properties of gas exchange surfaces

• aerobic respiration requires oxygen and produces carbon dioxide as a waste product
• photosynthesis requires carbon dioxide and produces oxygen as a waste product
• • single celled organisms have a high SA:V ratio which allows the exchange of substances to occur by simple diffusion
  • large surface area allows maximum absorption of nutrients and gases and removal of waste products
  • small volume = short diffusion distance
• size of organism increases = SA:V ratio decreases

  • there is less surface area for the absorption of nutrients and gases and removal of waste products than volume and requirements of the organism
  • greater volume = longer diffusion distance to the cells and tissues of the organism
• • Alveoli increase the surface area of mammalian lungs
• • the surface often contains only one layer of epithelial cells
  • the cells can also be flattened in shape to further reduce the distance across them
  • this means that substances have a very short diffusion pathway
• concentration gradient: the difference in concentration of a substance between two places. e.g between the air inside the alveoli and the blood
• greater difference in concentration = greater rate of diffusion as gas molecules move across the exchange surface
• the concentration gradient is maintained by:

  • alveoli having a good blood supply; this constantly removes oxygen from the capillary side of the exchange surface and supplies carbon dioxide
  • the ventilation system in mammals ensures constant inhalation and exhalation; this supplies oxygen and removes carbon dioxide from the alveoli side of the exchange surface
• as the overall size of the organism increases, the surface area becomes smaller in comparison to the organism's volume and the organism's surface area: volume ratio decreases

  this is because volume increases much more rapidly than surface area as size increases
• fick's law

  rate of diffusion ∝ (surface area x concentration difference) ÷ thickness of membrane
• ficks law

  rate = P X A ((C1 - C2) ÷ T)
• proportionality means the rate of diffusion will double if:

  • the surface area or concentration difference doubles
  • the diffusion pathway halves
• alveoli

  located at ends of bronchioles

  alveolar wall consists of a single layer of flattened/squamous epithelium
• • squamous epithelium forms the alveolar wall and is very thin and permeable for the easy diffusion of gases
  • alveoli are surrounded by elastic fibres, allowing them to stretch during inhalation
• alveoli are surrounded by an extensive capillary network

  • carbon dioxide diffuses out of the capillaries and into the alveoli to be exhaled, while oxygen diffuses from the alveoli and into the capillaries to be carried around the body
• a layer of moisture lines the alveoli, facilitating the diffusion of gases

  • oxygen and carbon dioxide are able to dissolve in the layer of moisture, so exchange occurs in solution rather than with the air inside the alveoli
• explain why emphysema affects the rate of gas exchange between the alveolus and the blood (4)

  • reduction in rate of gas exchange
  • due to reduced rate of diffusion
  • bc of smaller surface area (for diffusion)
  • greater diffusion distance pathway
• evaluate effects of smoking on respiratory system

  • smoking/increasing age decreases FEV/volume of air breathed out in 1 second
  • smoking decreases surface area of alveoli
  • smoking reduces volume of air in alveoli/lungs
• evaluate effects of smoking on respiratory system

  • smoking/reduced surface area reduces gas exchange
  • smoke contains inflammatory chemicals which damages alveolar septum
  • non smoker didn't develop emphysema; smokers develop emphysema
• evaluate effects of smoking on respiratory system

  • smokers develop symptoms of emphysema around age 70 / have severe disability due to emphysema around age 88
  • increasing age may increase likelihood of other respiratory issues
  • reduced blood flow to lungs reduces gas exchange