CIE A Level Biology (9700) 2019-2021

Revision Notes

14.1.1 Homeostasis


  • In order to function properly and efficiently, organisms have different control systems that ensure their internal conditions are kept relatively constant
  • The process of maintaining constant internal body conditions is known as homeostasis
  • Homeostasis is critically important for organisms as it ensures the maintenance of optimal conditions for enzyme action and cell function
  • Sensory cells can detect information about the conditions inside and outside of the body
  • Examples of physiological factors that are controlled by homeostasis in mammals include:
    • Core body temperature
    • Metabolic waste (eg. carbon dioxide and urea)
    • Blood pH
    • Concentration of glucose in the blood
    • Water potential of the blood
    • Concentration of the respiratory gases (carbon dioxide and oxygen) in the blood
  • Homeostasis in mammals relies on two different coordination systems to transfer information between different parts of the body:
    • Nervous system – information is transmitted as electrical impulses that travel along neurones
    • Endocrine system – information is transmitted as chemical messengers called hormones that travel in the blood

Exam Tip

Learn the following definition for homeostasis:

Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.

Negative Feedback

  • The majority of homeostatic control mechanisms in organisms use negative feedback to maintain homeostatic balance (ie. to keep certain physiological factors, such as blood glucose concentration, within certain limits)
  • Negative feedback control loops involve:
    • A receptor (or sensor) – to detect a stimulus that is involved with a condition / physiological factor
    • A coordination system (nervous system and endocrine system) – to transfer information between different parts of the body
    • An effector (muscles and glands) – to carry out a response
  • Outcome of a negative feedback loop:
    • The factor / stimulus is continuously monitored
    • If there is an increase in the factor, the body responds to make the factor decrease
    • If there is a decrease in the factor, the body responds to make the factor increase


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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