Chapter 38: The Human Endocrine System
- 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 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.
Hypothalamus: secretes hormones that control the secretions of the pituitary, e.g. growth hormone releasing hormone
Pineal: secretes melatonin – regulates biorhythms such as sleep and menstrual cycle
Pituitary (master gland) – controls all other glands: secretes many hormones – one example is growth hormone (GH) stimulates protein synthesis and bone elongation (growth)
Thymus: secretes thymosin which helps white blood cells (that are made in the bone marrow) to mature into active immune cells
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
Adrenals: secrete adrenaline (‘fight or flight’ hormone) which is secreted in times of stress or danger
– 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: 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: secretes thyroxine which increases metabolism
- Parathyroid: secretes parathormone which stimulates release of calcium from bone
- 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
- Increased appetite
- Heat intolerance
- Surgical removal of part of the thyroid
- Anti-thyroid drugs
- Administration of radioactive iodine
- 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