The Endocrine System: Endocrine Glands and Hormone Actions

CHAPTER 6: THE ENDOCRINE SYSTEM: ENDOCRINE GLANDS AND HORMONE ACTIONS

I. Introduction to the Endocrine System

The endocrine system is a network of glands that secrete hormones to regulate various physiological processes throughout the body. Hormones act as long-distance chemical messengers, influencing target cells and organs. Endocrine organs are classified based on whether hormone secretion is their primary or secondary function.

• Primary Endocrine Organs: Main function is hormone secretion (e.g., Pituitary Gland).

• Secondary Endocrine Organs: Secrete hormones as a secondary function (e.g., Kidneys).

• Hormones: Chemical messengers secreted into the blood, acting on distant target cells at low concentrations to initiate biochemical responses.

II. Primary Endocrine Glands

A. Hypothalamus and Pituitary Gland

The hypothalamus and pituitary gland work together to regulate nearly every system in the body. The hypothalamus is part of the brain and a primary endocrine gland, influencing the pituitary gland through hormone secretion.

• Pituitary Gland (Hypophysis): Pea-sized, connected to the hypothalamus by the infundibulum.

• Divided into two lobes:

  • Anterior Lobe (Adenohypophysis): Epithelial tissue; secretes tropic hormones that regulate other endocrine glands.

  • Posterior Lobe (Neurohypophysis): Nerve tissue; stores and releases neurohormones produced by the hypothalamus.

1. Posterior Pituitary Hormones

• Antidiuretic Hormone (ADH, Vasopressin): Increases water reabsorption in the kidneys; released in response to increased plasma solute concentration.

• Oxytocin: Stimulates uterine contractions during childbirth and milk ejection during breastfeeding; released in response to uterine pressure and infant suckling.

• Both hormones are synthesized in the hypothalamus and released from the posterior pituitary via neuroendocrine reflexes.

• These are neurohormones because they are released by neurons.

2. Anterior Pituitary Hormones

• Secretes tropic hormones that regulate other endocrine glands.

• Release is controlled by hypothalamic hormones via the hypothalamic-pituitary portal system (two capillary beds connected by a portal vein).

• Examples of hypothalamic and anterior pituitary hormones:

  • Prolactin Releasing Hormone (PRH): Stimulates prolactin release (breast development, milk secretion).

  • Prolactin Inhibiting Hormone (PIH, Dopamine): Inhibits prolactin release.

  • Thyrotropin Releasing Hormone (TRH): Stimulates TSH release, which stimulates thyroid hormone secretion (regulates metabolism).

  • Corticotropin Releasing Hormone (CRH): Stimulates ACTH release, which stimulates glucocorticoid (e.g., cortisol) secretion (stress response).

  • Growth Hormone Releasing Hormone (GHRH): Stimulates growth hormone (GH) secretion (growth, energy metabolism).

  • Growth Hormone Inhibiting Hormone (GHIH, Somatostatin): Inhibits GH secretion.

  • Gonadotropin Releasing Hormone (GnRH): Stimulates FSH and LH release (gamete development, ovulation).

• Hormone release is often regulated by negative feedback loops.

B. Pineal Gland

• Located in the brain; secretes melatonin.

• Melatonin regulates circadian rhythms (sleep-wake cycles); secretion is stimulated by darkness and inhibited by light.

• May enhance immune function.

C. Thyroid Gland

• Butterfly-shaped, located on the ventral surface of the trachea.

• Secretes:

  • Thyroid Hormones (Tetraiodothyronine/T4 and Triiodothyronine/T3): Regulate metabolic rate, growth, and development.

  • Calcitonin: Lowers blood calcium by stimulating bone formation.

• Hyperthyroidism: Weight loss, rapid heart rate, anxiety, excessive sweating.

• Hypothyroidism: Fatigue, muscle aches, weight gain, dry skin.

D. Parathyroid Glands

• Four small glands on the posterior thyroid.

• Secrete Parathyroid Hormone (PTH): Increases blood calcium by stimulating bone resorption and enhancing calcium reabsorption in the kidneys.

E. Thymus Gland

• Located near the heart in the thoracic cavity.

• Secretes Thymosin: Programs T lymphocytes for immune function.

• Most active in childhood; atrophies with age.

F. Adrenal Glands (Suprarenal Glands)

• Paired glands atop the kidneys; consist of two regions:

  • Adrenal Cortex (outer): Secretes steroid hormones.

  • Adrenal Medulla (inner): Secretes catecholamines.

• Adrenal Cortex Hormones:

  • Mineralocorticoids (Aldosterone): Regulate sodium and potassium balance.

  • Glucocorticoids (Cortisol): Regulate stress response and metabolism of lipids, carbohydrates, and proteins.

  • Androgens: Sex hormones involved in reproductive function.

• Addison's Disease: Hyposecretion of cortisol (fatigue, low blood pressure, skin bronzing).

