Homeostasis (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Definition of 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

• Homeostasis maintains optimal conditions for enzyme action and all cell functions

• In the human body, these include control of:

  • Blood glucose concentration

  • Body temperature

  • Water levels

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

Examples of Homeostasis

• Some examples of homeostasis are control of blood glucose concentration (see the next topic 'Insulin & Diabetes'), control of body temperature and the control of body water content

Control of body temperature in humans

• The core body temperature of humans is kept close to 37 °C

• Body temperature is tightly controlled as a change of more than 2 °C can be fatal because enzymes would not function optimally

• The body must make a coordinated response to any rise or fall in temperature

• Body temperature is monitored and controlled by the thermoregulatory centre in the base of the brain:

  • The thermoregulatory centre contains receptors that are sensitive to the temperature of the blood

  • Temperature receptors in the skin also send signals to the thermoregulatory centre

• The brain coordinates a cooling or heating response, depending on what is required

Control of body water content in humans

• Water loss via the lungs (during breathing) or skin (during sweating) cannot be controlled, but the volume of water lost in the production of urine can be controlled by the kidneys

• The kidneys contain tiny structures known as nephrons through which filtrate, which forms urine, passes through on its way to the bladder

• Water can be reabsorbed from this filtrate as it passes along the nephron (specifically from a part of the nephron called the collecting ducts)

  • If the water content of the blood is too high then less water is reabsorbed, if it is too low then more water is reabsorbed

  • This causes either a large amount of dilute urine to be produced, or a small amount of concentrated urine

  • A hormone called ADH controls this process