Endocrine: Adrenal Anatomy & Physiology

Adrenal Anatomy

Structure

  • Adrenal Cortex
    • Embryologic Origin: Mesoderm
    • No Innervation
  • Adrenal Medulla
    • Embryologic Origin: Ectoderm
    • Innervated by Sympathetic Splanchnic Nerves
    • Secretes: Catecholamines (Epinephrine, Norepinephrine & Dopamine)
    • Extraadrenal Sites:
      • Usually in Retroperitoneum
      • Most Common Site: Organ of Zuckerkandl
        • At Aortic Bifurcation
  • Gerota’s Fascia
    • Encapsulates Kidney & Adrenal Glands

Adrenal Gland 1

Adrenal Cortex Layers Mn

  • Zona Glomerulosa
    • Outer Layer
    • Secretes Aldosterone
  • Zona Fasciculata
    • Middle Layer (Widest Zone)
    • Secretes Glucocorticoids (Cortisol)
  • Zona Reticularis
    • Inner Layer
    • Secretes Androgens/Estrogens

Adrenal Gland Layers 2

Vascular Supply

  • Arterial Supply – More Variable
    • Superior Adrenal Artery – From the Inferior Phrenic Artery
    • Middle Adrenal Artery – From the Aorta
    • Inferior Adrenal Artery – From the Renal Artery
  • Venous Drainage – Generally Constant
    • Left Adrenal Vein – Drains into the Left Renal Vein
    • Right Adrenal Vein – Drains into the IVC

Lymphatic Drainage

  • Subdiaphragmatic & Renal Lymph Nodes

Adrenal Vasculature 3

Adrenal Hormones

Aldosterone

  • Mineralocorticoid Hormone
  • Secreted by Adrenal Cortex – Zona Glomerulosa
  • Stimulated By: Hyperkalemia (Strongest), Angiotensin II & ACTH (Minimally)
  • Function: Increased BP, Hypernatremia, Hypokalemia & Metabolic Alkalosis
    • Directly Upregulates Na/K-ATPase in Kidney (Distal Tubule & Collecting Duct)
      • Increases Sodium Reabsorption & Potassium Secretion
      • Increases Water Reabsorption
    • Indirectly Causes Loss of Hydrogen Ions
      • Increased Potassium Secretion Effects Alpha Intercalated Cells of Late Distal Tubule & Collecting Duct
      • Potassium Ions Are Exchanged for Hydrogen Ions

Glucocorticoids (Cortisol)

  • Secreted by Adrenal Cortex – Zona Fasciculata
  • Stimulated by: ACTH
  • Steroidogenesis Derived from Cholesterol
  • Functions:
    • Increases Gluconeogenesis & Glycogenolysis
    • Increases Inotropy & Chronotropy
    • Increases Peripheral Vascular Resistance
    • Immune Suppression

Catecholamines

  • Secreted by Adrenal Medulla
  • Stimulated by: Sympathetic Stimulation
  • Hormone Synthesis:
    • Amino Acid Conversion: Mn
      • Tyrosine > L-DOPA > Dopamine > Norepinephrine > Epinephrine
    • Enzymes: Mn
      • Tyrosine Hydroxylase: Tyrosine > DOPA (Rate Limiting Step)
      • DOPA-Decarboxylase: DOPA > Dopamine
      • Dopamine-β-Hydroxylase: Dopamine > Norepinephrine
      • Phenylethanolamine N-Methyltransferase (PNMT): Norepinephrine > Epinephrine
  • Hormones:
    • Epinephrine (85%)
    • Norepinephrine (15%)
      • Extraadrenal Sites of Synthesis Can Only Produce Norepinephrine, Not Epinephrine Because PNMT is Only Present in the Adrenal Medulla
  • Break Down:
    • Half-Life Only 2-3 Minutes
    • Primary Enzymes of Degradation:
      • Monoamine Oxidase (MAO)
      • Catechol-O-Methyltransferase (COMT)
    • Degradation Metabolites:
      • Vanillylmandelic Acid (VMA)
      • Homovanillic Acid (HVA)
      • Metanephrine
    • Metabolites are Secreted in the Urine

Sex Hormones

  • Adrenal Cortex Produces a Small Amount of Testosterone & Estrogen
  • Production is Usually Overshadowed by the Amount Produced in Testes & Ovaries

Adrenal Hormone Synthesis 4

Mnemonics

Layers of the Adrenal Cortex

  • G.F.R.
    • Zona Glomerulosa
    • Zona Fasciculata
    • Zona Reticularis

Hormone Production of the Adrenal Cortex

  • “Salt, Sugar, Sex – The Deeper You Go the Sweeter it Gets”
    • Zona Glomerulosa – Secretes Aldosterone
    • Zona Fasciculata – Secretes Glucocorticoids (Cortisol)
    • Zona Reticularis – Secretes Androgens/Estrogens

Catecholamine Amino Acid Conversion

  • With the Fight-or-Flight Excitement “True Love Does Not Exist”
    • T>L>D>N>E
  • Tyrosine > L-DOPA > Dopamine > Norepinephrine > Epinephrine

Catecholamine Amino Acid Conversion – Enzymes

  • Enzymes are Generally “-oxylase” of the Enzyme They Degrade
  • Exception: PNMT
  • Enzymes:
    • Tyrosine Hydroxylase: Tyrosine > DOPA (Rate Limiting Step)
    • DOPA-Decarboxylase: DOPA > Dopamine
    • Dopamine-β-Hydroxylase: Dopamine > Norepinephrine
    • Phenylethanolamine N-Methyltransferase (PNMT): Norepinephrine > Epinephrine

References

  1. Gray H. Anatomy of the Human Body (1918). Public Domain.
  2. OpenStax College. Wikimedia Commons. (License: CC BY-SA-3.0)
  3. Uludağ M, Aygün N, İşgör A. Surgical Indications and Techniques for Adrenalectomy. Sisli Etfal Hastan Tip Bul. 2020 Mar 24;54(1):8-22. (License: CC BY-NC-4.0)
  4. Colo M. Wikimedia Commons. (License: CC BY-SA-3.0)