Adrenal Gland Lecture Notes

Location and Anatomy

• Location: On top of the kidneys
  • Also known as suprarenal glands
• Shape: Generally pyramid-shaped
• Position in Body:
  • Liver on the right side
  • Spleen on the left side (typically)
  • Two kidneys and adrenal glands on top

Structure of the Adrenal Gland

• Adrenal Cortex: Outer part with three layers
  • Zona Glomerulosa (outermost, orange layer)
  • Zona Fasciculata (middle, largest section, green layer)
  • Zona Reticularis (innermost, purple layer)
• Adrenal Medulla: Central part, neural tissue (black component)
• Tissue Types:
  • Cortex: Glandular cuboidal epithelial tissue
  • Medulla: Neural tissue

Zona Glomerulosa

• Function: Produces aldosterone
• Stimuli for Aldosterone Release:
  • Primary: Low blood pressure leads to the release of renin by juxtaglomerular (JG) cells in the kidney.
    • Renin converts angiotensinogen (from the liver) to angiotensin I. ACE (angiotensin-converting enzyme) in the lungs converts angiotensin I to angiotensin II.
    • Angiotensin II stimulates aldosterone production.
  • Secondary: Low sodium (hyponatremia) or high potassium (hyperkalemia) in the blood.
  • Tertiary: Adrenocorticotropic hormone (ACTH) in stressful situations.

Production Pathway of Aldosterone

1. Hormone Pathway Activation: Angiotensin II binds to receptors, activating G stimulatory protein, which then:
   • Converts GDP to GTP
   • Activates adenylate cyclase, turning ATP into cAMP
   • cAMP activates protein kinase A (PKA)
2. Cholesterol Conversion: Cholesterol is converted through several stages into aldosterone, with key enzymes like 21-hydroxylase involved.
3. Protein Kinase A’s Role: Phosphorylates enzymes in the pathway to regulate aldosterone production.
4. ACTH Pathway: Similar pathway but weaker stimulus.

Target Organs and Effects of Aldosterone

• Primary Target: Distal convoluted tubule in the kidneys
  • Mechanism: Aldosterone binds to intracellular receptors, initiating gene transcription for specific proteins (e.g., sodium-potassium pumps, sodium channels, potassium channels).
  • Effects:
    • Increases sodium reabsorption into the blood, raising sodium levels (and blood volume/pressure).
    • Promotes water retention (due to increased sodium).
    • Lowers potassium levels by excreting it into the urine.

Regulation and Inhibition

• Inhibitor: Atrial natriuretic peptide (ANP) from the heart during high blood pressure; activates G inhibitory pathways, causing hyperpolarization and inhibition of aldosterone production.
• ACTH: Weak stimulus, primarily in stress, has minimal effect on aldosterone but significantly affects other cortical zones (Zona Fasciculata and Zona Reticularis).

Summary of Stimuli and Effects

• Primary Stimuli: Low blood pressure (via renin-angiotensin pathway)
• Secondary Stimuli: Hyponatremia and hyperkalemia
• Tertiary Stimuli: ACTH (stress-related)
• Inhibitor: High blood pressure (via ANP)

Key Takeaways

• Adrenal gland location, structure, and anatomy emphasize its vital function in endocrine regulation.
• Detailed aldosterone production and regulation pathways highlight the complex interplay of physiological signals.
• Understanding the stimuli and inhibitors helps in diagnosing and addressing adrenal gland disorders.

Next topic: Zona Fasciculata and Cortisol.