The Endocrine System

The Endocrine System

• The endocrine system is made up of endocrine glands that release chemical substances, known as hormones, into the blood stream
  • A gland is a group of cells that produces and releases one or more substances (a process known as secretion)
• Compared to the nervous system the effects of hormones are slower but they act for longer, so the endocrine system is used to control functions that do not need instant responses

Structures that make up the endocrine system

• Important structures in the endocrine system are:
  • Pituitary gland: a ‘master gland’ that makes hormones such as FSH and LH
  • Thyroid: produces thyroxine, which controls metabolic rates and affects growth
  • Pancreas: produces insulin, which regulates blood glucose levels
  • Adrenal glands: produces adrenaline
  • Testes (males): produce testosterone
  • Ovaries (females): produce oestrogen

The major endocrine glands of the body. These glands secrete hormones that circulate around the body in the bloodstream.

Exam Tip

Make sure you can recognise and recall all the structures shown in the diagram above. Although you will not be expected to recall the thyroid gland and the role of thyroxine in the body, this endocrine gland may be referred to in an exam question that asks more generally about the endocrine system or hormones, so it is worth being aware of!

Hormones

• A hormone is a chemical substance produced by a gland
• Hormones are carried by the blood and can therefore circulate around the whole body
  • They are chemicals that transmit information from one part of the organism to another and bring about a change (they provide a signal that triggers a response)
• They alter the activity of specific target cells, tissues or organs
• The following hormones are of great importance in humans:

• Adrenaline
• Insulin
• Testosterone
• Progesterone
• Oestrogen

Adrenaline and how it prepares the body for action

• 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 and how it controls blood sugar

• 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 does 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
• If the blood glucose concentration gets too low:
  • Cells in the pancreas detect the decreased blood glucose levels
  • The pancreas produces the hormone glucagon
  • Glucagon causes the glycogen stored in the liver to be converted into glucose and released into the blood
  • This increases the concentration of glucose in the blood back to normal levels, at which point the pancreas stops secreting glucagon

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

Important Hormones in the Body Table

Exam Tip

It is worth learning this list of effects of adrenaline on the body as it is a fairly common exam question and can be worth several easy marks.

Also, be careful when answering questions on the control of blood glucose levels – the terms glucagon and glycogen are very often mixed up by students as they sound similar.

• Glucagon is a hormone
• Glycogen is a polysaccharide (i.e. the polymer of glucose that acts as a glucose storage molecule)

Learn the differences between the spellings and what each one does so you don’t get confused in the exam!