lung volume

• Spirometer


  Device used to measure lung volumes and capacities

  
• Tidal volume (TV): The volume of air that moves into the lungs with each inspiration or that moves out with each expiration (0.5 L in both sexes)

  
• Inspiratory reserve volume (IRV)

  The extra volume of air that can be inspired with a maximal inspiratory effort over and beyond the normal inspiration (M=3.3L, F=1.9L)

  
• Expiratory reserve volume (ERV)

  The volume of air that can be expired with a maximal expiratory effort after normal expiration (M=1.0L, F=0.7L)
• Residual volume (RV)


  The volume of air remaining in the lungs even after maximum voluntary expiration (M=1.2L, F=1.1L)

  
• Vital capacity (VC)


  The maximum volume of air that can be expelled by the lungs by forceful effort following a maximum inspiration (the sum of TV +IRV+ERV) (M=4.8L, F=3.1L)

  
• Inspiratory capacity (IC)


  The maximum volume of air that can be inspired from the resting end expiratory level (M=3.8L, F=2.4L)

  
• Functional residual capacity (FRC)

  The volume of air remaining in the lungs at the end expiratory level (M=3.3L, F=1.9L)

  
• Total lung capacity(TLC)


  The maximum volume to which the lungs can be expanded with the greatest possible effort (the sum of all 4 lung volumes) (M=6L, F=4.2L)

  
• Dead space


  The area of the respiratory tract where gas exchange doesn't occur
• Types of dead space

  • Anatomical dead space
  • Physiological dead space
• Anatomical dead space


  Volume of air occupying the space from the external nares to the terminal bronchioles, i.e. in the conducting zone (150 ml in both sexes)

  
• Physiological dead space


  Volume of gas not equilibrated with blood (e.g. in lung diseases)
• Pulmonary ventilation


  Also called respiratory minute volume (RMV), the volume of air entering the lungs each minute, depends on RR and tidal volume

  
• Calculating pulmonary ventilation


  Pulmonary ventilation = TV x RR

  
• Alveolar ventilation


  The amount of air entering the alveoli per minute (exchange of gas between the alveoli and the external environment), always less than pulmonary ventilation

  
• Calculating alveolar ventilation


  Alveolar ventilation = (TV – DS) x RR

  
• Rapid shallow respiration produces much less alveolar ventilation than slow deep respiration at the same RMV
• Pulmonary circulation


  Pulmonary vascular system is a low pressure distensible system

  
• Pulmonary arterial pressure


  • About 24/9 mmHg, mean pressure about 15 mmHg

  
• Pulmonary blood flow


  Cardiac output of the right ventricle, about 5.5 L/min at rest

  
• When a person is supine, blood flow is nearly uniform throughout the lung

  
• When a person is standing (upright posture), blood flow is unevenly distributed because of the effect of gravity

  
• Blood flow distribution in upright posture

  Lowest at the apex of the lung (zone 1), highest at the base of the lung (zone 3)

  
• Calculating blood flow in different zones of the lung

  1. Zone 1: PA > Pa > Pv

  2. Zone 2: Pa > PA > Pv

  3. Zone 3: Pa > Pv > PA

  
• Ventilation per unit lung volume is greater at the base of the lung than at the apex in the upright position

  
• Ventilation (V) and Perfusion (Q)

  V/Q ratio is higher at the apex and lower at the base

  
• Regional differences in V/Q ratio lead to differences in the efficiency of gas exchange and resulting pulmonary capillary PO2 and PCO2

  
• Regional differences for PO2 are greater than those for PCO2
• At the apex (higher V/Q), PO2 is highest and PCO2 is lowest because gas exchange is more efficient
• At the base (low V/Q), PO2 is lowest and PCO2 is highest because gas exchange is less efficient
• Dead Space


  Parts of the respiratory system where air is present but no gas exchange occurs (e.g., trachea, bronchi)
• Blood flow, or perfusion, is lowest at the apex and highest at the base


  Because of gravitational effects on arterial pressure

  
• Ventilation is lower at the apex and higher at the base


  Because of gravitational effects in the upright lung
• The regional differences for ventilation are not as great as for perfusion

  
• As a result of the regional differences in V/Q ratio

  There are corresponding differences in the efficiency of gas exchange and in the resulting pulmonary capillary PO2 and PCO2
• At the apex (higher V/Q)

  PO2 is highest and PCO2 is lowest because gas exchange is more efficient
• At the base (lower V/Q)

  PO2 is lowest and PCO2 is highest because gas exchange is less efficient
• Alveolar Ventilation = 4 L/min , Pulmonary Blood Flow = 5 L/min

  
• Overall V/Q = 0.8, but linear increase in V/Q from bases (0.63) to apices (3.3)