Endocrine System Overview

Endocrine System Overview

• Endocrine glands release hormones directly into the bloodstream
• Hormones regulate various bodily functions
• The pituitary gland is often called the master gland because it controls other glands

Adrenal Gland

• Located near the kidneys
• Composed of cortex and medulla
• Cortex produces steroid hormones (e.g., mineralocorticoids, glucocorticoids, androgens)
• Medulla produces catecholamines (e.g., epinephrine, norepinephrine)
• Pituitary ACTH stimulates adrenal gland growth and hormone synthesis
• Stress (hypoglycemia, hypothermia, shock) stimulates ACTH release
• Adrenal cortex is controlled by ACTH from the pituitary gland
• Adrenal medulla is a part of the sympathetic nervous system

Adrenal Gland Hormones and Functions

• Mineralocorticoids (Zona glomerulosa): Primarily aldosterone, regulates fluid and electrolyte balance (e.g. sodium, potassium)
  • Released in response to hyponatremia, hyperkalemia, and angiotensin II
  • Maintains salt balance
  • Increases sodium absorption and potassium excretion
• Glucocorticoids (Zona fasciculata): Primarily cortisol, regulates metabolism, immune response, and stress response. Vital for life.
• Affects many organs.
• Androgens (Zona reticularis): Regulates sexual function
• Catecholamines (Adrenal Medulla): Epinephrine and norepinephrine, are involved in the fight-or-flight response.
  • Increase blood pressure, heart rate, and cardiac output
  • Bronchodilation
  • Glycogenolysis (breakdown of glycogen to glucose)

Mineralocorticoid Hormones from Zona Glomerulosa

• Aldosterone is a key mineralocorticoid
• Secretion is stimulated by low blood sodium, high potassium, and angiotensin II
• Increases sodium retention and potassium excretion in the kidneys
• Imbalances in aldosterone secretion cause specific medical conditions (hyperaldosteronism, hypoaldosteronism)

Glucocorticoids

• Cortisol is a crucial glucocorticoid
• Regulates metabolism of carbohydrates, fats, and proteins
• Important in prolonged stress responses
• Essential for life
• Effects on organs like adipose tissue, liver, and muscle.
• Diurnal rhythm: highest levels before waking, lowest at midnight

Hypothalamic-Pituitary-Adrenal (HPA) Axis Regulation

• Cortisol secretion is tightly regulated by feedback loops
• The hypothalamus releases corticotropin-releasing hormone (CRH)
• CRH stimulates the pituitary gland to release adrenocorticotropic hormone (ACTH)
• ACTH stimulates the adrenal cortex to release cortisol
• Cortisol inhibits the release of CRH and ACTH (negative feedback)

Function of Glucocorticoids

• Metabolic effects: Raises blood sugar levels (increases glucose production via gluconeogenesis), reduces glucose use by tissues, and increases protein breakdown. Influences metabolism of carbs, fats, and proteins.
• Maintenance of circulatory functions
• Hematological effects: Decreases other blood cells (except RBCs and platelets), has anti-inflammatory effects, stabilizes lysosomes of certain white blood cells.

Effects of Glucocorticoids

• Gastric effects: Increases hydrochloric acid and decreases mucosal cell proliferation, making the stomach more acidic
• Psychological effects: Can cause various psychological effects (euphoria, psychosis, depression, etc.)
• Lung surfactant: Can increase lung surfactant levels in late pregnancy
• Aldosterone-like effects: Plays a role in fluid balance akin to aldosterone

Hypercortisolism (Cushing Syndrome)

• Excessive cortisol production
• Characterized by specific symptoms (e.g. moon face, buffalo hump, muscle wasting, thin skin, poor wound healing)

Hypocortisolism (Addison's Disease)

• Insufficient cortisol production
• Characterized by specific symptoms (e.g., fatigue, weakness, low blood pressure, weight loss, skin darkening, altered electrolyte levels)

Adrenal Medulla

• Releases catecholamines (epinephrine and norepinephrine)
• Responds to sympathetic nervous system stimulation
• Actions: vasoconstriction, increased heart rate, cardiac output, bronchodilation, glycogenolysis, and increased BMR

Antidiuretic Hormone (ADH)

• Released from posterior pituitary
• Regulates water balance in the body
• Increases water reabsorption by the kidneys
• Promotes water retention

Oxytocin

• Synthesized in the hypothalamus and stored in the posterior pituitary

• Released in response to nerve signals from the cervix during childbirth, nipple stimulation during breastfeeding

• Functions include initiating labor contractions, and stimulating milk ejection

• Regulation of uterine muscle contractions during childbirth

• Promotes milk ejection (let-down reflex) in nursing