A6 - Endocrine System

• The endocrine system consists of a number of glands that release hormones. These hormones allow the glands to send signals to parts of the body to inform it how to respond in order to regulate metabolism, temperature, growth, ect.
• The glands that release the hormones have target organs that respond in specific ways when they are in contact with the hormone
• The endocrine system is similar to the nervous system. However, rather then sending electrical impulses which allow for very quick messages to be sent in the nervous system, the endocrine system is much slower at producing reactions as the hormones are released into the blood stream and carried to the target organ via the blood, which takes longer than nervous impulses
• The main hormones that are going to be examined are:

  • adrenaline
  • noradrenaline
  • cortisol
  • testosterone
  • human growth hormone (HGH)
  • oestrogen
• Adrenaline and noradrenaline are produced by the adrenal glands and adrenaline is produced from breaking down noradrenaline
• Adrenaline and noradrenaline, but primarily adrenaline, are responsible for the fight-or-flight response - getting the body ready to be able to run away quickly or to fight, both of which require the muscles to contract quickly, so energy must be supplied quickly
• Target Organ: Heart

  Effect of Adrenaline: Increase heart rate

  Purpose: Pumps more blood around the body to supply the muscles with oxygen and glucose for energy production
• Target Organ: Liver

  Effect of Adrenaline: Increase rate of conversion of glycogen to glucose

  Purpose: Increases blood sugar levels so more energy is available for muscle contraction
• Target Organ: Blood Vessels

  Effect of Adrenaline: Vasodilation of blood vessels leading to the skeletal muscle. Vasoconstriction of blood vessels to the gut, kidney and skin

  Purpose: Redistributes blood flow to the skeletal muscle
• Target Organ: Eye

  Effect of Adrenaline: Pupil dilates

  Purpose: Allows more light into the eye so vision is improved
• Target Organ: Lungs

  Effect of Adrenaline: Bronchioles relax

  Purpose: Increase amount of air that can be inhaled and exhaled during inspiration and expiration
• Cortisol is also known as hydrocortisone and is sometimes called the 'stress' hormone as more of it is produced when a person is stressed
• Cortisol is also released in response to emotional or physical threat in the same way that adrenaline and noradrenaline are released in order to prepare the body to respond to a perceived danger
• Cortisol increases the release of glucose into the blood stream to help to supply energy for muscular contraction
• Cortisol is released in the morning to help us to wake up and get out of bed and then function for the day ahead
• If a person is regularly experiencing stress, then more cortisol is produced which can have negative implications for a person as too much glucose in the blood stream is not good
• Excessive cortisol production can also lead to a lowering of a person's immune system so they are more prone to being ill
• Cortisol also has the effect of stopping the body from taking amino acids into the muscle cells which are required for muscle growth. It also stops the bone remodelling stage so will stop bone growth and repair as well as inhibiting calcium absorption from the diet
• Sleep deprivation, excessive intake of caffeinated drinks and excess alcohol intake all increase cortisol production
• Testosterone is produced by the testes in males and the ovaries in females, and some is also produced by the adrenal glands in both males and females
• Males have higher levels of testosterone is responsible for many of the changes that happen during puberty such as deepening of the voice, growth of body hair and production of sperm
• In both males and females however, testosterone increases muscle growth by increasing muscle protein synthesis
• Human growth hormone is also known as human chorionic gonadotropin (hCG). It is produced by the pituitary gland and is important for normal human growth and development, especially in children and teenagers
• Low hCG levels in children and teenagers result in dwarfism
• Growth hormone stimulates the development of natural male and female sex hormones
• In males, growth hormone acts to increase testosterone levels and results in increased muscle development
• Excessive growth hormone levels increase muscle mass by stimulating protein synthesis, strengthen bones by stimulating bone remodelling, and reduce body fat by stimulating the breakdown of fat cells
• Oestrogen is produced by the ovaries in females and the testes in males. In females it is responsible for regulating the menstrual cycle. In both males and females it plays a major role in regulating the process for bone remodelling
• Oestrogen has the effect of regulating osteoclast activity.
• In females there is less oestrogen produced by the body after the menopause which results in osteoclasts living longer, resulting in greater re-absorption of bone
• This greater re-absorption of bone can lead to a decrease in bone mass which is a condition known as osteoporosis and leaves bones weaker and less able to withstand impacts, so they are more likely to fracture