Lab #7: Respiratory Mechanics Purpose: To Determine Relative Contributions

Lab #7: Respiratory Mechanics Purpose: to determine relative contributions of elastic factors and surface tension factors to lung compliance Pressures •PB= Barometric Pressure •Relative PB assumed to be 0 •Palv= Alveolar Pressure  cannot be measured However: if there is NO FLOW, that means that Palv=PB so Palv=0 •Ppl= Pleural Pressure •Palv - Ppl= Transpulmonary Pressure Transpulmonary Pressure Ptp= Palv–Ppl = 0 -(-5) at FRC* = 5 cm H2O at FRC* where pressures are expressed relative to PB. *FRC is Functional Residual Capacity-the volume of air remaining in the lungs after a quiet expiration (About 2500 ml in a normal adult.)

What pressure was being measured in the lab? In the lab we measured the transpulmonary pressure by measuring the “inside” pressure (Ptp=Palv in the lab) Compliance Curve The static compliance of the lung is the change in volume for a given change in transpulmonary pressure with zero gas flow. Dynamic compliance measurements are made by monitoring the tidal volume used, while intra thoracic pressure measurements are taken during the instance of zero air flow that occur at the end inspiratory and expiratory levels with each breath.

Lung compliance (C) is the slope of the lung volume versus transpulmonary pressure curve and is a measure of the elastic behavior of the lung. The greater the lung compliance, the more easily the lung is distended. The elastic behavior of the lung is due to elastic tissue and to surface tension. SC (Specific Compliance) = CL/FRC  Compliance changes based on the size of the lung. Chest Wall Compliance The chestwall transmural pressure is used: Pwall= Ppl-PB = (-5) -0 at FRC = -5 cm H2O at FRC

Total Compliance 1/Ctotal=1/Clung+1/Cwall Given: Clung= 200 ml/cm H2O Cwall= 200 ml/cm H2O Calculate Ctotal

Hysteresis: Air filled lung versus saline filled lung

The Law of LaPlace (applies to each individual alveolus) P=2T/r  P∝T, P∝1/r

Point B: radius at minimum, pressure at maximum Surfactant High radius, high surface tension

Low radius, low surface tension

Surface Tension and Elasticity Combined

Critical Pressures (we don’t reach critical closing pressure until after our very first breath)

Absorption Atelectasis Respiratory Quotient ° Assume respiratory exchange rate for O2 and CO2 equals the respiratory quotient: RQ = CO2 produced/O2 consumed (in humans: normally around 0.8) Dissociation Curves