BIO 169: The Endocrine System - Comprehensive Study Notes

Endocrine System Overview

Introduction to the Endocrine System

The endocrine system is one of the body's two major regulatory systems, alongside the nervous system. It synthesizes and secretes chemical messengers called hormones into the bloodstream, which bind to receptors on target tissues to elicit changes in cellular function. The endocrine system maintains fluid, electrolyte, and acid-base homeostasis, promotes growth, regulates metabolic reactions, and responds to stressors.

• Hormones: Chemical messengers secreted by endocrine glands.

• Target tissues: Cells with specific receptors for hormones.

• Functions: Regulation of metabolism, growth, homeostasis, and response to stress.

Endocrine vs. Exocrine Glands

• Endocrine glands: Release hormones directly into the bloodstream.

• Exocrine glands: Release substances into ducts (e.g., sweat, digestive enzymes).

Nervous System vs. Endocrine System

The nervous system acts rapidly via electrical impulses, while the endocrine system acts more slowly via hormones.

• Nervous system: Fast, short-lived responses.

• Endocrine system: Slow, long-lasting effects.

Anatomy of the Endocrine System

Primary, Secondary, and Neuroendocrine Organs

• Primary organs: Pituitary, thyroid, parathyroid, adrenal glands, pancreas, gonads.

• Secondary organs: Heart, kidneys, adipose tissue, thymus.

• Neuroendocrine organs: Hypothalamus, pineal gland, adrenal medulla.

Classes of Hormones

Steroid vs. Non-Steroid Hormones

Hormones are classified based on their chemical structure and solubility.

• Steroid hormones: Derived from cholesterol, hydrophobic, require carrier proteins, can pass through cell membranes, bind to intracellular receptors.

• Non-steroid hormones: Amino acid-based, hydrophilic, freely carried in blood, cannot pass through cell membranes, bind to extracellular receptors and use second messengers.

Hormone Mechanisms and Effects

Mechanism of Action

Hormones exert their effects by binding to specific receptors, which can be located on the cell surface or inside the cell.

• Stimulate secretions from endocrine and exocrine cells

• Activate or inhibit enzymes

• Stimulate or inhibit mitosis/meiosis

• Open/close ion channels, alter membrane potential

• Activate or inhibit gene transcription/expression

Hormone Interactions

• Synergists: Hormones that work together to amplify effects.

• Antagonists: Hormones that oppose each other's actions.

Regulation of Hormone Secretion

Feedback Mechanisms

Most hormone secretion is regulated by negative feedback, which maintains homeostasis by adjusting hormone levels in response to changes in the internal environment.

• Negative feedback: Hormone release decreases as its effects are achieved.

Endocrine Glands and Their Hormones

Hormone Chart

The following table summarizes the main glands, their hormones, target tissues, and functions.

Gland	Part of gland/cell	Hormone	Target Tissue	Function
Thyroid	Follicles	T3/T4	Most body tissues	Regulates metabolism, thermoregulation
Thyroid	Parafollicular cells	Calcitonin	Osteoclasts	Decreases blood calcium
Parathyroid	Chief cells	PTH	Osteoclasts, kidneys, intestines	Increases blood calcium
Adrenals	Cortex	Aldosterone, cortisol, androgens	Kidneys, most tissues	Regulates BP, stress, sexual characteristics
Adrenals	Medulla	Epinephrine, norepinephrine	Various organs	Fight-or-flight response
Pancreas	Alpha cells	Glucagon	Liver, muscle, adipose	Raises blood glucose
Pancreas	Beta cells	Insulin	Liver, muscle, brain	Lowers blood glucose
Pineal	-	Melatonin	Brainstem	Regulates sleep/wake cycles
Thymus	-	Thymosin, thymopoietin	Lymph organs	T cell maturation
Testes	-	Testosterone	Reproductive organs	Male sex characteristics
Ovaries	-	Estrogen, progesterone	Reproductive organs	Female sex characteristics, pregnancy
Hypothalamus	-	Releasing/inhibiting hormones	Pituitary	Regulates pituitary hormones
Pituitary	Anterior	TSH, PRL, GH, FSH, LH, ACTH	Various	Regulates other glands and growth
Pituitary	Posterior	Oxytocin, ADH	Mammary glands, kidneys	Milk release, water retention

Hypothalamus and Pituitary Gland

Anatomy and Function

The hypothalamus and pituitary gland are part of the diencephalon and are connected by the infundibulum. The hypothalamic-hypophyseal portal system allows communication between these structures.

• Anterior pituitary: Glandular epithelial tissue

• Posterior pituitary: Nervous tissue

Posterior Pituitary Hormones

The posterior pituitary stores hormones made by the hypothalamus:

• Oxytocin: Stimulates milk release and uterine contraction; known as the "cuddle hormone".

• Antidiuretic hormone (ADH): Increases water retention by kidneys, affecting blood pressure.

