Constant Internal Environment (OCR Gateway GCSE Biology: Combined Science)

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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 of what is considered ‘normal’

Negative feedback loop

A negative feedback control loop

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