chapter  6
26 Pages

Lung Gas-Transfer Performance Analysis

WithDhanjoo N. Ghista, Kah Meng Loh

CONTENTS 6.1 Objectives ................................................................................................... 174 6.2 Respiratory System................................................................................... 174 6.3 Gas Transfer between Lung Alveoli and Pulmonary Capillaries .... 174 6.4 Lung Air Composition Analysis (O2 Consumption and

CO2 Production Rates) ............................................................................. 176 6.4.1 Calculation of O2 Consumption Rate and CO2

Production Rate............................................................................. 177 6.4.2 Dead Space Air Composition ..................................................... 179 6.4.3 Alveolar Air Composition and Partial Pressures .................... 180

6.5 Lung Gas-Exchange Model and Parametric Analysis ........................ 180 6.5.1 Expressions for (i) O2 and (ii) CO2

Diffusion Coefficients (DO2 and DCO2), Alveolar and Blood Partial Pressures ........................................................ 180

6.5.2 Quantifying Alveolar O2 and CO2 Partial Pressure Expressions .................................................................................... 186

6.5.3 Quantifying Arterial and Venous O2 and CO2 Partial Pressure Expressions ....................................................... 188

6.5.4 Determining DPO2av and DP CO2 av ..................................................... 190

6.6 Sequential Procedure to Compute DO2 and DCO2 ............................... 192 6.7 Case Studies............................................................................................... 194

6.7.1 Case Study 1 .................................................................................. 194 6.7.2 Case Study 2 .................................................................................. 195

6.8 Nondimensional Gas-Transfer Index (NDGTI) ................................... 197 References ........................................................................................................... 197

The primary function of the lung is to (1) oxygenate the blood and thereby provide oxygen to the cells for metabolization purposes, and (2) to remove the collected CO2 from the pulmonary blood. Herein, we will analyze the compositions of the inspired and expired air per breath, and from there compute the O2 consumption and CO2 production rates. Next, we derive expressions for diffusion coefficients DO2 and DCO2 in terms of the evaluated cardiacoutput, O2 and CO2 concentrations in arterial and venous blood, alveolar and blood O2 and CO2 partial-pressures. We then take up a typical case study, and demonstrate the computation of DO2 and DCO2 , to represent the lung performance capability to oxygenate the blood. This chapter (along with the figures) is based on our earlier Chapter 3 on

lung gas composition and transfer analysis in Human Respiration edited by V. Kulish and published by WIT Press [1].*

The respiratory system is the system of the body that deals with breathing. When we breathe, the body takes in the oxygen that it needs and removes the carbon dioxide that it does not need. First, the body breathes in the airwhich is sucked through the nose ormouth

and down through the trachea (windpipe). The trachea is a pipe shaped by rings of cartilage. It divides into two tubes called bronchi (Figure 6.1), which carry air into each lung. Inside the lung, the airway tubes divide into smaller and smaller

tubes called bronchioles. At the end of each of these tubes are small air sacs called alveoli (Figure 6.2). Capillaries are wrapped around these alveoli. The walls are so thin and close to each other that the air easily seeps through. In this way, oxygen diffuses into the bloodstream, and carbon dioxide (in the bloodstream) diffuses into the alveoli and is then removed from the body when we breathe out (Figure 6.3).