Endocrine System

Endocrine System: Key Notes

Objectives

• Understand the following:
  • Location, histology, hormones, and functions of the adrenal gland and pancreatic islets.
  • Properties, functions, location, and hormones of the pineal gland and thymus.
  • The body's response to stress.

Adrenal Gland

• Known as suprarenal glands.
• Structure:
  • Flattened pyramidal shape.
  • Located superior to each kidney in the retroperitoneal space.
  • Weighs approximately 3.5 to 5 g.
• Functional Regions:
  • Adrenal Cortex:
  • Comprises 80–90% of the gland, peripherally located.
  • Secretes steroid hormones essential for life.
  • Adrenal Medulla:
  • Comprises a smaller portion, centrally located.
  • Secrets catecholamines (epinephrine, norepinephrine, dopamine).
• Innervation:
  • Innervated by the sympathetic nervous system only; no parasympathetic innervation.

Hormones Produced by Adrenal Glands

Adrenal Cortex

• Mineralocorticoids (e.g., aldosterone):

  • Regulate homeostasis of sodium (Na+) and potassium (K+) ions.
  • Controlled via the renin–angiotensin–aldosterone (RAA) pathway.

• Glucocorticoids (e.g., cortisol):

  • Regulate metabolism and stress resistance.
  • Main actions:
  • Convert proteins into amino acids for ATP synthesis.
  • Break down triglycerides into fatty acids (lipolysis).
  • Stimulate gluconeogenesis from amino acids and fats, bypassing carbohydrates.
  • Provide resistance to stress by increasing glucose levels for ATP production.
  • Exhibit anti-inflammatory effects but can inhibit immune responses when in high doses.

• Adrenal Androgens: Weak androgens produced in small amounts.

Adrenal Medulla

• Functions as a modified sympathetic ganglion.
• Chromaffin Cells: Produce mainly epinephrine (80%) and norepinephrine (20%).
• Major Functions:
  • Increase heart rate and force of contraction.
  • Elevate cardiac output and blood pressure.
  • Dilate airways and increase plasma glucose and fatty acids.

Pancreatic Islets (Islets of Langerhans)

• The pancreas has both endocrine and exocrine functions.
• Endocrine Tissues:
  • Contain four types of hormone-secreting cells:
  • Alpha Cells (A cells):
    • Secrete glucagon to increase plasma glucose levels by converting glycogen to glucose.
  • Beta Cells (B cells):
    • Secrete insulin to decrease plasma glucose levels, facilitating glucose uptake into cells and glycogenesis.
  • Delta Cells (D cells):
    • Secrete somatostatin to inhibit both insulin and glucagon release and may slow nutrient absorption.
  • F Cells:
    • Secrete pancreatic polypeptide, which decreases food intake and is affected by meal times.

Pineal Gland

• Secretes melatonin, regulated by light exposure:
  • Synthesis and Release:
  • Stimulated by darkness; integrates with the day-night cycle.
  • Induces sleepiness; inhibited by light exposure.

Thymus

• Located behind the sternum, between the lungs.
• Functions as a lymphoid organ for the immune system.
• Hormones Produced:
  • Thymosin, thymic humoral factor (THF), thymic factor (TF), thymopoietin.
  • Promote maturation of T cells, essential for immune responses without antibodies.

Stress Response

• Involves increased glucose production and ATP generation to respond to stressors.
• Key processes:
  • Glycogenolysis: Breakdown of glycogen to glucose.
  • Gluconeogenesis: Production of glucose from non-carbohydrate sources, including proteins.

Sample Questions