Lung Volumes and Capacities

Question 1

Tidal Volume

Answer 1

• Symbol = V_T
• The volume of air that fills the alveoli + the volume of air that fills the conducting airways
• Normal, quiet breathing involves inspiration and expiration of the V_T
• Usually V_T ≈ 500 mL

Question 2

Inspiratory and Expiratory Reserve Volumes

Answer 2

• Inspiratory reserve volume = the additional volume that can be inspired above V_T (≈ 3000 mL)
• Expiratory reserve volume = the additional volume than can be inspired below V_T (≈ 1200 mL)

Question 3

Residual Volume

Answer 3

= the volume of gas remaining in the lungs after a maximal forced expiration

• Symbol: RV
• Cannot be measured by spirometry

Question 4

Inspiratory Capacity

Answer 4

= V_T + inspiratory reserve volume ≈ 3500 mL

Symbol: IC

Question 5

Functional Residual Capacity

Answer 5

• Symbol: FRC
• FRC = expiratory reserve volume + residual volume ≈ 2400 mL
• FRC = the volume present in the lungs after a person has expired a normal tidal breath
  
  • can be thought of as the equilibrium volume of the lungs
• Since it includes residual volume, it also cannot be measured by spirometry

Question 6

Vital Capacity

Answer 6

• Symbol: VC
• VC = IC + expiratory reserve volume ≈ 4700 mL
• VC = the volume that can be expired after maximal expiration
• VC value increases with body size, male gender, and physical conditioning; decreases with age

Question 7

Total Lung Capacity

Answer 7

• Symbol: TLC
• TLC = VC + residual volume ≈ 5900 mL
• Since it includes residual volume, it also cannot be measured my spirometry

Question 8

Physiologic Dead Space (V_D)

Answer 8

V_D = V_T x [(Pa_CO2 - PE_CO2)/Pa_CO2]

The volume of physiologic dead space is the tidal volume multiplied by a fraction that represents the dilution of alveolar P_CO2 by dead space air.

Question 9

Minute Ventilation

Answer 9

= the total rate of air movement into and out of the lungs (per unit time)

= V_T x (breaths/minute)

Question 10

Alveolar Ventilation

Answer 10

= minute ventilation corrected for the physiologic dead space

V_A = (V_T - V_D) x (breaths/min)

Question 11

Surface Tension

Answer 11

= tension that develops when the attractive forces between adjacent liquid molecules is greater than the attractive forces between adjacent liquid and gas molecules (in the alveoli)

• Surface tension (T) generates a pressure that tends to collapse the alveolus
  
  • this pressure is given by the law of Laplace
    
    • P = 2T/r (where r is the radius of the sphere/alveolus)
  • P and T are directly proportional, so the more T you have the more collapsing P there will be (if the radius stays the same)
  • Without surfactant, the only way to prevent alveolus collapse is the ⇡ its radius
• However, alveoli need to be small for purposes of gas exchange; surfactant solves the surface tension/collapsing pressure problem

Question 12

Surfactant

Answer 12

• Surfactant is a mixture of phospholipids (amphipathic!) that line the alveoli and reduce their T and collapsing P
  
  • Most impt constituent = dipalmitoyl phosphatidylcholine (DPPC) [40%]
    
    • DPPC molecules align themselves on the alveolar surface
      
      • hydrophobic portions attract each other
      • hydrophilic portions repel
    • Intermolecular forces b/w DPPC molecules break up the attracting forces between the liquid molecules lining the alveoli → ⇣T → ⇣collapsing P → small alveoli remain open
    • ⇣T also → ⇡lung compliance, which makes it easier for the lungs to expand
  • Other constituents
    
    • Cholesterol (50%)
    • Surfactant proteins (10%)
      
      • SP-A and SP-B combine w/ DPPC in the lamellar bodies
      • SP-B and SP-C are important for the maturation of DPPC and stabilize the surfactant coat
      • SP-A and SP-D are important for innate immune protection in the lungs