A level Biology exam revision resources written by A level Examiners


The cells of mammals are provided with a constant supply of the things they need and the removal of substances produced. So the cells in a mammal may live in constant moderate conditions even though the conditions outside may fluctuate, or be too extreme.

Such environmental fluctuations include variations in:

  • water
  • pH
  • oxygen and carbon dioxide
  • temperature
  • nutrients (e.g.. glucose, ions)
  • toxins and waste products

Homeostasis is the process, which maintains all these factors at the levels that are required to sustain life. It is defined as the maintenance of the internal environment. Where the `internal environment' = the interstitial fluid (the medium in which body cells are bathed) - also known as tissue fluid.

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