Constant Internal Environment (OCR GCSE Combined Science A (Gateway)): Revision Note

The Importance of Homeostasis

• Homeostasis is the regulation of the internal conditions of a cell or organism

• Some examples of these internal conditions include:

  • Water content (of an individual cell or of the body fluids of an organism)

  • Body temperature

  • pH

  • Blood pressure

  • Blood glucose concentration

• It is important for an organism to respond to internal and external environmental changes whilst maintaining optimum internal conditions for enzyme action and healthy cell functions

• If the homeostatic limits are exceeded, the organism may die

Control of homeostasis

• Maintaining controlled conditions within the body is under involuntary (automatic) control

• This means that the brain stem (or non-conscious part of the brain) and the spinal cord are involved in maintaining homeostasis – you don’t consciously maintain your body temperature or blood glucose level

• These automatic control systems may involve nervous responses or chemical responses

• All control systems include:

  • Cells called receptors, which detect stimuli (changes in the environment)

  • Coordination centres (such as the brain, spinal cord and pancreas) that receive and process information from receptors

  • Effectors (muscles or glands) which bring about responses which restore optimum levels

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)

• 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

  • The system is restored to its original level

• Negative feedback mechanisms are usually a continuous cycle of bringing levels down and then bringing them back up so that overall, they stay within a narrow range

A negative feedback control loop