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Endocrine System Functions and Hormone Classifications

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The Endocrine System

Function: Responsible for metabolic regulation.

Hormones: Chemical substances secreted into internal body fluids that exert physiological control over target cells throughout the body.

Types of Hormones

  • Local Hormones: Acetylcholine, which is secreted by nerve endings, and secretin, which stimulates the duodenal wall to produce aqueous pancreatic secretion.
  • General Hormones: Secreted by specific endocrine glands and carried by the blood throughout the body to produce various reactions.

Chemistry of Hormones

1. Steroid Hormones (Derived from Cholesterol)

  • Adrenal Cortex: Cortisol, aldosterone.
  • Ovaries: Estrogen, progesterone.
  • Testicles: Testosterone.
  • Placenta: Estrogen, progesterone.

2. Tyrosine Amino Acid Derivatives

  • Thyroid Hormones: Thyroxine and triiodothyronine (metabolic regulation).
  • Catecholamines: Derived from tyrosine and secreted by the adrenal medulla.

3. Proteins or Peptides

  • Antidiuretic hormone, oxytocin, insulin, glucagon, and parathormone.

Hypothalamic and Pituitary Hormones

Hypothalamic Hormones

Corticotropin-releasing hormone, growth hormone-releasing hormone, growth hormone-inhibiting hormone (somatostatin), and gonadotropin-releasing hormone.

Anterior Pituitary Hormones

  • ACTH: Stimulates the adrenal cortex.
  • Thyrotropin: Stimulates the thyroid.
  • Follicle-Stimulating Hormone (FSH): Promotes ovarian follicle growth.
  • Luteinizing Hormone (LH): Triggers ovulation and female hormone secretion.
  • Prolactin: Breast development and milk secretion.
  • Growth Hormone: Regulates body growth.

Posterior Pituitary Hormones

  • Antidiuretic Hormone (ADH): Promotes water retention.
  • Oxytocin: Stimulates uterine contractions during labor and myoepithelial cell contraction to expel milk.

Glandular Functions

Adrenal Glands

  • Adrenocortical: Mineralocorticoids (Aldosterone) affect Na and K electrolytes; Glucocorticoids (Cortisol) increase blood glucose and control carbohydrate and protein metabolism.
  • Adrenal Medulla: Secretes adrenaline and noradrenaline.

Thyroid and Parathyroid

  • Thyroid: Thyroxine and triiodothyronine increase the speed of chemical reactions; Calcitonin reduces calcium concentration in bones and extracellular fluid.
  • Parathyroid Glands: Parathyroid hormone controls calcium ion concentration via absorption in the kidneys, intestines, and release from bones.

Pancreatic Islets of Langerhans

  • Insulin: Facilitates glucose entry into cells.
  • Glucagon: Stimulates glucose release and synthesis in the liver.

Reproductive Glands and Placenta

  • Ovaries: Estrogen stimulates female sex organ development; Progesterone stimulates uterine endometrial gland secretion.
  • Testicles: Testosterone stimulates male sexual organ development.
  • Placenta: Human chorionic gonadotropin favors corpus luteum growth, estrogen secretion, and progesterone production for endometrial development.

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