Physiology Chapter 20: Adrenal Gland Endocrine System

Adrenal Gland Endocrine System

• The adrenal gland has two endocrine organs: the inner adrenal medulla and the outer adrenal cortex.

Adrenal Medulla

• The adrenal medulla constitutes 28% of the adrenal gland's mass and has two cell types: epinephrine-secreting (90%) and norepinephrine-secreting (10%).
• The main secretions of the adrenal medulla are the catecholamines: epinephrine, norepinephrine, and dopamine.

Adrenal Cortex

• The adrenal cortex secretes steroid hormones: glucocorticoids, mineralocorticoids, and androgens.
• Glucocorticoids have widespread effects on carbohydrate and protein metabolism.
• Mineralocorticoids are essential for maintaining Na+ balance and extracellular fluid (ECF) volume.
• Androgens, such as testosterone, can exert effects on reproductive function.

Adrenocortical Hormones

• Glucocorticoids:
  • Cortisol is the primary glucocorticoid in humans.
  • Cortisol is bound to protein to a significant extent and has a half-life of about 1-2 hours.
• Mineralocorticoids:
  • Aldosterone is the primary mineralocorticoid in humans.
  • Aldosterone is bound to protein to only a slight extent and has a short half-life (about 20 minutes).

Physiologic Effects of Glucocorticoids

• Glucocorticoids:
  • Promote protein catabolism and gluconeogenesis.
  • Increase plasma glucose levels, providing extra glucose to the brain and heart.
  • Inhibit ACTH secretion, representing a negative feedback response on the pituitary.
  • Have effects on the nervous system, including changes in electroencephalographic waves and personality changes.
  • Affect water metabolism, preventing water intoxication.
  • Decrease the number of circulating eosinophils and increase the number of neutrophils, platelets, and red blood cells.

Regulation of Glucocorticoid Secretion

• Glucocorticoid secretion is regulated by ACTH from the anterior pituitary.
• ACTH has a half-life in the circulation of about 10 minutes.
• ACTH is necessary to restore normal adrenal responses to ACTH.
• Circadian rhythm: ACTH is secreted in irregular bursts throughout the day, with the most frequent bursts in the early morning.
• Glucocorticoid feedback: free glucocorticoids inhibit ACTH secretion, and the degree of pituitary inhibition is proportional to the circulating glucocorticoid level.

Regulation of Aldosterone Secretion

• Aldosterone secretion is regulated by the renin-angiotensin system.
• Aldosterone has a short half-life (about 20 minutes).

Physiologic Effects of Mineralocorticoids

• Mineralocorticoids:
  • Increase the reabsorption of Na+ from the urine, sweat, saliva, and the contents of the colon.
  • Cause retention of Na+ in the ECF, expanding ECF volume.
  • Increase the exchange of Na+ for K+ and H+ in the renal tubules, producing a K+ diuresis and an increase in urine acidity.
  • Bind to a cytoplasmic receptor, altering the transcription of mRNAs.
  • Stimulate the synthesis of proteins that increase the activity of epithelial sodium channels (ENaCs) and their synthesis.