The Role of Hormones: Basic (Edexcel IGCSE Biology)

• A hormone is a chemical substance produced by a gland and carried by the blood, which alters the activity of one or more specific target organs
  • They are chemicals that transmit information from one part of the organism to another and bring about a change
• The following hormones are of great importance in humans:
  • Adrenaline
  • Insulin
  • Testosterone
  • Progesterone
  • Oestrogen

Adrenaline

• Adrenaline is known as the 'fight or flight' hormone as it is produced in situations where the body may be in danger
• It causes a range of different things to happen in the body, all designed to prepare it for movement (i.e. fight or flight)
• These include:
  • An increase in heart rate and breathing rate - ensures glucose and oxygen can be delivered to muscle cells (and carbon dioxide can be taken away from muscles cells) at a faster rate
  • Diverting blood flow towards muscles and away from non-essential parts of the body such as the alimentary canal - ensures an increased supply of the reactants of respiration (glucose and oxygen)
  • Dilation of the blood vessels inside muscles - ensures more blood can circulate through them (again, supplying more glucose and oxygen)
  • Breaking down of stored glycogen to glucose in the liver and muscle cells, with glucose released by the liver being transported to active muscle cells - ensures a higher blood glucose concentration for increased respiration in muscle cells (providing greater energy for movement)

Insulin

• Blood glucose concentration must be kept within a narrow range, so it’s another example of homeostasis (like the control of core body temperature)
  • Too high a level of glucose in the blood can lead to cells of the body losing water by osmosis, which can be dangerous
  • Too low a level of glucose in the blood can lead to the brain receiving insufficient glucose for respiration, potentially leading to a coma or even death
• The pancreas and liver work together to control blood glucose levels
• To carry out this role, the pancreas acts as an endocrine gland (making and secreting hormones into the bloodstream), although it also plays a vital (but separate) role in digestion (making and secreting enzymes into the digestive system)
• If the blood glucose concentration gets too high:
  • Cells in the pancreas detect the increased blood glucose levels
  • The pancreas produces the hormone insulin, secreting it into the blood
  • Insulin stimulates muscles and the liver to take up glucose from the bloodstream and store it as glycogen (a polymer of glucose)
  • This reduces the concentration of glucose in the blood back to normal levels, at which point the pancreas stops secreting insulin

The regulation of blood glucose levels

Testosterone

• Testosterone is produced in the male testes
• It is responsible for the development of secondary sexual characteristics in males

Progesterone

• Progesterone is produced in the female ovaries
• It is responsible for maintaining the uterine lining during pregnancy

Oestrogen

• Oestrogen is produced by the female ovaries
• It is responsible for the development of secondary sexual characteristics in females and regulating the menstrual cycle

Summary of hormones and their functions table