Chapter 16: The Endocrine System – Study Notes

Endocrine System Overview

Nervous System vs. Endocrine System

The nervous system and endocrine system are the two main control systems of the body. The nervous system uses electrical impulses and neurotransmitters for rapid, short-term responses, while the endocrine system uses hormones for slower, longer-lasting regulation.

• Nervous System: Fast, short-lived responses; uses neurons and neurotransmitters.

• Endocrine System: Slow, long-lasting effects; uses hormones released into the bloodstream.

Endocrine System Functions and Components

• Controls and integrates: Reproduction, growth, metabolism, electrolyte balance, and stress responses.

• Glands: Exocrine glands secrete products into ducts (e.g., sweat, saliva), while endocrine glands secrete hormones directly into the bloodstream.

• Hormones: Chemical messengers that regulate target cell activity.

• Autocrines: Chemicals that exert effects on the same cells that secrete them.

• Paracrines: Chemicals that act locally on nearby cells.

Hormone Classification

• Amino acid-based hormones: Most hormones; water-soluble; cannot cross the plasma membrane. Examples: Insulin, epinephrine, growth hormone.

• Steroid-based hormones: Derived from cholesterol; lipid-soluble; can cross the plasma membrane. Examples: Cortisol, aldosterone, estrogen, testosterone.

Water-soluble hormones include all amino acid-based hormones except thyroid hormone. Lipid-soluble hormones include steroid hormones and thyroid hormone.

Target Cell Specificity and Hormone Half-life

• Target cells: Cells with specific receptors for a hormone.

• Half-life: The time required for a hormone's blood level to decrease by half; water-soluble hormones have shorter half-lives than lipid-soluble hormones.

Hormone Mechanisms of Action

Second Messenger (G Protein) System

Most water-soluble hormones act via second messengers. The most common is the cAMP pathway.

• Hormone binds to receptor on plasma membrane.

• Receptor activates G protein.

• G protein activates adenylate cyclase.

• Adenylate cyclase converts ATP to cAMP.

• cAMP activates protein kinases, leading to cellular responses.

Example: Epinephrine acting on liver cells to promote glycogen breakdown.

Key Enzyme: Adenylate cyclase converts ATP to cAMP.

Direct Gene Activation

Lipid-soluble hormones (steroids and thyroid hormone) act by entering the cell and binding to intracellular receptors.

• Hormone diffuses through plasma membrane.

• Binds to intracellular receptor (in cytoplasm or nucleus).

• Hormone-receptor complex binds to DNA, activating specific genes.

• Leads to mRNA synthesis and protein production.

Example: Cortisol stimulating synthesis of glucose-metabolizing enzymes.

Summary Table: Hormone Mechanisms

Hormone Type	Solubility	Receptor Location	Mechanism
Amino acid-based	Water-soluble	Plasma membrane	Second messenger (e.g., cAMP)
Steroid-based	Lipid-soluble	Intracellular	Direct gene activation
Thyroid hormone	Lipid-soluble	Intracellular	Direct gene activation

Endocrine Gland Stimuli and Nervous System Modulation

• Humoral stimuli: Changes in blood levels of ions/nutrients (e.g., low Ca2+ stimulates PTH release).

• Neural stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic stimulation of adrenal medulla).

• Hormonal stimuli: Hormones stimulate other endocrine glands (e.g., hypothalamic hormones stimulate pituitary).

The nervous system can override or modulate endocrine controls during stress or emergencies.

Major Endocrine Organs and Hormones

Hypothalamus

• Produces releasing and inhibiting hormones that regulate the anterior pituitary.

• Releasing hormones: Stimulate pituitary hormone release (e.g., TRH, CRH, GnRH, GHRH).

• Inhibiting hormones: Inhibit pituitary hormone release (e.g., GHIH, PIH).

Pituitary Gland

• Posterior pituitary: Neural tissue; stores and releases hormones made by hypothalamus.

• Hormones: Oxytocin (OXY) (uterine contraction, milk ejection), Antidiuretic hormone (ADH) (water retention by kidneys).

