Endocrine System: Glands and Hormones

Endocrine System Overview

• The endocrine system is a major regulatory system, maintaining body functions and homeostasis.
• Endocrine glands are ductless, secreting hormones directly into the bloodstream.
• Hormones include amines (e.g., thyroxine, epinephrine, norepinephrine), proteins (e.g., insulin, growth hormone, calcitonin, ADH, oxytocin), and steroids (e.g., cortisol, aldosterone, estrogen, progesterone, testosterone).

Endocrine Glands & Hormones

• Pituitary Gland (Hypophysis): Plays a role in regulating several body functions. Composed of two parts:

  • Anterior pituitary (adenohypophysis): Separate glandular tissue.
  • Posterior pituitary (neurohypophysis): Extension of the hypothalamus, storing hormones produced there.

• Hypothalamus: Regulates anterior pituitary hormone secretion through releasing hormones.

• Thyroid Gland: Located on the front and sides of the trachea. Produces thyroxine (T4) and triiodothyronine (T3), crucial for regulating metabolic rate. Also produces calcitonin.

• Parathyroid Glands: Four glands on the back of each thyroid lobe. Secrete parathyroid hormone (PTH), which antagonizes calcitonin. PTH elevates blood calcium levels.

• Pancreas: Located in the upper left quadrant of the abdominal cavity. Contains the islets of Langerhans, which produce hormones. Alpha cells produce glucagon, while beta cells produce insulin.

• Glucagon is released when blood sugar levels are low and stimulates the liver to convert glycogen into glucose.

• Adrenal Glands (Suprarenal): Located on top of each kidney. Composed of two parts:

  • Adrenal medulla: Secretes epinephrine and norepinephrine (catecholamines), which are triggered by sympathetic impulses from the hypothalamus. These hormones duplicate and prolong the effects of the sympathetic nervous system.
  • Adrenal cortex: Secretes steroid hormones:
    • Mineralocorticoids (e.g., aldosterone): Regulate blood pressure and electrolyte balance. Stimulated by low blood sodium and high blood potassium.
    • Glucocorticoids (e.g., cortisol): Help the body respond to stress; increases metabolism of fats and proteins, and regulates blood glucose. Stimulated by ACTH.
    • Sex hormones (e.g., estrogen, androgens): Involved in reproductive functions and secondary sex characteristics.

• Ovary: Produces estrogen and progesterone, hormones required for reproduction, development of secondary sex characteristics, and the menstrual cycle..

• Testes: Produce testosterone, promoting the maturation of sperm and male secondary sex characteristics. Secrete inhibin, which regulates FSH release from the anterior pituitary.

Hormone Action Mechanisms

• Two-messenger system: Protein hormones bind to membrane receptors, initiating second messenger (e.g., cAMP) which activates other enzymes within the cell.
• Steroid hormones: Diffuse through cell membrane and bind to cytoplasmic receptors. The steroid-receptor complex moves into the nucleus, activating specific genes for protein synthesis.

Hormone Regulation

• Negative feedback: Information on the hormone's effects is sent back to the gland, decreasing hormone secretion when appropriate.
• Positive feedback: Hormone's effects stimulate further production, as seen in oxytocin and childbirth.

Regulation of Hormone Secretion

• Hormone secretion is primarily regulated by negative feedback loops. The effects of the hormone reverse the stimulus and decrease the secretion of the hormone.