Endocrine System - Chapter 17 (2026)

What is the endocrine system?

A communication system that uses hormones released into the blood to control body functions such as growth, metabolism, reproduction, stress, and homeostasis.

What is a hormone?

A chemical messenger released by an endocrine gland that travels through the blood to affect specific target cells.

What is a target cell?

A cell that has the specific receptor needed to respond to a particular hormone.

Why doesn't every cell respond to every hormone?

Only cells with the correct receptor for that hormone can respond.

What are the 6 major functions of the endocrine system?

Growth and development, metabolism and energy, reproduction, homeostasis, stress response, and involvement in diseases.

What does homeostasis mean?

Keeping the body's internal conditions relatively stable and balanced.

How does the endocrine system communicate?

Endocrine glands release hormones into the bloodstream.

How does the nervous system communicate?

Neurons release neurotransmitters into synapses.

Which system usually acts faster: endocrine or nervous?

The nervous system.

Which system usually lasts longer: endocrine or nervous?

The endocrine system.

Which system usually has a wider effect?

The endocrine system.

Which system usually has a more localized effect?

The nervous system.

What do the endocrine and nervous systems have in common?

Both release chemical signaling molecules called ligands that bind to receptors on target cells.

What is an easy way to remember endocrine vs nervous?

Endocrine = slow, widespread, long-lasting. Nervous = fast, localized, short-lasting.

What are the 3 major chemical categories of hormones?

Steroids, biogenic amines, and proteins.

What are steroid hormones made from?

Cholesterol.

Are steroid hormones water-soluble or lipid-soluble?

Lipid-soluble.

What are examples of steroid hormones?

Estrogen, testosterone, cortisol, and aldosterone.

What are biogenic amines?

Hormones made by modifying amino acids.

What are examples of biogenic amines?

Catecholamines, thyroid hormones, and melatonin.

What is the important exception with thyroid hormone?

Most biogenic amines are water-soluble, but thyroid hormones are lipid-soluble.

What are protein hormones made of?

Chains of amino acids.

Are protein hormones generally water-soluble or lipid-soluble?

Water-soluble.

How do water-soluble hormones travel through the blood?

They generally travel freely because they dissolve in blood plasma.

Where do water-soluble hormones bind?

Receptors on the cell membrane.

Why can't water-soluble hormones easily enter the cell?

They do not easily pass through the cell's lipid membrane.

What happens when a water-soluble hormone binds to its receptor?

It activates a signaling pathway inside the cell.

How do lipid-soluble hormones travel through the blood?

They travel attached to carrier proteins.

Why do lipid-soluble hormones need carrier proteins?

They do not dissolve well in the watery blood plasma.

Where do lipid-soluble hormones usually bind?

Inside the cell, usually to intracellular receptors.

Why can lipid-soluble hormones enter cells?

They can pass through the cell's lipid membrane.

What is the easiest way to remember hormone receptors?

Water-soluble hormones → membrane receptors. Lipid-soluble hormones → intracellular receptors.

What is a local hormone?

A chemical messenger that acts close to where it was produced rather than traveling throughout the entire body.

What is autocrine signaling?

A cell releases a chemical signal that acts on the same cell that released it.

How do you remember autocrine?

AUTO = self.

What is paracrine signaling?

A cell releases a signal that affects nearby cells.

How do you remember paracrine?

PARA = nearby.

What are eicosanoids?

Local signaling molecules made through an enzymatic pathway.

What are prostaglandins?

A type of eicosanoid involved in processes such as pain and inflammation.

What determines how much hormone is in the blood?

How quickly the hormone is released compared with how quickly it is eliminated.

What happens if hormone release increases?

Blood hormone concentration increases.

What happens if hormone release decreases?

Blood hormone concentration decreases.

How are hormones eliminated?

They can be broken down by the liver, removed by the kidneys, or taken up by target cells.

What happens to hormone concentration when elimination becomes faster?

Hormone concentration decreases.

What happens when hormone elimination slows down?

Hormone concentration increases.

What is up-regulation?

A target cell increases the number of receptors it has for a hormone.

What happens to sensitivity during up-regulation?

Sensitivity increases.

