The 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.

Hormones of the Posterior Pituitary

Oxytocin

• Definition: Oxytocin is a peptide hormone produced by the hypothalamus and released by the posterior pituitary gland.

• Functions:

  • Stimulates strong uterine contractions during labor, regulated by a positive feedback mechanism.

  • Triggers milk ejection ("letdown reflex") in lactating women.

  • Plays a role in sexual arousal and satisfaction in both males and nonlactating females.

• Clinical Application: Synthetic and natural oxytocic drugs are used to induce or hasten labor.

Antidiuretic Hormone (ADH, Vasopressin)

• Definition: ADH is a hormone that helps regulate water balance in the body by controlling the amount of water reabsorbed by the kidneys.

• Functions:

  • Prevents urine formation, thus conserving body water.

  • Osmoreceptors in the hypothalamus monitor blood solute concentration to regulate ADH release.

  • High solute concentration stimulates ADH release, promoting water reabsorption.

  • Low solute concentration inhibits ADH release, resulting in water loss.

  • Alcohol inhibits ADH release, causing increased urine output.

Disorders of ADH Secretion

• Diabetes Insipidus:

  • Caused by ADH deficiency; kidneys cannot reabsorb filtered water.

  • Symptoms: Large volume of dilute urine (polyuria), intense thirst (polydipsia).

  • Types: Central (pituitary origin) or nephrogenic (renal origin).

• Syndrome of Inappropriate ADH Secretion (SIADH):

  • Caused by excessive ADH secretion.

  • Symptoms: Fluid retention, brain edema, disorientation, headache, weight gain, decreased blood solute concentration.

  • Treatment: Fluid restriction, monitoring blood sodium levels.

Major Endocrine Structures & Their Hormones

Summary Table

Other Endocrine Structures

• Adipose tissue

• Kidneys

• Pineal gland

• Gastrointestinal tract

• Placenta

• Skin

• Thymus gland

The Thyroid Gland

Structure and Function

• Largest pure endocrine gland, located in the anterior neck, with two lobes connected by the isthmus.

• Composed of follicles that produce the glycoprotein thyroglobulin.

• Colloid (thyroglobulin + iodine) fills the follicle lumen and is the precursor to thyroid hormone.

• Parafollicular (C) cells produce the hormone calcitonin.

Thyroid Hormones (T3 and T4)

• Thyroxine (T4): Contains two tyrosine molecules and four iodine atoms; major hormone secreted by the thyroid.

• Triiodothyronine (T3): Contains two tyrosines and three iodine atoms; more active form, produced at target tissues from T4.

• Functions:

  • Stimulate glucose oxidation and increase metabolic rate.

  • Promote heat production (calorigenic effect).

  • Maintain blood pressure, regulate tissue growth, and support development of skeletal, nervous, and reproductive systems.

• Transport: T3 and T4 bind to thyroxine-binding globulins (TBGs) produced by the liver for transport in the blood.

• Regulation: Controlled by negative feedback involving hypothalamic thyrotropin-releasing hormone (TRH) and pituitary TSH.

Thyroid Disorders

• Hypothyroidism:

  • Cretinism: Congenital hypothyroidism in children; causes growth retardation, mental sluggishness, delayed puberty.

  • Myxedema: Adult hypothyroidism; symptoms include low metabolic rate, chilliness, mental sluggishness.

  • Goiter: Enlarged thyroid gland, often due to iodine deficiency.

• Hyperthyroidism:

  • Graves Disease: Autoimmune disorder; antibodies mimic TSH, causing excess thyroid hormone production. Symptoms: elevated metabolic rate, rapid heartbeat, nervousness, weight loss, exophthalmos (protruding eyes).

Calcitonin

• Produced by: Parafollicular (C) cells of the thyroid.

• Function: Lowers blood calcium levels, especially in children.

• Mechanism:

  • Inhibits osteoclast activity, reducing bone resorption and calcium release.

  • Stimulates calcium uptake and incorporation into bone matrix.

• Regulation: Controlled by blood calcium levels (humoral negative feedback).

• Antagonist: Parathyroid hormone (PTH).

Parathyroid Glands

Structure and Function

• Small glands embedded in the posterior aspect of the thyroid.

• Chief (principal) cells secrete parathyroid hormone (PTH).

• PTH Function: Regulates calcium balance in the blood.

• Actions:

  • Stimulates osteoclasts to digest bone matrix and release calcium.

  • Enhances reabsorption of calcium and excretion of phosphate by the kidneys.

  • Increases absorption of calcium by intestinal mucosa.

• High blood calcium inhibits PTH release (negative feedback).

