Surface area to volume ratio

• As organisms get larger, the surface area : volume ratio falls sharply which presents a huge problem for multicellular organisms
• Cells on the surface can get enough oxygen simply by diffusion, however not enough oxygen can diffuse into the cells in the centre of the organism as they are too far away from the surface.
• Fish get their oxygen from the water which passes into their mouth, flows over the gills and is then transported into the bloodstream.
• The gills of a fish are covered in very large numbers of fine filaments, which is where gases pass in and out of the blood. Deoxygenated blood passes into the filament, where oxygen diffuse from the water into the blood, allowing for oxygenated blood to then return to the body.
• The filaments in fish gills have 3 main adaptations to increase the rate of diffusion:

  1. Give gills a massive surface area
  2. Have thin membrane to provide a short diffusion pathway
  3. Efficient blood supply to take oxygenated blood away