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

• 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

• 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’

A negative feedback control loop