Gas exchange is a vital function in all living organisms as it facilitates the interchange of materials required for metabolic reactions

• Oxygen is required for aerobic respiration, whereby cells produce large amounts of chemical energy (ATP)
  

• Carbon dioxide is a waste product of this process and excess levels must be removed from the organism
  

  • In photosynthesising organisms, carbon dioxide is a required substrate for the synthesis of organic molecules via carbon fixation
    

In animals, respiratory gases are transported within the bloodstream and gas exchange is mediated by specialised ventilation systems (e.g. lungs, gills, etc.)

• In larger organisms, the distance from the centre of the organism to the exterior is greater – meaning gas exchange would not be efficient without a means of continuous ventilation
  

• Larger organisms also have a smaller SA:Vol ratio – this makes necessary a specialised ventilation system to ensure there is adequate gas supply to the tissues
  

Different types of organisms have evolved distinct systems for gas exchange that are suited to their environment – the complexity of these systems is typically correlated to organism size

• For small multicellular organisms (such as flatworms), simple diffusion across the external surface is sufficient to meet their respiratory needs
  

• Insects have a respiratory system that is independent of the circulatory system – instead they have a tracheal system to move gases throughout the organism 
  

• Aquatic organisms (such as fish) have evolved heavily folded structures called gills to take up the dissolved oxygen from within the water
  

• In mammals, ventilation is mediated by spongy respiratory structures called lungs, which exchange gases between the bloodstream and the atmosphere
  

Concentration Gradients

Because gas exchange is a passive process, a ventilation system is needed to maintain a concentration gradient at exchange surfaces

• Oxygen is constantly being consumed by cells for aerobic respiration, while carbon dioxide is continually being released

Ventilation systems function to continually cycle fresh gases into the tissues from the atmosphere (terrestrial animals) or water (aquatic animals)

• This means oxygen levels stay high within the respiring tissues and carbon dioxide levels remain low

• Respiratory gases are transported within the bloodstream, meaning that ventilation systems need to be connected to a dense network of blood vessels with continuous blood flow

• Ventilation systems are also structured to have a very large surface area, so as to increase the overall rate of gas exchange

Concentration Gradients