Adrenal Gland and Hormones Overview

Adrenal Gland

• Located near the kidneys
• Growth and hormone synthesis are stimulated by Pituitary ACTH
• ACTH release is triggered by stress (e.g. hypoglycemia, low body temperature, shock)
• Adrenal cortex is controlled by Pituitary gland ACTH
• Adrenal medulla is part of the sympathetic nervous system

Adrenal Gland Hormones

• Mineralocorticoid Hormones (zona glomerulosa):
  • Aldosterone (95%) and Desoxycorticosteron (5%)
  • Aldosterone is the primary mineralocorticoid secreted by the adrenal cortex
  • Acts mainly in the kidneys (distal convoluted tubule) to increase sodium absorption in exchange for hydrogen or potassium
  • Released in response to:
    • Low sodium levels (Hyponatremia)
    • High potassium levels (Hyperkalemia)
    • Angiotensin II
• Hyperaldosteronism: Increased aldosterone secretion
  • Clinical picture:
    • Low potassium levels (Hypokalemia)
    • Metabolic alkalosis
• Hypoaldosteronism: Decreased aldosterone secretion
  • Clinical picture:
    • High potassium levels (Hyperkalemia)
    • Metabolic acidosis
• Glucocorticoid Hormones (zona faciculata):
  • Cortisol (95%) and corticosterone (5%)
  • Essential for life
  • Influences various organs:
    • Metabolic Effects:
      • Carbohydrate Metabolism:
        • Anti-insulin effect
        • Increases blood sugar levels by:
          • Increasing gluconeogenesis from amino acids
          • Decreasing glucose utilization by peripheral tissues
      • Decreases protein synthesis and increases protein catabolism
      • Enhances lipolysis and causes fat redistribution
    • Circulatory Functions: Maintains normal circulatory functions and cardiovascular responsiveness to catecholamines
    • Hematological Effects:
      • Decreases all blood cells except red blood cells and platelets
      • Anti-inflammatory effect by decreasing eosinophils, basophils, and stabilizing their lysosomal membranes
    • Gastric Effects: Increases gastric hydrochloric acid (HCl) and decreases mucosal cell proliferation, leading to gastritis and peptic ulceration
    • Psychological Effects: Euphoria, psychosis, depression, paranoia
    • Lung Function: Increases lung surfactant in late pregnancy
    • Aldosterone-like Effect: Exerts aldosterone-like effects
    • Plasma Cortisol Levels: Follow a diurnal pattern or circadian rhythm (highest just before waking, lowest at midnight)
• Hypercortisolism: Causes Cushing syndrome
  • Clinical picture:
    • Moon face
    • Buffalo hump
    • Pendulous abdomen
    • Muscle wasting
    • Osteoporosis
    • Poor wound healing
• Hypocortisolism: Causes Addison's disease

Adrenal Medulla

• Secretes catecholamines (e.g. epinephrine, norepinephrine) in response to sympathetic nervous system stimulation during stress
• Actions of catecholamines:
  • Vasoconstriction and increased blood pressure
  • Increased heart rate and cardiac output
  • Bronchodilation
  • Glycogenolysis
  • Increased basal metabolic rate (calorigenic effect)
• Tumors of the adrenal medulla lead to over-secretion of catecholamines, causing episodes of hypertension

Antidiuretic Hormone (ADH)

• Functions:
  • Regulation of Body Water Balance: In the absence of ADH, the distal nephron becomes impermeable to water, resulting in the excretion of large volumes of diluted urine (20 ml/minute).
  • Vascular Smooth Muscle Contraction: High concentrations of ADH stimulate vascular smooth muscle contraction (pressor effect), which is a crucial compensatory reaction in hypovolemic shock.

Oxytocin

• Synthesized by the hypothalamus as part of a prohormone
• Transported to the posterior pituitary for storage
• Release Triggered By:
  • Stretch receptors in the cervix during labor
  • Touch receptors in the nipple during suckling (milk let down reflex)
• Functions:
  • Initiation of Labor: Contractile response of uterine muscles to oxytocin is enhanced by estrogen and antagonized by progesterone levels. In late pregnancy, uterine smooth muscles become more sensitive to circulating oxytocin.
  • Milk Ejection: Suckling stimulates touch receptors on the nipple, sending nerve signals to the hypothalamus, leading to oxytocin secretion and contraction of myoepithelial cells around the alveoli of the mammary gland.

Reminder

• Endocrine glands are ductless glands that secrete regulatory molecules called hormones.
• The pituitary gland is the master gland of the endocrine system.