When might a cell up-regulate receptors?

When hormone levels are low.

Why would a cell increase its receptors?

To become more sensitive and respond better to a small amount of hormone.

What is down-regulation?

A target cell decreases the number of receptors it has for a hormone.

What happens to sensitivity during down-regulation?

Sensitivity decreases.

When might down-regulation occur?

When hormone levels are high or the cell has been exposed to the hormone for a long time.

Why does down-regulation happen?

To prevent the cell from being overstimulated.

How do you remember up-regulation vs down-regulation?

Up = more receptors = more sensitivity. Down = fewer receptors = less sensitivity.

What is a synergistic interaction?

Two or more hormones work together to produce a stronger effect.

How do you remember synergistic?

Synergy = teamwork.

What is a permissive interaction?

One hormone allows another hormone to produce its full effect.

How do you remember permissive?

Permissive = permits or allows.

What is an antagonistic interaction?

Two hormones have opposite effects.

What is a classic example of antagonistic hormones?

Insulin lowers blood glucose while glucagon raises blood glucose.

How do you remember the 3 hormone interactions?

Synergistic = together. Permissive = allows. Antagonistic = opposite.

Why is the pituitary called the master gland?

It releases hormones that control several other endocrine glands and body functions.

Where is the pituitary gland located?

At the base of the brain, just below the hypothalamus.

What connects the hypothalamus and pituitary gland?

The infundibulum, or pituitary stalk.

What are the two parts of the pituitary gland?

The anterior pituitary and posterior pituitary.

How does the hypothalamus control the anterior pituitary?

Through hormones carried by the hypothalamic-hypophyseal portal system.

How does the hypothalamus control the posterior pituitary?

Through nerve axons extending from hypothalamic neurons.

What is the easiest way to remember anterior vs posterior pituitary?

Anterior = blood. Posterior = nerves.

What are the major anterior pituitary hormones?

GH, TSH, ACTH, PRL, FSH, and LH.

What does GH stand for?

Growth hormone.

What does growth hormone do?

Stimulates growth and cell reproduction in bones, muscles, and other tissues.

What does GH cause the liver to produce?

Insulin-like growth factors, especially IGF-1.

What does TSH stand for?

Thyroid-stimulating hormone.

What does TSH do?

Stimulates the thyroid gland to produce thyroid hormones.

What hypothalamic hormone stimulates TSH release?

TRH.

What does ACTH do?

Stimulates the adrenal cortex to release cortisol.

What hypothalamic hormone stimulates ACTH?

CRH.

What does prolactin do?

Stimulates milk production in mammary glands.

What hormone stimulates prolactin?

PRH.

What hormone inhibits prolactin?

PIH.

What do FSH and LH control?

Reproductive functions in the ovaries and testes.

What hypothalamic hormone stimulates FSH and LH?

GnRH.

Does the posterior pituitary make ADH and oxytocin?

No. The hypothalamus makes them; the posterior pituitary stores and releases them.

What does ADH stand for?

Antidiuretic hormone.

What is another name for ADH?

Vasopressin.

Where is ADH made?

The supraoptic nucleus of the hypothalamus.

What does oxytocin do?

It is involved in uterine contractions and milk ejection.

Where is oxytocin made?

The paraventricular nucleus of the hypothalamus.

What does the posterior pituitary mainly do?

Stores and releases ADH and oxytocin into the blood.

What is the key posterior pituitary fact to memorize?

ADH and oxytocin are made in the hypothalamus, transported down axons, and stored and released by the posterior pituitary.

Where is the thyroid gland?

In the front of the neck, below the larynx and around the trachea.

What shape is the thyroid gland?

Butterfly-shaped.

What connects the two thyroid lobes?

The isthmus.

What are thyroid follicles?

Structures lined by follicular cells that make and store thyroid hormones.

What do thyroid follicular cells produce?

T3 and T4.

What do T3 and T4 do?

Increase metabolic rate, energy use, body temperature, growth, and development.

What hormone controls thyroid follicular cell activity?

TSH.

What are C cells?

Parafollicular cells of the thyroid.

What hormone do C cells produce?

Calcitonin.

What does calcitonin do?

Helps lower blood calcium levels.