• Anterior pituitary: Glandular tissue; produces and releases its own hormones under hypothalamic control.

• Hormones: Growth hormone (GH), Prolactin (PRL), Follicle-stimulating hormone (FSH), Luteinizing hormone (LH), Adrenocorticotropic hormone (ACTH), Thyroid-stimulating hormone (TSH).

• Tropic hormones: Hormones that regulate other endocrine glands (FSH, LH, ACTH, TSH).

Connections: The hypothalamus communicates with the anterior pituitary via the hypophyseal portal system (primary capillary plexus), and with the posterior pituitary via nerve fibers.

Thyroid Gland

• Located in the neck; consists of follicles filled with colloid.

• Thyroid hormone (TH): Major hormone secreted by follicles; regulates metabolism.

• At target tissues, most T4 is converted to T3 (the more active form).

• Calcitonin: Lowers blood calcium levels.

• Iodide must be oxidized to iodine upon entering the colloid for TH synthesis.

Parathyroid Gland

• Secretes parathyroid hormone (PTH), which increases blood calcium by acting on:

  • Bone: Stimulates osteoclasts to release Ca2+.

  • Kidneys: Increases Ca2+ reabsorption and activates vitamin D.

  • Intestine: Increases Ca2+ absorption (via activated vitamin D).

Adrenal Gland

• Cortex: Three layers, each producing different corticosteroids:

  • Zona glomerulosa: Mineralocorticoids (e.g., aldosterone; regulates Na+ and K+ balance).

  • Zona fasciculata: Glucocorticoids (e.g., cortisol; regulates metabolism and stress response).

  • Zona reticularis: Gonadocorticoids (e.g., androgens).

• Medulla: Produces catecholamines (epinephrine and norepinephrine) in response to sympathetic stimulation.

Pineal Gland

• Secretes melatonin, which regulates sleep-wake cycles.

Pancreas

• Both endocrine and exocrine functions.

• Exocrine: Acinar cells secrete digestive enzymes.

• Endocrine: Pancreatic islets (islets of Langerhans) secrete hormones:

  • Insulin: Lowers blood glucose.

  • Glucagon: Raises blood glucose.

• Diabetes mellitus: Results from hyposecretion or hypoactivity of insulin.

Gonads and Placenta

• Ovaries: Produce estrogens and progesterone.

• Testes: Produce testosterone.

• Placenta: Produces hormones such as hCG, estrogens, and progesterone during pregnancy.

Other Hormone-Producing Organs

• Many organs not considered major endocrine organs produce hormones (e.g., heart, kidneys, GI tract).

Homeostatic Imbalances and Hormone Effects

• For each hormone, know the effects of hypo- and hypersecretion (e.g., diabetes insipidus from low ADH, gigantism from excess GH).

• Recognize the target organs and physiological effects of each hormone.

Summary Table: Major Endocrine Glands and Hormones

Gland	Hormone(s)	Main Target(s)	Main Effect(s)
Anterior Pituitary	GH, PRL, FSH, LH, ACTH, TSH	Various	Growth, lactation, gonadal function, adrenal cortex, thyroid
Posterior Pituitary	OXY, ADH	Uterus, kidneys	Labor, milk ejection, water retention
Thyroid	TH, Calcitonin	Most cells, bone	Metabolism, lowers blood Ca2+
Parathyroid	PTH	Bone, kidney, intestine	Raises blood Ca2+
Adrenal Cortex	Aldosterone, Cortisol, Androgens	Kidney, most cells	Na+ balance, stress response, sex characteristics
Adrenal Medulla	Epinephrine, Norepinephrine	Heart, blood vessels	Fight-or-flight response
Pancreas	Insulin, Glucagon	Liver, muscle, fat	Blood glucose regulation
Ovaries/Testes	Estrogen, Progesterone, Testosterone	Reproductive organs	Sexual development, reproduction

Additional info: For exam preparation, be able to match each hormone to its gland, target organ, and physiological effect, and recognize symptoms of hypo- and hypersecretion.