Adrenal Gland Structure and Function

Adrenal Gland

• The adrenal glands are triangle-shaped glands located on top of the kidneys.
• Each gland has two main parts: the cortex and the medulla.
• The medulla is the center of the gland and produces epinephrine and norepinephrine (adrenaline).
• The cortex surrounds the medulla and produces other hormones necessary for fluid and electrolyte balance, including cortisone and aldosterone.
• The adrenal cortex is divided into three distinct zones: zona glomerulosa, zona fasciculata, and zona reticularis.

Location and Structure

• The adrenal glands sit atop the kidneys.
• Each gland consists of a central medulla surrounded by a cortex.

Adrenal Cortex

• The cortex produces various steroid hormones.
• The three zones of the cortex each produce specific hormones:
  • Zona glomerulosa: mineralocorticoids (e.g., aldosterone)
  • Zona fasciculata: glucocorticoids (e.g., cortisol)
  • Zona reticularis: androgens (e.g., androstenedione, testosterone)

Hormones

• Corticosteroids are produced from cholesterol.
  • Glucocorticoids: responsible for increasing plasma glucose. Cortisol is the most important one.
  • Mineralocorticoids: regulate fluid and electrolyte balance. Aldosterone is the primary one.
  • Androgens: male sex hormones.

Corticosteroids

• Synthesized from cholesterol through enzymatic modifications.
• Glucocorticoids and mineralocorticoids are important classes.

Glucocorticoids (e.g., Cortisol)

• Increase plasma glucose levels.
• Essential for life.
• Primarily synthesized in the zona fasciculata of the adrenal cortex.
• Regulation of metabolism, immunity, and stress response.
• Cortisol deficiency leads to hypoglycemia.

Mineralocorticoids (e.g., Aldosterone)

• Maintain fluid and electrolyte balance.
• Primarily synthesized in the zona glomerulosa.
• Regulate sodium and potassium reabsorption in the kidneys.

Androgens (e.g., Dehydroepiandrosterone (DHEA))

• Play a minor role as sex hormones.
• Synthesized in the zona reticularis.

Aldosterone Biosynthesis

• Synthesized in the zona glomerulosa of the adrenal cortex.
• It's a crucial hormone for electrolyte balance and blood pressure regulation.
• Four enzymes are involved in the synthesis.

Cortisol Metabolism

• 20% converted to cortisone by renal and other tissues.
• Cortisol and cortisone are inactive in the liver.

Mechanism of Action

• Corticosteroids act through glucocorticoid receptors.

Physiology

• The hypothalamus releases corticotropin-releasing hormone (CRH).
• CRH stimulates the anterior pituitary gland to release adrenocorticotropic hormone (ACTH).
• ACTH targets the adrenal cortex to produce and release corticosteroid hormones.
• Stress and circadian rhythm influence hormone release.

Additional effects of Glucocorticoids

• They impact metabolism by influencing gluconeogenesis, amino acid uptake, glycogen synthase, glucose production from proteins, and insulin release.
• They affect adipocytes by regulating glucose uptake, lipolysis, and lipogenesis.
• They increase bone catabolism, which can lead to osteoporosis.
• They influence other physiological systems.

Corticosteroid effects & regulation:

• Negative feedback mechanisms regulate steroid release.

Mineralocorticoids

• Aldosterone is the primary mineralocorticoid.
• Its main actions include promoting sodium reabsorption and potassium excretion in the kidneys.
• It influences fluid and electrolyte balance and blood pressure.

Adrenal gland malfunction

• Cushing's syndrome results from overproduction of glucocorticoids.
• Addison's disease is caused by the hyposecretion of glucocorticoids.

Body Fluid Compartments

• Body fluids constitute about 55-60% of body mass in lean adults.
• Body fluids are divided into intracellular and extracellular compartments.

Important Molecules

• Cholesterol is a precursor to steroid hormones.
• Cortisol, aldosterone, and testosterone are important steroid hormones with specific functions.