Exchange surfaces

• The function of the lungs is to transfer oxygen from the air to blood and remove carbon dioxide from the blood
• Lungs contain millions of alveoli (tiny air sacs) where gas exchange takes place
• Alveoli maximise diffusion by:

  • Having millions of alveoli which increase surface area
  • A moist lining for dissolving gases
  • Very thin walls for a short diffusion distance
  • A good blood supply to maintain a steep concentration gradient
• The inside of the small intestine is covered with millions of tiny projections called villi
• Villi increase the surface area to speed up diffusion of nutrients through the wall of the small intestine into the blood
• Villi have thin walls for a short diffusion distance
• Villi have a good blood supply to maintain a steep concentration gradient
• The underneath of the leaf is an exchange surface. It's covered in biddy little holes called stomata which the carbon dioxide diffuses in through
• Oxygen (produced in photosynthesis) and water vapour also diffuse out 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 absorbed
• The flattened shape of a leaf creases the area of this exchange surface so that it's more effective
• The walls of 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 water vapour evaporates from the cells inside the leaf. Then it escapes by diffusion
• 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 filaments are covered in lots of tiny structures called lamellae which increase the surfce area even more
• The lamellae have lots of blood capillaries to speed up diffusion
• 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
• The concentration of oxygen in the water is always higher than in the blood, so as much oxygen as possible diffuses from the water to the blood
• Plant roots have specialised cells called root hair cells, which increase the surface area to speed up the absorption of water through osmosis and mineral ions through diffusion and active transport
• Water also constantly moves away from the roots through the stem in the transpiration stream, which maintains a steep concentration gradient