Adrenal (Suprarenal) Glands

Structure

• Paired, pyramid-shaped organs atop the kidneys.

• Two regions: Adrenal cortex (glandular tissue) and adrenal medulla (nervous tissue).

Adrenal Cortex

• Produces steroid hormones called corticosteroids.

• Three layers:

  • Zona glomerulosa: Mineralocorticoids (chiefly aldosterone)

  • Zona fasciculata: Glucocorticoids (chiefly cortisol)

  • Zona reticularis: Gonadocorticoids (chiefly androgens)

Mineralocorticoids (Aldosterone)

• Regulate electrolyte concentrations, especially sodium and potassium.

• Aldosterone: Most important mineralocorticoid; stimulates sodium reabsorption and potassium excretion by the kidneys.

• Regulation: Stimulated by rising blood potassium, low sodium, decreasing blood volume/pressure, and the renin-angiotensin-aldosterone system (RAAS).

• Inhibition: Atrial natriuretic peptide (ANP) from the heart inhibits aldosterone secretion.

Glucocorticoids (Cortisol)

• Help the body resist stress by maintaining blood glucose and volume.

• Cortisol: Stimulates gluconeogenesis (glucose formation from noncarbohydrates), increases blood glucose, fatty acids, and amino acids, and enhances vasoconstriction.

• Excess: Depresses cartilage/bone formation, inhibits inflammation, depresses immune system, and affects cardiovascular/neural/gastrointestinal function.

Gonadocorticoids (Androgens)

• Weak androgens (e.g., DHEA) are precursors to sex steroids.

• Contribute to puberty onset, secondary sex characteristics, and female sex drive.

• Can be converted to estrogens after menopause.

Adrenal Medulla

• Composed of chromaffin cells that secrete epinephrine and norepinephrine.

• Part of the sympathetic nervous system (SNS); mediates fight-or-flight response.

• Effects: Increase blood glucose, constrict blood vessels, increase heart rate, and divert blood to essential organs.

• Epinephrine: More potent on heart and metabolism.

• Norepinephrine: More influential on blood vessel constriction and blood pressure.

Pancreas

Structure and Function

• Triangular gland behind the stomach with both exocrine (digestive enzymes) and endocrine (hormones) functions.

• Pancreatic islets (Islets of Langerhans): Contain alpha (α) cells (produce glucagon) and beta (β) cells (produce insulin).

Glucagon

• 29-amino-acid polypeptide; potent hyperglycemic agent.

• Major target: Liver.

• Actions:

  • Stimulates glycogenolysis (breakdown of glycogen to glucose).

  • Stimulates gluconeogenesis (formation of glucose from noncarbohydrates).

  • Promotes release of glucose into the blood.

Insulin

• 51-amino-acid protein; lowers blood glucose levels.

• Actions:

  • Enhances glucose transport into cells.

  • Stimulates glucose oxidation for ATP production.

  • Promotes glycogen synthesis and fat storage.

• Insulin receptor: Tyrosine kinase enzyme; triggers cellular uptake and metabolism of glucose.

Diabetes Mellitus (DM)

• Results from hyposecretion or hypoactivity of insulin.

• Three cardinal signs:

  • Polyuria: Excessive urine output

  • Polydipsia: Excessive thirst

  • Polyphagia: Excessive hunger

• Hyperinsulinism: Excessive insulin secretion, resulting in hypoglycemia.

Gonads

Ovaries

• Produce estrogens and progesterone.

• Functions: Maturation of reproductive organs, secondary sexual characteristics, breast development, and regulation of uterine cycle.

Testes

• Produce testosterone.

• Functions: Initiates maturation of male reproductive organs, secondary sexual characteristics, sex drive, sperm production, and maintenance of sex organs.

Other Endocrine Organs

• Pineal Gland: Secretes melatonin, involved in circadian rhythms and physiological cycles.

• Thymus: Produces thymopoietins and thymosins, essential for T cell development and immune function.

• Heart: Produces atrial natriuretic peptide (ANP), which reduces blood pressure and blood volume.

• Gastrointestinal Tract: Enteroendocrine cells release local-acting digestive hormones (e.g., gastrin, secretin, CCK, incretins).

• Placenta: Releases hormones that influence pregnancy (progesterone, estrogens, hCG, placental lactogen, relaxin).

• Kidneys: Secrete erythropoietin (stimulates red blood cell production) and renin (RAAS system).

• Skin: Produces cholecalciferol, precursor of vitamin D.

• Adipose Tissue: Releases leptin, involved in satiety and energy expenditure.

Summary Table: Major Hormones and Their Functions