Gas exchange bio

• The lungs are the gas exchange surface in humans
• Gas exchange surfaces


  • Large surface area to allow faster diffusion of gases across the surface
  • Thin walls to ensure diffusion distances remain short
  • Good ventilation with air so that diffusion gradients can be maintained
  • Good blood supply to maintain a high concentration gradient so diffusion occurs faster
• Structures in the human breathing system

  • Cilia and mucus
  • Trachea
  • Bronchi
  • Bronchioles
  • Alveoli
• Cilia


  Tiny hairs on the end of epithelial cells that beat and push mucus up the passages towards the nose and throat where it can be removed
• Goblet cells


  Mucus-producing cells that are shaped like a goblet or cup
• Mucus traps particles, dust and pathogens and cilia beat and push it up and away from the lungs
• External intercostal muscles


  Pull the rib cage up
• Internal intercostal muscles


  Pull the ribcage down
• Diaphragm


  A thin sheet of muscle that separates the chest cavity from the abdomen
• Ventilation during inhalation


  1. Diaphragm contracts and flattens

  2. External intercostal muscles contract to pull ribs up and out

  3. Increases volume of chest cavity

  4. Decreases air pressure inside lungs relative to outside

  5. Air is drawn in
• Ventilation during exhalation


  1. Diaphragm relaxes and moves upwards

  2. External intercostal muscles relax so ribs drop down and in

  3. Decreases volume of chest cavity

  4. Increases air pressure inside lungs relative to outside

  5. Air is forced out
• The external and internal intercostal muscles work as antagonistic pairs
• During forced exhalation, the internal intercostal muscles also work to pull the ribs down and in to decrease the volume of the thorax more, forcing air out more forcefully and quickly
• Alveoli


  • Many rounded alveolar sacs which give a very large surface area to volume ratio
  • Thin, single layers of cells to minimise diffusion distance
  • Ventilation maintains high levels of oxygen and low levels of carbon dioxide in the alveolar air space
  • Good blood supply ensures constant supply of blood high in carbon dioxide and low in oxygen
  • Layer of moisture on the surface helps diffusion as gases dissolve
• Smoking causes chronic obstructive lung disease (COPD), coronary heart disease and increased risks of several different types of cancer, including lung cancer
• Chemicals in cigarettes


  • Tar
  • Nicotine
  • Carbon monoxide
• Nicotine


  • Narrows blood vessels leading to an increased blood pressure
  • Increases heart rate
• Carbon monoxide


  Binds irreversibly to haemoglobin, reducing the capacity of blood to carry oxygen
• Tar


  • A carcinogen that stimulates goblet cells and mucus glands to enlarge, producing more mucus
  • Destroys cilia so mucus builds up blocking the smallest bronchioles and leading to infections
  • Contributes to emphysema by breaking down the elastic fibres in the alveoli
• Exercise causes the frequency of breathing to increase in order to provide more oxygen for respiration and to pay off any subsequent oxygen debt
• Investigating the effect of exercise on breathing

  1. Measure breathing rate at rest

  2. Student exercises for a set time

  3. Measure breathing rate immediately after exercise and every minute for 5 minutes after

  4. Repeat for a second student

  5. Repeat the whole investigation after a period of rest
• Fitness, breathing rate and exercise

  Unfit individuals may have a higher breathing rate while resting, a more rapid increase in breathing rate during exercise, and a longer recovery period for their breathing rate to return to normal
• It is important to control variables when investigating the effect of exercise on breathing