Endocrine System: Hormones, Glands, and Physiological Effects

Endocrine System Overview

The endocrine system is a network of glands that secrete hormones directly into the bloodstream to regulate various physiological processes. These hormones control growth, metabolism, reproduction, and homeostasis. The following notes summarize the major endocrine glands, their hormones, and the target organs and effects.

Hypothalamus and Pituitary Gland

Hypothalamus

The hypothalamus is a region of the brain that links the nervous system to the endocrine system via the pituitary gland. It produces releasing and inhibiting hormones that regulate the anterior and posterior pituitary.

• GnRH (Gonadotropin-releasing hormone): Stimulates release of FSH and LH from anterior pituitary.

• GHRH (Growth hormone-releasing hormone): Stimulates release of GH.

• GHIH (Growth hormone-inhibiting hormone): Inhibits release of GH.

• TRH (Thyrotropin-releasing hormone): Stimulates release of TSH.

• CRH (Corticotropin-releasing hormone): Stimulates release of ACTH.

• PIH (Prolactin-inhibiting hormone): Inhibits release of prolactin.

Pituitary Gland

The pituitary gland is divided into anterior and posterior lobes, each releasing different hormones.

Posterior Pituitary

• Oxytocin: Stimulates uterine contractions during childbirth and milk ejection during breastfeeding.

• ADH (Antidiuretic hormone): Regulates water balance (not shown in diagram).

Anterior Pituitary

• PRL (Prolactin): Stimulates milk production in breasts.

• GH (Growth hormone): Stimulates growth, cell reproduction, and regeneration; acts on liver, skeletal muscle, and bone to increase IGFs (insulin-like growth factors).

• TSH (Thyroid-stimulating hormone): Stimulates thyroid gland to release thyroid hormones (TH).

• ACTH (Adrenocorticotropic hormone): Stimulates adrenal cortex to release corticosteroids.

• FSH (Follicle-stimulating hormone): Stimulates sperm production in testes and egg production in ovaries.

• LH (Luteinizing hormone): Stimulates testosterone production in testes and estrogen/progesterone production in ovaries.

Major Endocrine Glands and Their Hormones

Thyroid Gland

• Thyroid Hormones (TH): Increase metabolic rate, regulate growth and development.

• Calcitonin: Lowers blood calcium levels by increasing calcium storage in bones.

Parathyroid Gland

• PTH (Parathyroid hormone): Raises blood calcium levels by stimulating calcium release from bones and reabsorption in kidneys; activates vitamin D.

Adrenal Glands

• Adrenal Cortex: Releases corticosteroids for long-term stress response.

• Adrenal Medulla: Releases norepinephrine (NE) for short-term stress response.

Pancreas

• Insulin: Lowers blood glucose by increasing cellular uptake and promoting glycogenesis.

• Glucagon: Raises blood glucose by stimulating glycogenolysis and gluconeogenesis in the liver.

Pineal Gland

• Melatonin: Regulates circadian rhythms and day/night cycles.

Reproductive Glands

Testes

• Testosterone: Stimulates sperm production and development of male secondary sexual characteristics.

Ovaries

• Estrogens & Progesterone: Regulate menstrual cycle, egg production, and development of female secondary sexual characteristics.

Summary Table: Major Endocrine Glands, Hormones, and Effects

Gland	Hormone	Target Organ	Main Effect
Hypothalamus	GnRH, GHRH, TRH, CRH, PIH	Pituitary	Regulates pituitary hormone release
Anterior Pituitary	GH, TSH, ACTH, FSH, LH, PRL	Various	Growth, metabolism, reproduction, lactation
Posterior Pituitary	Oxytocin	Uterus, breasts	Contractions, milk ejection
Thyroid	TH, Calcitonin	Body cells, bones	Metabolism, calcium storage
Parathyroid	PTH	Bones, kidneys	Increase blood calcium
Adrenal Cortex	Corticosteroids	Body cells	Long-term stress response
Adrenal Medulla	Norepinephrine	Body cells	Short-term stress response
Pancreas	Insulin, Glucagon	Liver, tissues	Regulate blood glucose
Pineal	Melatonin	Brain	Regulate circadian rhythm
Testes	Testosterone	Body cells	Sperm production, male traits
Ovaries	Estrogens, Progesterone	Body cells	Egg production, female traits

Key Terms and Definitions

• Hormone: Chemical messenger secreted by endocrine glands, transported by blood to target organs.

• Gland: An organ that synthesizes substances for release, often hormones.

• Target Organ: The organ or tissue affected by a specific hormone.

• Glycogenesis: Formation of glycogen from glucose.

• Glycogenolysis: Breakdown of glycogen to glucose.

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources.

Example: Regulation of Blood Glucose

• After a meal, insulin is released from the pancreas, promoting glucose uptake by cells and storage as glycogen.

• During fasting, glucagon is released, stimulating the liver to break down glycogen and produce glucose via gluconeogenesis.

Important Equations

• Gluconeogenesis (simplified): $\text{Pyruvate} \rightarrow \text{Glucose}$

• Glycogenolysis: $\text{Glycogen} \rightarrow \text{Glucose}$

Additional info: Some hormone names and pathways were inferred based on standard anatomy and physiology knowledge to provide complete context.