Gaseous Exchange.

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BY03 - Gaseous Exchange

Small, simple organisms (single cell) have a relatively small oxygen demand. All the oxygen they need diffuses into their body through it's surface. This strategy of diffusion would not work through the surface of a larger organism.

As organisms get larger, their volume increases at a faster rate than their surface area; resulting in the need to exchange materials quickly outstripping their ability to do so (The material an organism can exchange is proportional to it's surface area). Therefore, larger organisms can only survive if they evolve strategies to increase their surface area:volume ratio. These strategies involve specialist exchange organs called respiratory organs.

Respiratory Organs
Gases pass into and out of the body of an organism by diffusion. Respiratory organs simply increase the efficiency of diffusion by having the following features in common:

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  • A large surface area. The greater the area of tissue in contact with the environment, the greater the rate of diffusion. 
  • Thin permeable walls. This ensures that the diffusion distance is as short as possible. 
  • Moist. Only gases in solution can pass into cells. 
  • A good blood supply. A circulatory system supplies the respiratory organs with plenty of blood. Blood takes up oxygen and transports it to other parts of the organism. This ensures that a diffusion gradient is always present. 
  • Good ventilation. The physical pumping mechanism which continually brings ...

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