Exchanging Substances

• The job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it.
• For the lungs to transfer oxygen and remove carbon dioxide, they contain millions of little air sacs called alveoli where gas exchange takes place.
• Alveoli are specialised to maximise the diffusion of 02 and CO2. They have:

  1. An enormous surface area (about 75m^2 in humans).
  2. A moist lining for dissolving gases.
  3. Very thin walls.
  4. A good blood supply.
• The inside of the small intestine is covered in millions and millions of these tiny little projections called villi.
• Villi increase the surface area of the small intestine in a big way so that digested food is absorbed much more quickly into the blood.
• Villi have a single layer of surface cells and a very good blood supply to assist quick absorption.
• In plants, carbon dioxide diffuses into the air spaces within the leaf, then it diffuses into the cell where photosynthesis happens. The leafs structure is adapted so that this can happen easily.
• The underneath of a leaf is its exchange surface. It’s covered in little holes called stomata which the carbon dioxide diffuses through.
• Carbon dioxide, oxygen and water vapour diffuse out through the stomata.
• In plants, 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. Without these guard cells the plant would soon wilt.
• A leaf is a flat shape to increase the area of its exchange surface so that it’s more effective.
• The gills on a fish are their gas exchange surface. 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.
• In a fish, each gill is made of lots of thin plates called gill filaments, which give a big surface area for exchanges of gases.
• In a fish, the gill filaments are covered in lots of tiny structures called lamellas, which increase the surface area.
• In a fish, the lamellae have lots of blood capillaries to speed up diffusion.
• In fish, 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.
• The concentration of oxygen in sea water is always higher than that in a fishes blood so as much oxygen as possible diffuses from the water into the blood.