Lung Volumes and Capacities

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Lung Volumes[edit]

  • Primary lung volumes:
  • RV - residual volume
  • ERV - expiratory reserve volume
  • TV - tidal volume
  • IRV - inspiratory reserve volume
  • Secondary derived capacities
  • TLC - total lung capacity
  • VC - vital capacity
  • IC - inspiratory capacity
  • FRC - functional residual capacity

Volume refers to one of the 4 primary, non-overlapping subdivisions of TLC, each capacity includes two or more of the primary lung volumes

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  • Total ventilation is 7500ml/min
  • Alveolar ventilation is 5250ml/min
  • Pulmonary blood flow is 5000ml/min
  • Alveolar ventilation and pulmonary blood flow are roughly equal

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  • Neither FRC nor RV can be measured with a spirometer alone, so must be measured by gas dilution or plethysmograph:
  • Gas dilution technique:
  • A patient is connected to a spirometer with a known concentration of helium (virtually insoluble in blood), and the patient breathes until helium concentrations in spirometer and lungs equalise
  • C x V then used to determine total volume of the system
  • Body plethysmograph:
  • Patient sits in plethysmograph and is asked to inhale against a pressure sensor. Pressure in lungs drops and volume expands.
  • Boyle's law: pressure x volume is constant at a constant temperature - PV=K
  • P1V1 = P2(V1 - ΔV)
  • And V can be obtained and used in P3V2 = P4(V2 + ΔV)
  • V2 is the FRC.
  • Helium dilution only measures communicating gas/ventilated lung volume, whereas the body plethysmograph method measures all gas in lung including trapped volume

Functional Residual Capacity:

  • Definition: The volume of gas left in the lungs at the end of normal tidal expiration
  • The lung volume in which gas exchange takes place
  • Small fluctuations of alveolar and arterial gas tensions occur with each tidal breath as fresh gas mixes with alveolar air
  • Acts as a buffer:
  • Maintains constant A and a gas tensions with each breath
  • Prevents rapid changes in alveolar gas with changes in ventilation/inspired gas
  • Increased with increasing height, being male, emphysema, asthma, increasing PEEP
  • Decreased with decreased diaphragmatic muscle tone (in anaesthesia), lying down, increased abdominal contents