Homeostasis (Edexcel IGCSE Science (Double Award)): Revision Note

Homeostasis

• Homeostasis is the maintenance of a constant internal environment

• It is essential for an organism to keep internal conditions within set limits for it to remain healthy

• Homeostasis ensures optimal conditions for enzyme activity and cell function, allowing the organism to survive and respond to internal and external changes

• If internal conditions exceed these limits, the organism may die

• Two examples of homeostasis are 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