The Endocrine System (OCR A Level Biology)

• Homeostatic mechanisms help organisms to keep their internal body conditions near constant
• Three key homeostatic mechanisms are:
  • Thermoregulation – the control of body temperature
  • Osmoregulation – the control of the water potential of body fluids
  • The control of blood glucose concentration

• These homeostatic mechanisms in mammals require information to be transferred between different parts of the body
• One of the coordination systems in mammals that does this is the endocrine system

The endocrine system

• A hormone is a chemical messenger produced by an endocrine gland and carried by the blood
  • They are chemicals which transmit information from one part of the organism to another and bring about a change
  • They alter the activity of one or more specific target organs

• Hormones are used to control functions that do not need instant responses
• The endocrine glands that produce hormones in animals are known collectively as the endocrine system
  • A gland is a group of cells that produces and releases one or more substances (a process known as secretion)

The major endocrine glands in the body

• Endocrine glands have a good blood supply as when they make hormones they secrete them directly into the bloodstream (specifically the blood plasma)
• The hormones are then transported around the body via the blood to target cells/tissues to bring about a response
• Hormones come into contact with a lot of different cells in the body but they only affect cells with receptors that the hormone can bind to
  • These are either found on the cell surface membrane
  • Receptors have to be complementary to hormones for there to be an effect

Hormones are synthesized and released into the bloodstream from a gland (such as the pituitary gland) and circulate in the bloodstream where target cells and tissues detect them

Hormones: first and second messengers

• First and second messengers are used to carry out the action of a hormone
• The first messenger is the hormone that brings the "information or signal" from the endocrine gland
  • Hormones don't actually enter the cell, they bind to a receptor on the cell surface membrane
• A second messenger inside the cell causes the effect
• The mode of action for the "fight-or-flight" hormone adrenaline occurs as follows:
  • Adrenaline binds to specific receptors on the membrane of liver cells
  • This causes the enzyme adenylyl cyclase to change shape and become activated
  • Activated adenylyl cyclase catalyses the conversion of ATP to the second messenger molecule cyclic AMP (cAMP)
  • cAMP binds to protein kinase A enzymes, activating them
  • Active protein kinase A enzymes initiate a series of enzyme activations that result in the breakdown of glycogen to glucose; this process is known as glycogenolysis
• The enzyme cascade described above amplifies the original signal from adrenaline and results in the release of extra glucose by the liver to increase the blood glucose concentration

The hormone adrenaline (first messenger) does not enter the cell, it is the second messenger (cyclic AMP) which causes an effect in the cell by causing a cascade of reactions

• There are several key differences between the two coordination systems in mammals

Features of the nervous system & the endocrine system table

• Situated above each kidney is a gland known as the adrenal gland
  • Adrenal glands are an example of endocrine glands
• Both adrenal glands have the same structure and function, consisting of two main areas: a central medulla and an outer cortex
• The cortex produces steroid hormones:
  • Aldosterone regulates levels of salts (sodium and potassium) and the water balance in the blood, this has an impact on blood volume and pressure
  • Cortisol is the primary stress hormone which regulates metabolism of glucose, proteins and fats to release usable energy
• The medulla produces adrenaline, a hormone produced at times of stress or excitement that affects many body organs, preparing the body to respond to emergency situations
  • Adrenaline initiates the "fight or flight" response in the body

The location and structure of the adrenal glands