Study Notes — The Endocrine System

Main Function

• Purpose: Maintain homeostasis — the body’s internal balance (blood pressure, glucose, electrolytes, etc.).

• Mechanism: Uses hormones — chemical messengers transported in the bloodstream.

• Target Cells: Only respond if they have specific receptors for a hormone.

Major Control Centers

1. Hypothalamus

• Located at the base of the brain.

• Acts as the master regulator of homeostasis.

• Sends signals to the pituitary gland through hormones or direct neural control.

2. Pituitary Gland (Master Gland)

• Located just below the hypothalamus.

• Divided into:

  • Anterior Pituitary (Adenohypophysis)

  • Posterior Pituitary (Neurohypophysis)

Anterior Pituitary Hormones

Controlled by releasing hormones from the hypothalamus.

Hypothalamic Releasing Hormone	Pituitary Hormone Released	Target Organ	Function
Thyrotropin Releasing Hormone (TRH)	TSH (Thyroid Stimulating Hormone)	Thyroid	Stimulates T3, T4 secretion (metabolism).
Corticotropin Releasing Hormone (CRH)	ACTH (Adrenocorticotropic Hormone)	Adrenal Cortex	Stimulates cortisol release (stress).
Growth Hormone Releasing Hormone (GHRH)	GH (Growth Hormone)	Bones, muscles	Promotes growth.
Gonadotropin Releasing Hormone (GnRH)	LH, FSH	Ovaries/Testes	Controls sex hormone production and gametogenesis.

Other Anterior Pituitary Hormones (Not directly from hypothalamus):

• MSH (Melanocyte Stimulating Hormone): Increases melanin production for UV protection.

• Prolactin: Milk production and ejection during pregnancy and breastfeeding.

Posterior Pituitary Hormones

Secreted directly by hypothalamic neurons into the blood.

Hormone	Target	Function
Oxytocin	Uterus, mammary glands	Stimulates uterine contractions and milk ejection.
ADH (Vasopressin)	Kidneys	Promotes water reabsorption → prevents dehydration.

Other Endocrine Organs & Hormones

1. Pineal Gland

• Hormone: Melatonin

• Function: Regulates sleep-wake cycle (more in darkness).

2. Thyroid Gland

• Stimulated by: TSH

• Hormones:

  • T3 & T4: Regulate metabolism and energy use.

  • Calcitonin: Lowers blood calcium by storing calcium in bones.

3. Thymus Gland

• Hormone: Thymosin

• Function: Matures lymphocytes (immune defense).

• Note: Shrinks after puberty.

4. Kidneys

• Renin: Raises blood pressure (via RAAS system).

• Erythropoietin (EPO): Stimulates red blood cell production.

5. Adrenal Glands

Divided into Cortex and Medulla.

Adrenal Cortex:

Hormone	Function
Cortisol	Stress hormone; increases blood sugar, activates “fight or flight.”
Aldosterone	Retains salt & water; increases blood volume & pressure.
Androgens	Precursor sex hormones.

Adrenal Medulla:

Hormone	Function
Epinephrine (Adrenaline)	Increases HR, BP, and airway dilation (fight or flight).
Norepinephrine	Works with epinephrine to heighten alertness and circulation.

6. Pancreas

Hormone	Function
Insulin	Lowers blood sugar by allowing glucose into cells.
Glucagon	Raises blood sugar by promoting glucose release from liver.

7. Reproductive Organs

Ovaries:

Hormone	Function
Estrogen	Female secondary sex characteristics.
Progesterone	Prepares uterus for pregnancy.

Testes:

Hormone	Function
Testosterone	Male secondary sex characteristics, sperm production.

Summary Flow Chart

1. Hypothalamus → releases hormones →

2. Pituitary Gland → releases regulating hormones →

3. Target Glands (Thyroid, Adrenal, Gonads, etc.) → release final hormones →

4. Target Organs/Tissues → maintain homeostasis.

Quick Review

• Endocrine System = Hormonal Communication System.

• Key Goal: Maintain internal balance (homeostasis).

• Main Players: Hypothalamus + Pituitary + Target Glands.

• Hormones: Chemical messengers in the blood acting on specific receptors.