Anterior Pituitary Hormones

The hypothalamus secretes releasing and inhibiting hormones to regulate the anterior pituitary.

• TSH: Stimulates thyroid hormone secretion

• PRL: Milk production

• GH: Growth and cell division

• FSH: Egg and sperm production

• LH: Ovulation and testosterone production

• ACTH: Stimulates adrenal cortex

Thyroid and Parathyroid Glands

Thyroid Anatomy and Hormones

The thyroid is located in the anterior neck, superficial to the larynx. It consists of follicles (produce T3/T4) and parafollicular cells (produce calcitonin).

• T3/T4: Regulate metabolic rate and thermoregulation

• Calcitonin: Decreases blood calcium by inhibiting osteoclasts

Parathyroid Anatomy and Hormones

The parathyroid glands are located posterior to the thyroid. Chief cells produce parathyroid hormone (PTH), which increases blood calcium.

• PTH: Increases osteoclast activity, Ca2+ reabsorption in kidneys and intestines

Thyroid Disorders

• Hypothyroidism: Decreased metabolism, weight gain, cold intolerance, slow heart rate, low BP. Causes: iodine deficiency, Hashimoto thyroiditis.

• Hyperthyroidism: Increased metabolism, weight loss, heat intolerance, rapid heart rate, increased BP, exophthalmia. Cause: Graves disease.

• Goiter: Enlargement of the thyroid, can occur in both disorders.

Regulation of Thyroid Hormones

The TRH-TSH-Thyroid hormone pathway involves negative feedback to maintain metabolic balance.

Thyroid and Parathyroid Summary Table

Cell Type	Hormones	Stimulus for Secretion	Inhibitor of Secretion	Target Tissues	Effects
Follicle cells (Thyroid)	T3/T4	TSH from anterior pituitary	High T3/T4	Nearly every cell	Metabolic rate, thermoregulation, growth
Parafollicular cells (Thyroid)	Calcitonin	Increased blood Ca2+	Decreased blood Ca2+	Osteoclasts	Inhibits osteoclasts, lowers blood Ca2+
Chief cells (Parathyroid)	PTH	Decreased blood Ca2+	Increased blood Ca2+	Bone, kidneys, intestines	Raises blood Ca2+

Adrenal Glands

Anatomy and Hormones

The adrenal glands consist of an outer cortex and inner medulla.

• Zona glomerulosa: Mineralocorticoids (aldosterone)

• Zona fasciculata: Glucocorticoids (cortisol)

• Zona reticularis: Androgenic steroids

• Medulla: Epinephrine/norepinephrine

Adrenal Hormone Functions

• Aldosterone: Increases sodium and water reabsorption, raises BP

• Cortisol: Stress hormone, increases blood glucose, reduces inflammation

• Androgens: Secondary sexual characteristics

• Epinephrine/norepinephrine: Fight-or-flight response

Adrenal Disorders

• Hypercortisolism: Cushing disease/syndrome; muscle breakdown, weight gain, hyperglycemia, increased BP, weakened immunity

• Addison disease: Adrenal insufficiency; hyposecretion of cortisol/aldosterone, fluid/electrolyte imbalance

Pancreas

Anatomy and Hormones

The pancreas is both an endocrine and exocrine gland.

• Pancreatic islets: Alpha cells (glucagon), beta cells (insulin), delta cells (somatostatin)

• Acinar cells: Secrete digestive enzymes

Glucagon and Insulin

• Glucagon: Raises blood glucose; targets liver, muscle, adipose tissue

• Insulin: Lowers blood glucose; targets liver, muscle, brain

Diabetes Mellitus

• Type I: Autoimmune destruction of beta cells; decreased insulin production

• Type II: Insulin resistance; cells do not recognize insulin

• Symptoms: Glycosuria, polyuria, polydipsia, vascular damage, neuropathy, blindness, kidney failure

Blood Glucose Regulation

Other Endocrine Glands

Pineal Gland

• Melatonin: Regulates sleep/wake cycles and mood; targets brainstem

Thymus

• Thymosin, thymopoietin: T lymphocyte maturation; targets lymph organs

Gonads

• Testes: Testosterone; male sex characteristics, spermatogenesis

• Ovaries: Estrogen and progesterone; female sex characteristics, menstrual cycle, pregnancy

Hormones from Other Organs and Tissues

• Adipose tissue: Leptin; regulates appetite

• Heart: Atrial natriuretic peptide; lowers blood pressure

• Kidneys: Erythropoietin; stimulates red blood cell production

Summary and Study Tips

• Preview materials before class for better retention

• Use spaced repetition and immediate review

• Rewrite notes to enhance memory

Key Pathways and Feedback Mechanisms

TRH-TSH-Thyroid Hormone Pathway

Regulation of Blood Calcium

Control of Blood Glucose by Insulin & Glucagon