Bio- Gas Exchange Systems Internal

• Gas exchange is needed for cellular respiration, to produce energy needed for life processes.
• Gas exchange occurs across cell membranes through diffusion
• For gas exchange systems to be efficient, they must:

  • be moist
  • have a large surface area to volume ratio
  • have a thin membrane
  • operate in a way that maintains a favourable concentration gradient
• The gas exchange surface must be moist as gases must be dissolved in water to diffuse across a cell membrane.
• There must be a large surface area to volume ratio to maximise the rate of diffusion. Without a large SA:V ratio, there would not be enough sites along the specialised respiratory surface for the gases to diffuse across, reducing the rate of diffusion.
• The cell membrane must be thin so that there is a short diffusion distance so molecules can diffuse faster.
• The larger the concentration gradient, the higher the rate of diffusion. A concentration gradient must be present for diffusion to occur.
• Advantages of getting oxygen from air:

  • air has a much higher oxygen concentration than water (approximately 21%)
  • air is less dense and viscous, so diffusion occurs more quickly and less energy is needed to ventilate
• Limitations of getting oxygen from air:

  • water is continuously lost by evaporation
  • debris and dust from air may clog or block airways
• In a locust tracheal system, oxygen goes from the outside air to the spiracles, to the trachae, to the tracheoles, and then to the body cells.
• Limitations of locust gas exchange system (2):

  • tidal ventilation (air enters and exits the same what so the new oxygen-rich air mixes with the old air trapped in the tracheae or tracheoles, lowering the oxygen concentration and decreasing the concentration gradient)
  • size (they rely on diffusion to move air through system so can only grow so big. If they were to grow larger, the diffusion distance from spiracles to body cells would be greater, reducing the rate of diffusion
• The advantage of water as a gas exchange medium is that keeping the gas exchange surface moist isn't an issue.
• Disadvantages of water as a gas exchange medium are that dissolved oxygen concentrations in water are low and water is dense and viscous so more energy is required to ventilate.
• Limitations of gills:

  • Fish are restricted to water as air is not dense enough to support gills (gill filaments would collapse and stick together, reducing SA) and the gills would dry out as they are external and fish don't have adaptations to stay moist.
• Limitations for mammal gas exchange system:

  -Tidal ventilation (air enters and exits lands the same way, reducing concentration gradient)

  -Air is dry
• During inhalation in humans, the intercostal muscles contract, lifting the ribs up and outwards and the diaphragm contracts and lowers. The volume of the chest cavity increases and air is drawn into the lungs.
• During exhalation in humans, the diaphragm and the intercostal muscles relax, reducing the volume of the chest cavity, forcing air out of the lungs.
• Humans have closed circulation (the blood is in vessels).
• Surfactant is a liquid that coats the inside of the alveoli. It provides a moist membrane for gas exchange and decreases the surface tension of water (which ensures a large surface area).
• There are 300 million alveoli in the lungs.
• Bronchi are hollow tubes held open by cartilage rings and lined with mucus and ciliated cells.