Hormone Axis Physiology

• Hormones


  Messenger chemicals that travel in the blood between organs and tissues, regulating many processes
• Hormone axis


  A system that controls the amount of a hormone in the system
• Hypothalamus


  Brain region that releases hormones
• Pituitary gland


  An organ with anterior and posterior parts that releases hormones
• Anterior pituitary gland releases

  • Thyroid-stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
  • Follicle-stimulating hormone (FSH)
  • Luteinising hormone (LH)
  • Growth hormone (GH)
  • Prolactin
• Posterior pituitary releases


  • Oxytocin
  • Antidiuretic hormone (ADH)
• Thyroid Axis


  A system involving the hypothalamus, anterior pituitary, and thyroid gland to regulate thyroid hormones
• Thyroid Axis


  1. Hypothalamus releases thyrotropin-releasing hormone (TRH)

  2. TRH stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH)

  3. TSH stimulates the thyroid gland to release triiodothyronine (T3) and thyroxine (T4)

  4. Hypothalamus and anterior pituitary respond to T3 and T4 by suppressing the release of TRH and TSH, resulting in lower amounts of T3 and T4

  5. Lower T3 and T4 offer less suppression of TRH and TSH, causing more of these hormones to be released, resulting in a rise of T3 and T4
• Lower amounts of T3 and T4


  Result in less suppression of TRH and TSH, causing more of these hormones to be released, resulting in a rise of T3 and T4
• Thyroid hormone level is closely regulated to keep it within normal limits
• End hormone suppresses the release of controlling hormones


  Called negative feedback
• Cortisol is secreted by the two adrenal glands, which sit above each kidney
• Cortisol is released in pulses throughout the day and in response to a stressful stimulus


  It is a "stress hormone"
• Diurnal variation


  High and low at different times of the day
• Cortisol peaks in the early morning


  Triggering us to wake up and get going
• Cortisol is at its lowest late in the evening


  Prompting us to relax and fall asleep
• Hypothalamus releases corticotropin-releasing hormone (CRH)


  Stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH), which stimulates the adrenal glands to release cortisol
• Cortisol is sensed by the hypothalamus and anterior pituitary


  Suppressing the release of CRH and ACTH, resulting in lower amounts of cortisol
• Cortisol is closely regulated to keep it within normal limits
• Actions of Cortisol within the body

  • Increases alertness
  • Inhibits the immune system
  • Inhibits bone formation
  • Raises blood glucose
  • Increases metabolism
• Hypothalamus produces growth hormone-releasing hormone (GHRH)


  Stimulates the anterior pituitary to release growth hormone (GH)
• Functions of Growth Hormone

  • Stimulates muscle growth
  • Increases bone density and strength
  • Stimulates cell regeneration and reproduction
  • Stimulates growth of internal organs
• Parathyroid hormone (PTH) is released from the four parathyroid glands in response to a low calcium level in the blood


  The role of PTH is to increase serum calcium concentration
• PTH increases the activity and number of osteoclasts in bone


  Causing reabsorption of calcium from the bone into the blood, increasing serum calcium concentration
• PTH stimulates calcium reabsorption in the kidneys


  Less calcium is excreted in the urine
• PTH stimulates the kidneys to convert vitamin D3 into calcitriol


  The active form of vitamin D
• Vitamin D promotes calcium absorption from food in the intestine


  PTH increases serum calcium
• High serum calcium level suppresses the release of PTH


  Helping to reduce the serum calcium level
• Renin is an enzyme secreted by the kidney
• Organs (kidneys and intestine)


  Help to increase the serum calcium
• High serum calcium level

  Suppresses the release of PTH via negative feedback, helping to reduce the serum calcium level
• Renin-Angiotensin-Aldosterone System


  1. Renin is secreted by juxtaglomerular cells in the afferent (and some in the efferent) arterioles in the kidney

  2. Renin converts angiotensinogen (released by the liver) into angiotensin I

  3. Angiotensin I converts to angiotensin II in the lungs with the help of an enzyme called angiotensin-converting enzyme (ACE)

  4. Angiotensin II acts on blood vessels, causing vasoconstriction

  5. Angiotensin II stimulates the release of aldosterone from the adrenal glands and contributes to cardiac remodelling by promoting hypertrophy of heart muscle cells (myocytes)

  6. Aldosterone acts on the nephrons in the kidneys to increase sodium reabsorption from the distal tubule, increase potassium secretion from the distal tubule, and increase hydrogen secretion from the collecting ducts
• When sodium is reabsorbed in the kidneys, water follows it by osmosis, leading to increased intravascular volume and subsequently, blood pressure
• Understanding the renin-angiotensin-aldosterone system is essential to understanding the mechanism of action of ACE inhibitors and angiotensin II receptor blockers
• By blocking the action of angiotensin-converting enzyme or angiotensin II receptors, ACE inhibitors and angiotensin II receptor blockers reduce the activity of angiotensin II, reducing vasoconstriction, cardiac remodelling, and the secretion of aldosterone
• Reduced aldosterone leads to reduced sodium reabsorption in the kidneys and less water retention
• Reduced potassium secretion due to ACE inhibitors and angiotensin II receptor blockers can cause hyperkalaemia (raised potassium)
• TOM TIP: 'Understanding the renin-angiotensin-aldosterone system is essential to understanding the mechanism of action of ACE inhibitors and angiotensin II receptor blockers. By blocking the action of angiotensin-converting enzyme or angiotensin II receptors, they reduce the activity of angiotensin II, reducing vasoconstriction, cardiac remodelling, and the secretion of aldosterone. Reduced aldosterone leads to reduced sodium reabsorption in the kidneys and less water retention. However, the reduced potassium secretion means these medications can cause hyperkalaemia (raised potassium)'