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.
The hypothalamus is located towards the base of the brain just above the pituitary gland. It secretes hormones directly into the bloodstream that travel the short distance to the pituitary gland and therefore can regulate the secretions of the pituitary, e.g. growth hormone releasing hormone – which causes the release of GH from the pituitary.
The pineal gland is located deep within the centre of the brain and secretes melatonin which regulates biorhythms such as sleep and the menstrual cycle.
The pituitary (master endocrine gland) is located at the base of the brain and controls all other endocrine glands (outside of the central nervous system). It secretes many hormones; e.g. growth hormone (GH) stimulates protein synthesis and bone elongation (growth).
Giganitism is a symptom of excess secretion of growth hormone and is usually caused by a pituitary tumour which can be treated by surgery, if caught early.
Dwarfism is a symptom of growth hormone deficiency. It is treated by injections of growth hormone during childhood.
The thymus gland is located just in front of the heart and behind the sternum. It secretes thymosin which helps white blood cells (that are made in the bone marrow) to mature into active immune cells.
Pancreas endocrine gland:
The pancreas is located underneath the stomach on the left hand side of the abdomen.
It is both an endocrine gland and an exocrine gland. The endocrine part is composed of islets of Langerhans which secrete insulin. Insulin enters the bloodstream directly and stimulates all cells in the body to absorb glucose. The exocrine part of the pancreas is discussed in Chapter 33: The Human Digestive System
Symptoms of insulin deficiency and its treatment:
- 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
The adrenal glands are located on top of each kidney in the back of the abdomen. They secrete adrenaline (‘fight or flight’ hormone) which is secreted in times of stress or danger.
Functions of 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
- The parathyroids are present in each lobe of the thyroid gland in the neck. They secrete parathormone which stimulates increased blood calcium levels by releasing of calcium from bone.
- Thyroid: secretes thyroxine which increases metabolism
Symptoms of thyroxine deficiency:
- Goitre (swelling of the thyroid gland)
- Low metabolic rate and mental retardation (cretinism in children)
- Tiredness, fatigue
- Weight gain (fluid build up – oedema)
Treatment of thyroxine deficiency:
- Thyroxine is administered (tablets)
- Iodine is administered (tablets)
Symptoms of excess of thyroxine secretion:
- Bulging eyeballs (exophthalmia)
- Goitre (swelling in the front of the neck)
- Increased appetite
- 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