• Cushing's Disease: Hypersecretion of cortisol (weight gain, fat redistribution, cervical hump).

• Adrenal Medulla Hormones (Chromaffin Cells):

  • Epinephrine (Adrenaline): Increases heart rate, fight-or-flight response (80% of secretion).

  • Norepinephrine: Fight-or-flight response (19%).

  • Dopamine: Inhibits prolactin secretion (1%).

G. Pancreas

• Located behind and below the stomach; both exocrine (digestive enzymes) and endocrine (hormones) functions.

• Islets of Langerhans: Endocrine cell clusters.

  • Beta Cells: Secrete Insulin (lowers blood glucose by promoting cellular uptake and ATP production).

  • Alpha Cells: Secrete Glucagon (raises blood glucose by stimulating glycogen breakdown in the liver).

• Hyperinsulinism: Excess insulin secretion.

• Hypoinsulinism: Insufficient insulin secretion.

H. Ovaries and Testes

• Produce gametes and secrete sex hormones.

• Testes: Secrete testosterone and androstenedione.

• Ovaries: Secrete estrogen and progesterone.

• Placenta: Secretes human chorionic gonadotropin (hCG) during pregnancy (used in pregnancy tests).

III. Secondary Endocrine Organs

• Organs whose primary function is not hormone secretion but do produce hormones.

• Major Secondary Endocrine Organs:

  • Kidneys: Secrete erythropoietin (stimulates red blood cell production).

  • Digestive Tract: Secretes gastrin, cholecystokinin, and other hormones for digestion.

  • Liver: Secretes various growth factors.

IV. Hormone Actions at the Target Cell

The concentration of free hormone in the blood determines the magnitude of the hormone's effect on target cells. This concentration is regulated by secretion rate, transport, and metabolism.

• Factors Affecting Free Hormone Concentration:

  1. Rate of hormone secretion

  2. Amount of hormone bound to carrier proteins (only unbound hormones are active)

  3. Rate of hormone metabolism (degradation)

• Rate of Hormone Secretion: Controlled by neural or humoral (bloodborne) signals.

  • Humoral Signals: Include hormones, ions, and metabolites in the blood.

  • Example: Regulation of blood potassium:

    1. Increased blood potassium detected by sensory neurons.

    2. Adrenal cortex releases aldosterone.

    3. Aldosterone increases potassium excretion by the kidneys (negative feedback).

  • Example: Insulin release in response to blood glucose (negative feedback).

• Transport of Hormones: Only unbound (free) hormones can bind to target cell receptors.

• Hormone Metabolism: Hormones are rapidly degraded, mainly in the liver; metabolites are either stored or excreted by the kidneys.

Table: Major Endocrine Glands and Their Hormones

Gland	Hormone(s)	Main Function(s)
Pituitary (Anterior)	GH, TSH, ACTH, FSH, LH, Prolactin	Growth, metabolism, stress response, reproduction, lactation
Pituitary (Posterior)	ADH, Oxytocin	Water balance, uterine contraction, milk ejection
Pineal	Melatonin	Circadian rhythms, immune modulation
Thyroid	T3, T4, Calcitonin	Metabolism, growth, calcium regulation
Parathyroid	PTH	Increases blood calcium
Thymus	Thymosin	T cell maturation
Adrenal Cortex	Aldosterone, Cortisol, Androgens	Electrolyte balance, stress response, reproductive function
Adrenal Medulla	Epinephrine, Norepinephrine, Dopamine	Fight-or-flight response
Pancreas	Insulin, Glucagon	Blood glucose regulation
Ovaries	Estrogen, Progesterone	Female reproductive function
Testes	Testosterone, Androstenedione	Male reproductive function
Placenta	hCG	Pregnancy maintenance
Kidneys (secondary)	Erythropoietin	Red blood cell production
Digestive Tract (secondary)	Gastrin, CCK, others	Digestive regulation
Liver (secondary)	Growth factors	Growth, metabolism

Key Equations

• Negative Feedback Regulation (Generalized): $\text{Stimulus} \rightarrow \text{Endocrine Gland} \rightarrow \text{Hormone Release} \rightarrow \text{Target Cell Response} \rightarrow \text{Reduced Stimulus}$

• Hormone Concentration in Blood: $[\text{Free Hormone}] = \frac{\text{Rate of Secretion} - \text{Rate of Metabolism}}{\text{Volume of Distribution}}$

Example: Regulation of Blood Glucose

• After a meal, blood glucose rises, stimulating insulin release from pancreatic beta cells.

• Insulin promotes glucose uptake by cells, lowering blood glucose to normal levels (negative feedback).

• During fasting, blood glucose falls, stimulating glucagon release from alpha cells, which increases blood glucose by promoting glycogen breakdown in the liver.

Additional info: The hypothalamic-pituitary axis is a central regulatory system for many endocrine functions, and negative feedback is a common mechanism to maintain homeostasis in hormone levels.