Endocrine System: Hormones, Glands, and Regulation

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Endocrine System Overview

Definition and Function of Hormones

The endocrine system is a network of glands that produce and secrete hormones, which are chemical messengers. These hormones are released into the bloodstream and regulate the activity of target cells and organs throughout the body.

Hormones: Chemical messengers secreted by endocrine glands, affecting specific target cells with appropriate receptors.
Endocrine glands: Ductless glands that release hormones directly into the blood (e.g., pituitary, thyroid, adrenal glands).
Exocrine glands: Glands that release their secretions through ducts to the outside of the body or into body cavities (e.g., sweat, salivary glands).

Example: The pancreas acts as both an endocrine gland (releasing insulin into the blood) and an exocrine gland (releasing digestive enzymes into the small intestine).

Types of Hormones and Their Mechanisms

Amino Acid-Based vs. Steroid Hormones

Hormones can be classified based on their chemical structure, which determines how they interact with target cells.

Amino acid-based hormones: Water-soluble, cannot cross the plasma membrane, bind to receptors on the cell surface. Examples: insulin, epinephrine.
Steroid hormones: Lipid-soluble, derived from cholesterol, can cross the plasma membrane and bind to intracellular receptors. Examples: cortisol, estrogen, testosterone.

Key Point: Only cells with specific receptors for a hormone will respond to that hormone.

Second Messengers

Many hormones use second messengers to relay signals inside the cell after binding to a surface receptor.

Second messenger: A molecule inside the cell that transmits signals from a hormone-receptor complex to initiate a cellular response.
Examples: cAMP (cyclic adenosine monophosphate), IP3 (inositol triphosphate), DAG (diacylglycerol).

Example: Epinephrine binds to a receptor on a liver cell, activating cAMP, which triggers glycogen breakdown.

Regulation of Hormone Release

Control by the Nervous System

The nervous system can directly influence hormone release, especially via the hypothalamus and pituitary gland.

Posterior pituitary: Releases hormones (ADH, oxytocin) produced in the hypothalamus, transported via nerve fibers.
Anterior pituitary: Releases hormones in response to regulatory hormones from the hypothalamus via the hypophyseal portal system.

Example: Nerve impulses from the hypothalamus trigger the release of oxytocin during childbirth.

Factors Affecting Hormone Action

Blood levels of the hormone
Number of receptors for that hormone on/in the target cell
Affinity (strength) of the binding between hormone and receptor

Major Endocrine Glands and Their Hormones

Pituitary Hormones

The pituitary gland, often called the "master gland," releases several key hormones that regulate other endocrine glands and body functions.

Hormone	Released by (Posterior/Anterior Pituitary)	Target Cells	Effects
Oxytocin	Posterior Pituitary	Uterus and Breast	Stimulates uterine contractions during labor; initiates milk ejection
Antidiuretic hormone (ADH, vasopressin)	Posterior Pituitary	Kidneys	Stimulates kidney tubules to reabsorb water, reducing urine output
Growth hormone (GH)	Anterior Pituitary	Liver, muscle, cartilage, bone, other tissues	Stimulates growth, protein synthesis, increases blood glucose
Thyroid-stimulating hormone (TSH)	Anterior Pituitary	Thyroid gland	Stimulates thyroid hormone release
Adrenocorticotropic hormone (ACTH)	Anterior Pituitary	Adrenal cortex	Stimulates release of glucocorticoids (e.g., cortisol)
Follicle-stimulating hormone (FSH)	Anterior Pituitary	Ovaries and testes	Stimulates gamete production (ova, sperm)
Luteinizing hormone (LH)	Anterior Pituitary	Ovaries and testes	Triggers ovulation, stimulates production of sex hormones
Prolactin (PRL)	Anterior Pituitary	Breast secretory tissue	Promotes lactation

Other Major Endocrine Hormones

Hormone	Endocrine Gland	Target Cells	Effects
Thyroid hormone (TH, T3/T4)	Thyroid	Virtually every cell in body	Regulates metabolism, growth, and development
Calcitonin	Thyroid	Bone cells (osteoblasts, osteoclasts)	Lowers blood calcium by promoting bone deposition
Parathyroid hormone (PTH)	Parathyroid gland	Bones, kidneys, intestines	Raises blood calcium by stimulating release from bone, reabsorption in kidneys, and absorption in intestines
Aldosterone	Adrenal cortex	Kidney tubules	Increases sodium reabsorption, regulates blood pressure
Cortisol	Adrenal cortex	Most body cells	Stress response, increases blood glucose
Epinephrine (adrenaline)	Adrenal medulla	Most body cells, especially heart, liver, muscles	Fight or flight response: increases heart rate, blood pressure, blood glucose
Norepinephrine (noradrenaline)	Adrenal medulla	Most body cells, especially blood vessels	Enhances alertness, vasoconstriction
Melatonin	Pineal gland	Brain, hypothalamus	Regulates sleep/wake cycles
Glucagon	Pancreas (alpha cells)	Liver	Raises blood glucose by promoting glycogen breakdown
Insulin	Pancreas (beta cells)	Most body cells, especially liver, muscle, fat	Lowers blood glucose by promoting uptake and storage
Estrogen	Ovaries	Female reproductive organs	Promotes development of female secondary sex characteristics, regulates menstrual cycle
Testosterone	Testes	Male reproductive organs	Promotes male secondary sex characteristics, sperm production
Thymosin	Thymus	White blood cells (lymphocytes)	Promotes T cell development for immune function

Endocrine Glands and Lifespan Changes

Examples of Endocrine Glands That Change Over the Lifespan

Thymus gland: Shrinks after puberty and is replaced by connective and fatty tissue. Hormone production decreases, reducing immune function with age.
Pineal gland: Secretes melatonin, which is high during childhood and adolescence but declines with age, affecting sleep cycles.
Gonads (ovaries and testes): Produce sex hormones (estrogen, progesterone, testosterone). With age, hormone production drops (e.g., menopause in women, gradual testosterone decline in men), affecting reproductive capability and secondary sex characteristics.

Key Vocabulary and Concepts

Term	Definition	Example/Use in Sentence	Synonym
Hormone	Chemical messenger produced by endocrine glands	Her pituitary gland released the hormone FSH which stimulated her ovaries.	Chemical messenger
Exocrine gland	Gland that releases secretions through ducts to a surface	I have exocrine glands in my stomach that secrete acid.	Secretion glands
ACTH	Adrenocorticotropic hormone; stimulates adrenal cortex to release cortisol	ACTH signals the adrenal glands to release cortisol when we're stressed.	Stress response hormone
Prolactin	Hormone that stimulates milk production	Prolactin helps mothers produce milk after childbirth.	Milk producing hormone
Growth hormone (GH)	Stimulates growth of body tissues	Growth hormone helps children grow taller and build muscle.	Body growth stimulator
Oxytocin	Hormone that stimulates uterine contractions and milk ejection	Oxytocin helps with childbirth.	Love hormone
Pineal gland	Gland in the brain that secretes melatonin	The pineal gland releases melatonin to help us sleep better at night.	Sleep regulator

Summary

The endocrine system uses hormones to regulate body functions, growth, metabolism, and homeostasis.
Hormones act only on target cells with specific receptors and may use second messengers to exert their effects.
Major endocrine glands include the pituitary, thyroid, adrenal glands, pancreas, gonads, pineal gland, and thymus.
Hormone production and gland function can change over the lifespan, affecting growth, reproduction, and immune function.