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HBIO1 > The Lungs
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Lung structure

  • Nose
    • Air is filtered in nostrils with small hairs
    • Air is moistened and warmed by nasal cavities
    • Mucus traps foreign particles while cilia propels particles towards the throat
  • Air passes into the pharynx → larynx → trachea
    • The epiglottis is found within the larynx
    • Breathing: epiglottis projects upwards → larynx is open
    • Swallowing: larynx pulled up / epiglottis flaps back and blocks larynx / prevents food from entering airway
  • Trachea
    • Contains C-shaped cartilage rings / prevents collapse of tube
    • Divides into 2 tubes with smaller diameter called bronchi
    • Bronchus is supported with ciliated epithelia to prevent microorganisms
    • Right bronchus is bigger than the left one → common site for inhaled foreign bodies
  • Bronchi further divide into bronchioles
    • Their diameter can be controlled by smooth muscles
    • Form alveoli (100µm in diameter)

Fick’s law

  • Rate of diffusion is proportional to (surface area x conc. difference) / distance
  • Applies to exchange of food, waste, gases, and heat with surroundings
  • Large organisms
    • Have a small surface area : volume ratio
    • Decreases the rate of diffusion
    • Large animals loose less heat than small animals
    • Don't require a high metabolism to maintain body temperature
    • Feed only once
  • Small organisms
    • Lose heat very readily
    • Need a high metabolism to maintain body temp
    • Must feed continuously
  • More detail in Unit 2 Section 3.2.4
  • [exam] Efficient gas exchange requires:
    • Large surface area
    • Large concentration gradient (low O2 on one side, high O2 on the other site of the membrane)
    • Short diffusion pathway (thickness of membrane molecules must travel to diffuse across)

Alveolar Gas Exchange

  • Greater partial pressure of O2 in alveolar air / more O2 dissolves in blood (Henry's Law)
  • Two types of alveoli cells
  • Type I cells
    • Composed of endothelium - layer of two thin cells
    • This allows diffusion of gases (short diffusion pathway) down their conc. gradients
    • O2diffuses from air to blood; CO2diffuses from blood to air
  • Type II cells
    • Secrete surfactant that keep alveoli constantly moist
    • Allows oxygen to dissolveand to diffuse through the cells into the blood
    • In the blood, it is taken up by haemoglobin
  • Alveoli contain phagocytes to kill bacteria that have not been trapped by mucus
  • Ventilation
    • Flow of air in and out of alveoli
    • Maintains large concentration gradient


  • Tidal volume, VT, volume of air inhaled and exhaled in a normal single breath (≈0.5 L)
  • Functional residual capacity, FRC, volume remaining in lungs after exhalation of tidal volume (≈2.5 L)
  • Expiratory reserve volume, ER, volume of a maximal exhalation (≈1.5 L)
  • Residual volume, RV, volume remaining in lung after maximal exhalation (≈1L)
  • Inspiratory reserve volume, IR, additional volume that can be inhaled after inhalation of tidal volume
  • Vital capacity, VC, maximum volume of exhalation after lungs are maximally filled
    • Best clinical indicator of breathing
  • Minute ventilation is the overall flow of air into lungs (analogous to cardiac output)
    • Minute Ventilation = Tidal Volume x Respiratory Rate
    • (0.5 litre/breath * 10 breaths/min = 5 litres per minute)
  • "Dead space" - not all O2 available in air is available to alveoli
    • Fresh air mixes with exhaled air during inspiration
    • Alveolar ventilation takes dead space into account
    • Alveolar ventilation = (Tidal Volume - Dead Space) x Respiratory Rate
    • (350 ml x 10 breaths per minute = 3500 ml or 3.5 litres)

Measurements of Ventilation

  • A spirometer is used to measure expired breath
  • Restrictive disorders, such as pulmonary fibrosis, reduce compliance and vital capacity
  • Four measures are called respiratory volumes
    • Tidal volume
    • Inspiratory reserve volume
    • Expiratory reserve volume
    • Residual volume
  • Others, called respiratory capacities, are calculated by adding 2 or more of the respiratory volumes