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Suprarenal (Adrenal) Glands – Structure, Hormones, & Regulation

Anatomical Overview

  • Terminology
    • "Renal glands" = Suprarenal / Adrenal glands.
  • Location & Peritoneal Relationship
    • Sit on superior pole of each kidney: Right & Left suprarenal glands.
    • Retroperitoneal: positioned between parietal peritoneum & posterior abdominal wall.

Histological Organization: Cortex vs. Medulla

  • Cortex (outer) ≈ 80–90 % of gland mass.
  • Medulla (inner) ≈ 10–20 % of gland mass.
  • Each region secretes distinct hormone classes, reflecting embryological & functional differences.

Adrenal Cortex: Zones & Hormones

  • The cortex is stratified into three concentric zones (outer → inner):
    1. Zona Glomerulosa
    • Secretes Mineralocorticoids (chiefly aldosterone).
    • Mnemonic: "Salt" (regulates \text{Na}^+ & water balance).
    1. Zona Fasciculata
    • Secretes Glucocorticoids (chiefly cortisol).
    • Mnemonic: "Sugar" (elevates blood glucose).
    1. Zona Reticularis
    • Secretes weak Androgens / Gonadocorticoids (chiefly DHEA – dehydroepiandrosterone).
    • Mnemonic: "Sex" (secondary male characteristics). Notably not significant testosterone production.
  • Mnemonic summary: Salt → Sugar → Sex (outer to inner).

Adrenal Medulla: Catecholamines

  • Cell type: Chromaffin cells (modified post-ganglionic sympathetic neurons).
  • Hormone output: \approx 80\% epinephrine (adrenaline) & \approx 20\% norepinephrine (noradrenaline) → collectively catecholamines.
  • Vascularity: Highly vascular; rapid hormone release into blood.
  • Trigger: Direct sympathetic stimulation (fight-or-flight).
  • Acute physiological effects (“prepare to run” theme):
    • ↑ Heart rate → ↑ Cardiac Output \big(\text{CO}=\text{HR}\times\text{SV}\big) → ↑ Blood Pressure.
    • ↑ Bronchodilation → ↑ Airflow & \text{O}_2 delivery.
    • ↑ Blood flow to skeletal muscle (via vasodilation in muscle & systemic vasoconstriction elsewhere).
    • ↑ Blood glucose (glycogenolysis) & ↑ plasma fatty acids (lipolysis) → ATP production.

Aldosterone & The Renin-Angiotensin-Aldosterone System (RAAS)

  • Primary Mineralocorticoid: Aldosterone (from zona glomerulosa).
  • Key stimuli
    • Dehydration, \text{Na}^+ deficiency, or hemorrhage → ↓ Blood Volume & ↓ Blood Pressure.
  • Sequence of events
    1. ↓ Stretch in afferent arteriole (baroreceptors) & ↓ \text{Na}^+ at macula densa.
    2. Signals Juxtaglomerular (JG) cells → secrete renin.
    3. Renin converts angiotensinogen → Angiotensin I.
    4. ACE (lungs) converts Ang I → Angiotensin II.
    5. Ang II actions
    • Potent systemic vasoconstrictor → ↑ BP.
    • Stimulates zona glomerulosa → ↑ aldosterone.
    1. Aldosterone effects on kidney (distal tubule & collecting duct)
    • ↑ \text{Na}^+ & water reabsorption ("obligatory" water follows salt).
    • ↑ \text{K}^+ & \text{H}^+ secretion into urine.
    1. Result: ↑ Blood Volume & ↑ Blood Pressure → negative feedback shuts RAAS.
  • Clinical cross-links: ACE inhibitors for hypertension; hyperaldosteronism → hypokalemia.

Cortisol: Functions & Hypothalamic-Pituitary-Adrenal (HPA) Axis

  • Primary Glucocorticoid: Cortisol (from zona fasciculata).
  • Stimuli ("almost any disturbance"): heat, cold, trauma, infection, emotional stress, fear, bleeding, toxins → ↓ plasma cortisol triggers axis.
  • Regulatory pathway
    1. Hypothalamus: Neurosecretory cells release CRH (corticotropin-releasing hormone).
    2. Anterior Pituitary: CRH → ACTH secretion from corticotrophs.
    3. Adrenal Cortex: ACTH → ↑ cortisol output.
    4. Negative feedback: Rising cortisol inhibits both CRH & ACTH.
  • Metabolic & Physiologic actions (“all about ATP”):
    • Protein catabolism (esp. skeletal muscle) → ↑ plasma amino acids → gluconeogenesis.
    • Gluconeogenesis: Convert amino acids / lactic acid → glucose.
    • Lipolysis: Break triglycerides → fatty acids & glycerol for ATP.
    • ** stress resistance**: Ensures sustained glucose availability.
    • Anti-inflammatory (high levels):
    • Inhibits leukocyte migration & cytokine release → ↓ inflammation.
    • Side-effect: ↓ wound healing.
    • Immunosuppression (pharmacologic doses):
    • Basis for therapeutic glucocorticoids (e.g., prednisone) in transplants, autoimmune diseases (e.g., Crohn’s).
    • Long-term high dose → adverse effects; risks often > benefits.

Integrated Stress Response & Clinical Correlations

  • Fight-or-Flight (Acute): Catecholamines dominate → rapid cardiovascular & metabolic shifts.
  • Chronic stress: Sustained HPA activation → potential "adrenal fatigue" (lay term) & deleterious cortisol elevations.
  • Interconnected systems
    • RAAS links kidney perfusion & adrenal cortex.
    • Angiotensin II both vasoconstricts & drives aldosterone.
    • Cardiac unit concepts revisited: epinephrine boosts HR & CO.
  • Therapeutic angles
    • ACE inhibitors / ARBs, beta-blockers, corticosteroids, mineralocorticoid antagonists.
    • Ethical weighing of prolonged steroid therapy: quality of life vs. side-effects.

Memory Aids & Study Tips

  • "Salt – Sugar – Sex": Glomerulosa (Na(^+)), Fasciculata (glucose), Reticularis (androgens).
  • Medulla output ratio: "E > N (4 : 1)" → 80\%:20\% epinephrine:norepinephrine.
  • Think "RUN" for catecholamines → ↑ Rate, ↑ Uptake of O(_2), ↑ Nutrients.
  • Relate pathways back to earlier courses (cardiac physiology, renal handling) for integrated understanding.