Exchanging Substances

• Gas exchange happens in the lungs
• The job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it. To do this the lungs contain millions of little air sacs called alveoli where gas exchange takes place

  
• The alveoli are specialised to maximise the diffusion of O2 and CO2

  
• Alveoli have:

  • An enormous surface area (about 75 m² in humans)
  • A moist lining for dissolving gases
  • Very thin walls
  • A good blood supply
• The villi provide a really really big surface area
• The inside of the small intestine is covered in millions and millions of tiny little projections called villi

  
• Villi increase the surface area in a big way so that digested food is absorbed much more quickly into the blood

  
• Villi have:

  • A single layer of surface cells
  • A very good blood supply to assist quick absorbtion

  
• The digested food moves into the blood by diffusion and by active transport
• The structure of leaves lets gases diffuse in and out of cells
• Carbon dioxide diffuses into the air spaces within the leaf, then it diffuses into the cells where photosynthesis happens. The leaf's structure is adapted so that this can happen easily
• The underneath of the leaf is an exchange surface. It's covered in little holes called stomata which the carbon dioxide diffuses through

  
• Oxygen produced in photosynthesis and water vapour also diffuse out through the stomata

  
• Water vapour is actually lost from all over the leaf surface, but most of it is lost through the stomata

  
• The size of the stomata are controlled by guard cells. These close the stomata if the plant is losing water faster than it is being replaced by the roots. Withought these guard cells the plant would soon wilt

  
• The flattened shape of the leaf increases the area of this exchange surface so that it's more effective

  
• The walls of the cells inside the leaf form another exchange surface. The air spaces inside the leaf increase the area of this surface so there's more chance for carbon dioxide to get into the cells

  
• The vapour evaporates from the cells inside the leaf. Then it escapes by diffusion because there's a lot of it inside the leaf and less of it in the air outside
• Gills have a large surface area for gas exchange
• The gills are the gas exchange surface in fish
• Water (containing oxygen) enters the fish through its mouth and passes out through the gills. As this happens, oxygen diffuses from the water into the blood in the gills and carbon dioxide diffuses from the blood into the water

  
• Each gill is made of lots of thin plates called gill filaments, which give a big surface area for exchange of gases

  
• The gill finlaments are covered in lots of tiny structures called lamellae, which increase the surface area even more

  
• Lamellae also have a thin surface layer of cells to minimise the distance that the gases have to diffuse

  
• Blood flows through the lamellae in one direction and water flows over in the opposite direction. This maintains a large concentration gradient between the water and the blood

  
• In fish, the concentration of oxygen in the water is always higher than that in the blood so as much oxygen as possible diffuses from the water into the blood