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In simple organisms such as sponges and jellyfish, gas exchange is a direct and simple process. How and why is this process different for humans, fish, and most other large multicellular animals?
- MLv 43 months ago
Simple: it is all to do with diffusion.
First, our body's surface area is too small to take in enough oxygen and to excrete all the carbon dioxide we synthesise. That really is all that needs to be said. However, even if the surface area was big enough there is another problem.
The movement of molecules by diffusion is relatively slow. Therefore, even if your body's surface area were large enough molecules of oxygen could not be transported from the body's surface to cells in the centre of the body at a rate required to support life.
There are other problems, too, but I do not intend going through them all.
Because large organisms, in general, cannot use their body's surface area for gas exchange they have special organs for doing this: lungs or gills. As these organs are situated in one place another system, the circulatory system, is required to transport gases between cells and the specialist gas exchange organ.
- az_lenderLv 73 months ago
We don't have enough surface area, so must pump the fresh oxygen around to our organs in some fashion -- humans use blood to do so. We use lungs as a special set of surfaces for the gas exchange with the surroundings.