Exchange surfaces and breathing

• Diffusion is enough to supply single-celled organisms because their metabolic activity is usually low so oxygen demands and carbon dioxide production is relatively low, and the surface area to volume ratio is large.

  Specialised exchange surfaces are efficient due to -

  • Increased surface area
  • Thin layers so diffusion distance is short.
  • Good blood supply ensures substances are always delivered and removed.
  • Ventilation to maintain diffusion gradient
• The human gaseous exchange system - Mammals have a small SA:V ratio and a lot of cells. They have a high metabolic rate because they are active and maintain their body temperature independent of the environment.

  Key structures -

  • The nasal cavity
  • trachea
  • bronchus
  • bronchioles
  • alveoli
• The nasal cavity -

  • large surface area with a good blood supply, warming the air to body temp.
  • hairy lining which secretes mucus to trap dust and bacteria, protecting delicate lung tissue from irritation and infection.
  • Moist surfaces, which increase the humidity of the incoming air, reducing evaporation from the exchange surfaces.

  Trachea - wide tube supported by rings of strong, flexible cartilage. It is lined with a ciliated epithelium, with goblet cells which secrete mucus onto the lining of the trachea, to trap dust and microorganisms.
• Bronchus - left leads to left lung and right leads to right lung and are similar to the trachea with supporting rings of cartilage, but are smaller.

  Bronchioles - have no cartilage rings and contain smooth muscle. constrict and relax to change the flow of air. lined with flattened epithelium, making some gaseous exchange possible.

  Alveoli - main gaseous exchange surface of the body. thin flattened epithelial cells along with collagen and elastin fibres. elastic recoil to squeeze the air out. They contain a large surface area, thin layers, a good blood supply and good ventilation.
• Ventilating the lungs - air is moved in and out of the lungs as a result of pressure changes in the thorax (chest)

  Inspiration - taking in air. diaphragm contracts, flattening and lowering. External intercostals contract moving the ribs up and out. volume of thorax increases so pressure is reduced. It is now lower than the external air so air is drawn into the lungs.

  Expiration - breathing out air. Muscles of the diaphragm relax, going into its dome shape. External intercostals relax so ribs move down and inwards. decrease the volume of the thorax, increasing pressure so air moves out.
• measuring the capacity of the lungs -

  • peak flow meter - measures rate at which air can be expelled from the lungs.
  • vitalographs - breathing out quickly and a graph is produced showing the amount of air that is breathed out and how quickly.
  • spirometer - investigating breathing patterns. Soda lime in the water to remove carbon dioxide.
• Tidal volume - volume of air moved in or out with each resting breath.

  vital capacity - volume of air exhaled when the deepest possible intake of breath is followed by the strongest possible exhalation.

  Inspiratory reserve volume - maximum volume of air breathed in above normal inhalation.

  Expiratory reserve volume - The extra amount of air that can be forcibly exhaled after a normal breath out.

  Residual volume - volume of air left in the lungs after a forcible exhale.

  Total lung capacity - sum of vital capacity and residual volume
• Gaseous exchange in insects -

  tough exoskeleton where no gaseous exchange can take place. Do not have blood pigments that can carry oxygen.

  • spiracles along the thorax and abdomen. open or closed by sphincters to minimise water loss.
  • leading away from spiracles are tracheae and they are the largest tubes running into and along the body. They are lined with spirals of chitin and so no gaseous exchange can take place as it is impermeable to gases.
  • tracheae form into smaller tracheoles that are permeable as they do not contain chitin.
• Air moves along their tracheae and tracheoles by diffusion, reaching all the tissues. Vast number of tiny tracheoles give a very large surface area for diffusion. Oxygen dissolves in moisture on the walls of the tracheoles and diffuses into into the surrounding cells. Towards the end of the tracheoles there is tracheal fluid, which limits the penetration of air for diffusion. When oxygen demands build up, for example when the insect is flying, a lactic acid build up in the tissues results in water moving out of the tracheoles by osmosis. Exposing more surface area for diffusion.
• Some insects have high energy demands and therefore need alternative methods of increasing the level of gaseous exchange.

  • mechanical ventilation of the tracheal system - air actively pumped into the system by movements of the thorax and/or abdomen. Change the volume of the body and changes the pressure in the tracheae and tracheoles. Air drawn in or forced out as pressure changes.
  • collapsible enlarged tracheae or air sacs, which act as air reservoirs - Increase the amount of air moved through the gas exchange system. inflated and deflated by ventilating movements of thorax and abdomen.
• Respiratory systems in bony fish - water has a much lower oxygen content and is 100x more viscous than air, so fish do not have lungs. Instead, they have gills - they maintain a one directional flow of water over their gills which have a large surface area, good blood supply, and thin layers. They are covered by an operculum (bony flap).

  water flow over the gills - when fish are swimming, they keep a flow of water over their gills by opening and closing their mouth and operculum.
• Effective gaseous exchange in water -

  • the tips of adjacent gill filaments overlap which increases the resistance to the flow of water over the gill surfaces and slows down the movement of the water, more time for gaseous exchange to take place.
  • The water moving over the gill filaments and the blood in the gill filaments flow in different directions. Because of this, there is a countercurrent exchange system. This ensures that steeper concentration gradients are maintained than in parallel systems. The bony fish can remove 80% of the oxygen from the water flowing over their gills.