exchnage surfaces

• • cells use diffusion to take in substances and remove waste products
  • oxygen and carbon dioxide transferred between cells
  • urea diffuses from cells into plasma for removal from body
• • single celled organisms have a large surface area to volume ratio, so they can exchange large amounts of substances
• • multicellular organisms have small surface area to volume ratio, so not enough substances can diffuse to supply volume
  • so, exchange surfaces needed
• exchange surfaces:

  • thin membrane - short distance to diffuse
  • large surface area - many substances can diffuse at once
  • many blood vessels