Hormones in Humans (CIE IGCSE Biology)

What is a Hormone?

• A hormone is a chemical substance produced by a gland and carried by the blood
• The hormone alters the activity of one or more specific target organs i.e. they are chemicals which transmit information from one part of the organism to another and bring about a change
• The glands that produce hormones in animals are known collectively as the endocrine system

The major endocrine glands in the body

Transport around the body

• Endocrine glands have a good blood supply as when they make hormones they need to get them into the bloodstream (specifically the blood plasma) as soon as possible so they can travel around the body to the target organs to bring about the response
• Hormones only affect cells with target receptors that the hormone can bind to. These are either found on the cell membrane, or inside cells. Receptors have to be complementary to hormones for there to be an effect. 
• The liver regulates levels of hormones in the blood; transforming or breaking down any that are in excess. 

How hormones work

 Important hormones in the human body:

• Blood glucose levels are controlled by a negative feedback mechanism involving the production of two hormones - insulin and glucagon
• Both hormones which control blood glucose concentration are made in the pancreas
• Insulin is produced when blood glucose rises and stimulates liver and muscle cells to convert excess glucose into glycogen to be stored
• Glucagon is produced when blood glucose falls and stimulates liver and muscle cells to convert stored glycogen into glucose to be released into the blood

Negative feedback regulation of blood glucose levels

• Adrenaline is known as the 'fight or flight' hormone as it is produced in situations where the body may be in danger
  • Flight = remove oneself rapidly from a dangerous situation eg. run away
  • Fight = if flight is not possible, resort to physical combat to overcome danger
• It causes a range of different things to happen in the body, all designed to prepare it for movement (ie fight or flight). 
• These include: 
• Increasing blood glucose concentration for increased respiration in muscle cells
• Increasing pulse rate and breathing rate so glucose and oxygen can be delivered to muscle cells, and carbon dioxide taken away, from muscles cells more quickly
• Diverting blood flow towards muscles and away from non-essential parts of the body such as the alimentary canal; again to ensure the reactants of respiration are as available as possible
• Dilating pupils to allow as much light as possible to reach the retina so more information can be sent to the brain

More on Adrenaline: Extended

• Additional effects of adrenaline include;
  • Increasing the concentration of glucose in the blood
    • This help deliver more important glucose to muscles for respiration
  • Increasing heart rate
    • This has the same effect, to ensure that all muscles are well prepared for high levels of activity in a flight or fight situation