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.

This quiz covers the adrenal gland endocrine system, including the adrenal medulla and adrenal cortex, glucocorticoids, and regulation of glucocorticoid and aldosterone secretion.