Human Endocrine System: A Comprehensive Look

Posted on Jan 23, 2025 in Human Nutrition and Dietetics

Endocrine System: Hormones, Glands, and Functions

Hormone Classification and Function

The endocrine system comprises various glands that secrete hormones, which are chemical messengers that regulate numerous bodily functions. Hormones can be classified based on their chemical structure and solubility:

  • Lipid-soluble hormones (steroids, thyroid hormones, nitric oxide): These hormones are bound to transport proteins in the blood and bind to receptors within target cells.
  • Water-soluble hormones (amines, peptides/proteins, eicosanoids): These hormones circulate freely in the blood and bind to receptors on the exterior surface of target cells.

Major Endocrine Glands

  1. Pituitary gland: Located at the base of the brain.
  2. Pineal gland: Situated in the superior colliculus and brain, it produces melatonin.
  3. Thyroid gland: Located above the trachea.
  4. Parathyroid glands: Located behind the thyroid gland.
  5. Thymus: Located above the heart, it produces thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin, which contribute to the maturation of the immune system’s T cells.
  6. Adrenal glands: Located above the kidneys.
  7. Pancreas: Located behind the liver and stomach.
  8. Ovaries: Produce two estrogens (estradiol and estrone) that develop and maintain female sex organs, progesterone that maintains the uterus lining for pregnancy, relaxin, and inhibin.
  9. Testes: Produce testosterone, promoting male sex traits and sperm production.

Key Hormones and Their Functions

  • Human growth hormone (hGH): Promotes growth by enhancing lipolysis and diminishing glucose uptake.
  • Thyroid-stimulating hormone (TSH): Stimulates the thyroid gland to synthesize and secrete thyroid hormones.
  • Follicle-stimulating hormone (FSH): Initiates the development of oocytes and secretion of estrogen in females and stimulates the testes to produce sperm in males.
  • Luteinizing hormone (LH): Stimulates the secretion of estrogen and progesterone, ovulation, and menstruation in females and stimulates the testes to produce testosterone in males.
  • Prolactin (PRL): Stimulates the mammary glands to promote milk production.
  • Adrenocorticotropic hormone (ACTH): Stimulates the adrenal cortex to secrete glucocorticoids.
  • Melanocyte-stimulating hormone (MSH): Influences brain activity; excess amounts can darken the skin.
  • Oxytocin (OT)
  • Antidiuretic hormone (ADH)

Chemical Nature of Hormones

Hormones can be broadly categorized into two main chemical classes:

  • Amino acid-based: These include amino acid derivatives, peptides, and proteins.
  • Steroids: These are synthesized from cholesterol and include gonadal and adrenocortical hormones.

Thyroid Gland

  • Follicular cells: Produce thyroxine and triiodothyronine (iodide).
  • Parafollicular cells: Produce calcitonin (in response to high calcium levels) and parathyroid hormone (PTH) (in response to low calcium levels).

Adrenal Glands

  1. Zona glomerulosa: Secretes mineralocorticoids, which regulate mineral balance. Aldosterone, a key mineralocorticoid, promotes sodium retention and potassium excretion.
  2. Zona fasciculata: Secretes glucocorticoids, which affect glucose metabolism. Cortisol is the most produced glucocorticoid, and its secretion is regulated by negative feedback and ACTH.
  3. Zona reticularis: Secretes weak androgens (masculinizing hormones).
  4. Chromaffin cells: Secrete epinephrine (adrenaline) and norepinephrine (noradrenaline), which mediate the fight-or-flight response.

Pancreas

  • Acini: Clusters of exocrine cells that produce digestive enzymes released into the gastrointestinal tract.
  • Pancreatic islets: Clusters of endocrine tissue among the acini.
    • Alpha (A) cells: Secrete glucagon (under negative feedback control).
    • Beta (B) cells: Secrete insulin (under negative feedback control).
    • Delta (D) cells: Secrete somatostatin.
    • F cells: Secrete pancreatic polypeptide.

Glucose Regulation

  • Glucagon: Raises blood glucose levels by converting glycogen to glucose and forming glucose from lactic acid and certain amino acids.
  • Insulin: Lowers blood glucose levels by accelerating the facilitated diffusion of glucose into cells, speeding up the conversion of glucose into glycogen, increasing amino acid uptake and protein synthesis, and speeding up the synthesis of fatty acids.

Diabetes

  • Type 1 diabetes: Characterized by hyposecretion of insulin.
  • Type 2 diabetes: Characterized by hypoactivity of insulin.
  • Signs of diabetes:
    1. Polyuria: Excessive urination (glucose acts as an osmotic diuretic).
    2. Polydipsia: Excessive thirst (due to water loss from polyuria).
    3. Polyphagia: Excessive hunger (cells cannot take up glucose and are starved).