3.2 - Human gas exchange

• Lungs


  • Pair of lobed structures
  • Large surface area
  • Located in chest cavity
  • Able to inflate
  • Surrounded by rib cage
• Lubricating substance


  Secreted to prevent friction between rib cage and lungs during inflation and deflation
• Breathing


  1. External intercostal muscles contract to raise ribcage

  2. Internal intercostal muscles contract to lower ribcage
• Diaphragm


  Separates the lungs from abdomen area
• Air flow into lungs


  1. Enters through nose

  2. Travels along trachea

  3. Travels along bronchi

  4. Travels along bronchioles
• Alveoli


  • Tiny sacs filled with air
  • Gaseous exchange takes place in their walls
• Trachea, bronchi and bronchioles


  • Structures adapted to enable passage of air into the lungs
• Trachea and bronchi


  • Similar in structure, except for size - bronchi are narrower
• Trachea and bronchi wall


  • Composed of several layers including cartilage, glandular and connective tissue, elastic fibres, smooth muscle, blood vessels
• Bronchioles


  • Narrower than bronchi
  • Only larger bronchioles contain cartilage
  • Wall made of smooth muscle and elastic fibres
  • Smallest have alveoli clusters at ends
• Alveoli


  • Very thin, only around one cell thick
  • Surrounded by capillaries also one cell thick
  • Reduces diffusion pathway for gases
  • Constant blood supply maintains steep concentration gradient
  • ~300 million, giving ~70m2 surface area
• Cartilage


  • Supports trachea and bronchi, prevents lungs collapsing during exhalation
• Ciliated epithelium


  • Present in bronchi, bronchioles and trachea, moves mucus towards throat
• Goblet cells


  • Present in trachea, bronchi and bronchioles, secrete mucus to trap bacteria and dust
• Smooth muscle


  • Contracts to constrict airways, controlling air flow
• Elastic fibres


  • Stretch when exhaling, recoil when inhaling, controlling air flow
• Ventilation


  The flow of air in and out of the alveoli
• Ventilation


  1. Inspiration

  2. Expiration
• Ventilation


  • Composed of two stages: inspiration and expiration
  • Occurs with the help of two sets of muscles: the intercostal muscles and diaphragm
• Inspiration


  1. External intercostal muscles contract

  2. Internal intercostal muscles relax

  3. Ribs raise upwards

  4. Diaphragm contracts and flattens

  5. Volume inside the thorax increases, lowering the pressure

  6. Difference between pressure inside the lungs and atmospheric pressure creates a gradient, forcing air into the lungs
• Expiration


  1. Internal intercostal muscles contract

  2. External intercostal muscles relax

  3. Rib cage lowers

  4. Diaphragm relaxes and raises upwards

  5. Volume inside the thorax decreases, increasing the pressure, forcing air out of the lungs
• Spirometer


  A device used to measure lung volume
• Using a spirometer


  1. Person breathes in and out of the airtight chamber

  2. Causes the chamber to move up and down

  3. Leaves a trace on a graph

  4. Graph can then be interpreted
• Vital capacity


  The maximum volume of air that can be inhaled or exhaled in a single breath
• Vital capacity


  • Varies depending on gender, age, size as well as height
• Tidal volume


  The volume of air we breathe in and out at each breath at rest
• Breathing rate


  The number of breaths per minute, can be calculated from the spirometer trace by counting the number of peaks or troughs in a minute
• Residual volume


  The volume of air which is always present in the lungs
• Inspiratory reserve volume


  The additional volume of air that can be inhaled on top of the tidal volume, e.g. during exercise
• Expiratory reserve volume


  The additional volume of air that can be exhaled on top of the tidal volume