Distributed model for O2-CO2 transport and exchange between a three compartment lung and the pulmonary circulation, modeled as a blood tissue exchange (BTEX) unit.

Description

  Distributed model for O2-CO2 transport and exchange between a three 
  compartment lung and the pulmonary circulation, modeled as a blood
  tissue exchange (BTEX) unit.

  The lung model is composed of three serial compartments: 
    1) a low compliance compartment 
	  - the oral/nasal cavity and the upper (cartilaginous) airways (UA)
    2) a moderately compliant compartment
	  - the collapsible bronchial airways
    3) a high compliance compartment
	  - the alveolar space
  To ventilate the lung, positive pressure is applied at the airway opening 
  and is cycled on and off by a sequence of square waves.  This pressure in 
  conjunction with the sequence of square waves creates bi-directional  
  airflow and a corresponding inflation and deflation of the lung.  Either  
  respiratory rate or tidal volume can be controlled by altering the period  
  of the square wave or the pressure at the airway opening, respectively.
  Oxygen and carbon dioxide move between adjacent lung compartments via
  convective and diffusive mechanisms.  Each compartment is well-mixed.
  Respiratory gas exchange between the well-mixed alveolar space and the 
  plasma of the distributed BTEX unit occurs by diffusion.  Inhaled partial 
  pressure of O2 and CO2 are user specified.
  The pulmonary circulation model is composed of a blood-tissue exchange 
  (BTEX) unit.  The BTEX unit is composed of a core of red blood cells (RBC)
  surrounded by a layer of plasma.  Respiratory gas exchange initially
  occurs between the alveolar gas and the plasma which then exchanges 
  with the RBCs.  O2 and CO2 bind instantaneously with hemoglobin.  
  HbO2 saturation depends explicitly on pH and pCO2. Similarly, HbCO2 
  saturation depends explicitly on pH and pO2.  The user must specify 
  the incoming (pulmonary arterial) partial pressures of O2 and CO2.
  The subscripts, "pl" and "rbc", refer to "plasma" and "red blood cells"
  while the prefixes, "pa" and "pg", denote "pulmonary circulation" and
  "pulmonary gas".
  The lung model is stongly based on the three compartment model of gas 
  exchange developed by Jim Bassingthwaighte.  The pulmonary circulation
  model is solely based on the blood-tissue exchange (BTEX) model code
  developed by Ranjan Dash, Assistant Professor, Compuational Bioengineering 
  Group, Medical College of Wisconsin.  Modifications and unification of the
  two codes were made by Joseph C. Anderson, National Simulations Resource, 
  Department of Bioengineering, University of Washington.  

Figure: O2 and CO2 pressures at various points within the lung. P1: alveolus, P2: small bronchi, P3: oral cavity, pl: plasma (papO2pl is partial pressure of O2 in plasma), PBL: whole blood

Equations

The equations for this model may be viewed by running the JSim model applet and clicking on the Source tab at the bottom left of JSim's Run Time graphical user interface. The equations are written in JSim's Mathematical Modeling Language (MML). See the Introduction to MML and the MML Reference Manual. Additional documentation for MML can be found by using the search option at the Physiome home page.