glucose accumulates in plasma

This increases plasma toxicity and keeps sugar from cells, preventing efficient ATP production

Type 1 diabetics need insulin injections to replace pancreatic insulin failure

Type 2 diabetes

Often occurs in sedentary, overweight people with poor diets

Problem is systematic inflammation interfering with normal insulin signaling pathways triggering glucose uptake

This “insulin resistance” keeps blood glucose elevated, contributing to high blood pressure, cardiovascular disease, and cancer complications

Exercise can temporarily bypass thus - during/immediately after physical activity, muscle cells absorb glucose directly without requiring insulin

While temporary, consistent exercise and healthy diet can reverse type 2 diabetes by lowering inflammation and restoring insulin sensitivity

Pineal gland

Located at the brain base toward the skull back

Primary endocrine gland made of glandular tissue

Secretes melatonin, integral in establishing circadian rhythm modulating sleep

Melatonin production increases with darkness exposure, explaining nighttime sleepiness or sleepiness in dark classrooms

Plasma hormone levels: secretion rate

Some hormones (thyroid hormones) are secreted constantly, others only respond to stimuli (stress/pregnancy)

Secretion signals can be inhibitory or stimulators, with rates depending on body needs

Example: extremely low blood calcium causes parathyroid glands to produce PTH at high rates to restore calcium set points

Plasma hormone levels: hormone transport bound to carrier proteins

Hormones can be water soluble (epinephrine) or lipid soluble (steroids/thyroid hormones)

Water soluble hormones don’t need special blood transport, but lipid soluble (hydrophobic) hormones require carrier proteins

Some carriers are specific, others like albumin are not

Carriers are essential for lipid soluble messenger transportation because without them, lipid soluble molecules would clump and prevent blood flow

Plasma hormone levels: hormone removal rate

Hormones are removed through digestive enzyme breakdown

Hormones metabolize quickly, often by target cells through receptor-mediated endocytosis

Breakdown products are released into urine and expelled

Hydrophilic hormones degrade faster than bound, hydrophobic hormones

Example: insulin is removed within minutes, while testosterone remains for weeks

Abnormal hormone secretion

Involved too much or too little hormone release

When hormones are secreted abnormally, the entire body can be dramatically affected

Abnormal hormone secretion: primary secretion disorder

Abnormality originating in the hormone-secreting endocrine gland

Blood tropic hormone levels tend to be low due to enhanced negative feedback

Example: inadequate ACTH regulation may lead to adrenal glands to secrete excess cortisol

Negative feedback may still reduce hypothalamic CRH and anterior pituitary ACTH production, so these tropic hormones are present at low blood levels

The disorder causes continued adrenal cortisol overproduction (primary hypersecretion disorder involving too much cortisol)

Abnormal hormone secretion: secondary secretion disorder

Occurs when abnormalities originate in anterior pituitary or hypothalamic endocrine cells

Example: negative feedback mechanism dysfunction may prevent anterior pituitary ACTH production reduction when cortisol levels rise

This could result in persistently high cortisol production (secondary hypersecretion disorder involving too much cortisol)

Secondary endocrine organs: heart

Produces atrial natriuretic peptide (ANP), affecting kidney sodium handling

Secondary endocrine organs: kidneys

Produce erythropoietin, promoting bone marrow and red cell production

Secondary endocrine organs: liver

Produces insulin-like growth factors, promoting bone and soft tissue growth

Secondary endocrine organs: GI tract

Stomach and small intestines produce numerous digestion related hormones

Secondary endocrine organs: skin, liver, kidneys

Produce 1,25-dihydroxyvitamin D3, promoting calcium absorption

Hormone interaction: antagonism

Occurs when two hormones act opposingly

Example: insulin decreases plasma glucose levels; glucagon increases them

Example: PTH increases blood calcium concentration; calcitonin inhibits bone breakdown (calcium storage), producing opposite effects

Hormone interactions: additive reaction

Occurs when net effects equal individual effects sums (5+5=10)

Example: growth hormones glucose-sparing action is additively enhanced with cortisol present

Hormone interaction: synergistic reaction

Occurs when net effects are greater than individual sums (5+5=50)

Example: norepinephrine and epinephrine both increase heart rate but acting together produce even greater heart rate increases than independently

Example: both FSH and testosterone are involved in sperm production but are much more effective when acting together

Hormone interaction: permissiveness

Occurs when one’s hormones presence is necessary for another hormones effects

Example: thyroid hormone is required for adrenergic (beta) receptor production

Epinephrine binds these receptors causing bronchodilation

Without thyroid hormone, this pathway cannot occur