Mechanics of Breathing - Part 1 Charles L. Webber, Jr., Ph.D.

Learning Objectives: You should be able to:

• Give the formulae for alveolar, dead space and total ventilations and the effect each has on carbon dioxide levels in the systemic arterial blood.
• Define compliance and show how pulmonary versus chest wall compliances can vary in various respiratory diseases.
• Explain the origination of surface tension forces at air/water interfaces in terms of the Law of LaPlace.
• Show how pulmonary surfactants directly proportion lung surface tension forces to lung volume and how this helps maintain airway patency.

Rhoades & Tanner Text Readings: Chapter 19, Pages 352-362

Ventilatory Output     Elastic Properties     Surfactant      Compliance Curves     MainMenu

1. Definitions
   • ventilation = volume * frequency = volume per time ( in L/min)
   • minute volume = volume per minute
   • dead space ventilation (ineffective for gas exchange)
     _D = f * V_D
   • alveolar ventilation (effective for gas exchange)
     _A = f * V_A = f * (V_T - V_D)
   • total ventilation (sum of dead space and alveolar space ventilations)
     _E = _D + _A = f * V_T

    

2. Frequency * Volume Combinations for Constant Alveolar Ventilations

   f (min-1)	VT (mL)	VD (mL)	VA (mL)	E (L/min)	D (L/min)	A (L/min)
   10	670	170	500	6.7	1.7	5.0
   15	503	170	333	7.6	2.6	5.0
   20	420	170	250	8.4	3.4	5.0
   30	337	170	167	10.1	5.1	5.0

Ventilatory Output     Elastic Properties     Surfactant      Compliance Curves     MainMenu

Elastic Properties of the Lungs

1. Definition of Elastance
   • the ability to resist deformation
   • the ability to return to original shape after deformation
   • a steel rod is more elastic than a rubber band
   • Hooke's law: linear relationship between length and applied force

    

2. Volume/Pressure Relationships in the Lung 

   

   • curvilinear relationship between volume (length) & pressure (force)
   • superimposition of inflation & deflation curves in saline-filled lung
   • hysteresis of inflation & deflation curves in air-filled lung
3. Interpretation of Experimental Results
   • the lung behaves as an elastic structure that resists volume deformation
   • elastic properties are attributed to elastin, collagen, and surface tension forces
   • total elastance of the pulmonary system depends upon summed elastances of the lung & chest wall (E_total = E_lung + E_{chest wall})

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Surfactant and Alveolar Surface Tension

1. Origin of Surface Tension
   • contraction of surface molecules at liquid/gas interfaces
   • the imbalance of intermolecular cohesive forces shrinks the surface area
   • the "skin" of water will "float" a steel needle

    

2. Measurement of Surface Tension 

   

   • surface tension of pure water is independent of surface area exposed to air
   • detergents decrease surface tension by interfering with intermolecular cohesive forces
   • surface tension of lung extract (lavage) varies directly with area
   • surface tension forces are established at the alveolar level (air/blood interface)
   • this dependent relationship allows interconnected alveoli of various sizes to coexist

    

3. Pulmonary Surfactant
   • phospholipids, notably dipalmitoyl phosphatidylcholine (alveolar type II pneumocytes)
   • floats on the alveolar epithelium where is exerts its detergent action (monolayer effect)
   • benefits of pulmonary surfactant:
     • decreases the overall elastance of the lung
     • decreases the opening pressure of alveoli
     • permits over a two-fold size range of alveoli to coexist
   • detriments of lack of surfactant:
     • alveolar instability and collapse (atelectasis)
     • Hyaline membrane disease of the newborn (respiratory distress syndrome)

Ventilatory Output     Elastic Properties     Surfactant      Compliance Curves     MainMenu

1. Definition of Compliance
   • the ability to comply with deformation forces
   • compliance = reciprocal elastance (C = 1 / E)
   • a steel rod is less compliant than a rubber band

    

2. Pulmonary Compliances 

   

   • in general, C = change in volume / change in pressure = V/ P
   • relaxation pressure curve (total compliance curve of respiratory system)
   • there are three pulmonary compliances:
     • compliance of the chest wall (C_{chest wall})
     • compliance of the lung (C_lung)
     • combined compliance of the chest wall-lung system (C_total)

     Ventilatory Output     Elastic Properties     Surfactant      Compliance Curves     MainMenu