Chapter 38: Human Endocrine System

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The human endocrine system is composed of endocrine glands.

  • An endocrine gland is an organ that secretes a hormone directly into the bloodstream.
  • A hormone is a chemical messenger secreted by an endocrine gland directly into the bloodstream where it travels to a target organ/tissue where it exerts a specific effect.

The locations of the various endocrine glands are shown below:


 Exocrine versus endocrine glands:

  • Exocrine gland is an organ that secretes its product into a duct.
  • Endocrine gland secretes its product directly into the bloodstream.

Endocrine action versus nerve action:

  • Endocrine action is slow, prolonged, and chemical in nature.
  • Nerve action is fast, short-lived, and electrical in nature.

Endocrine glands

Hypothalamus: secretes hormones that control the secretions of the pituitary, e.g. growth hormone releasing hormone

Pineal gland:
Pineal: secretes melatonin – regulates biorhythms such as sleep and menstrual cycle

Pituitary gland:
Pituitary (master gland) – controls all other glands: secretes many hormones – one example is growth hormone (GH) stimulates protein synthesis and bone elongation (growth)


Thymus gland:
Thymus: secretes thymosin which helps white blood cells (that are made in the bone marrow) to mature into active immune cells

Pancreas endocrine gland:
Pancreas: the islets of Langerhans in the pancreas secrete insulin which stimulates all cells to absorb glucose

  • Diabetes results if there is no insulin or lack of insulin in the body
  • Insulin is used as a hormone supplement to treat type I diabetes

Adrenal glands:
Adrenals: secrete adrenaline (‘fight or flight’ hormone) which is secreted in times of stress or danger

  • Adrenaline:

– Increases blood flow to the brain and muscles
– Decreases blood flow to the skin and internal organs such as the intestines and kidneys
– Dilates the bronchioles allowing more air in
– Increases blood glucose levels
– Increases heart rate


Ovaries and testes:

  • Ovaries: secrete oestrogen (stimulates changes that occur at puberty in females) and progesterone which are both involved in the menstrual cycle and in preparing the female body for a possible conception
  • Testes: secrete testosterone which stimulates the changes that occur in the male at puberty and also help to maintain these changes (called secondary sexual characteristics)

– Anabolic steroids act in the same way in which testosterone acts – builds up muscle – therefore, anabolic steroids are used by body-builders and they           have also been used (illegally) by athletes to boost athletic performance

Thyroid and parathyroid glands

  • Thyroid: secretes thyroxine which increases metabolism
  • Parathyroid: secretes parathormone which stimulates release of calcium from bone

Thyroxine deficiency leads to myxoedema:

  • Goitre (swelling of the thyroid gland)
  • Low metabolic rate and mental retardation (cretinism in children)
  • Tiredness, fatigue
  • Weight gain (fluid build up – oedema)


  • Thyroxine is administered (tablets)
  • Iodine is administered (tablets)

Excess of thyroxine leads to:

  • Bulging eyeballs
  • Goitre
  • Increased appetite
  • Irritability
  • Heat intolerance


  • Surgical removal of part of the thyroid
  • Anti-thyroid drugs
  • Administration of radioactive iodine
Negative Feedback Mechanism of Thyroxine

  • When thyroxine levels rise above normal:
  • Pituitary stops secreting thyroid-stimulating hormone (TSH) thereby causing the thyroid to reduce secretion of thyroxine
  • When thyroxine levels fall below normal:
  • Pituitary starts secreting TSH causing the thyroid gland to secrete more thyroxine
  • When iodine is completely absent from diet thyroxine cannot be made – therefore, pituitary keeps secreting TSH which builds up to extreme levels in the thyroid causing goitre