14.2.1 Hormones in Humans

• 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 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

• 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 (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