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TECHNICAL PAPERS: Fluids/Heat/Transport

# Effect of Blood Flow on Gas Transport in a Pulmonary Capillary

[+] Author and Article Information
Ali A. Merrikh1

Laboratory for Porous Materials Applications, Mechanical Engineering Department,  Southern Methodist University, Dallas, TX 75275-0337

José L. Lage2

Laboratory for Porous Materials Applications, Mechanical Engineering Department,  Southern Methodist University, Dallas, TX 75275-0337

1

ASME Member, current address: Fellow, Internal Medicine, University of Texas-SW Medical Center, Dallas, TX 75390-9034.

2

ASME Fellow, corresponding author; e-mail: JLL@ENGR.SMU.EDU

J Biomech Eng 127(3), 432-439 (Oct 22, 2004) (8 pages) doi:10.1115/1.1894322 History: Received February 02, 2004; Revised October 22, 2004

## Abstract

The effects of blood velocity on gas transport within the alveolar region of lungs, and on the lung diffusing capacity $DL$ have for many years been regarded as negligible. The present work reports on a preliminary, two-dimensional investigation of CO convection-diffusion phenomenon within a pulmonary capillary. Numerical simulations were performed using realistic clinical and morphological parameter values, with discrete circular red blood cells (RBCs) moving with plasma in a single capillary. Steady-state simulations with stationary blood (RBCs and plasma) were performed to validate the model by comparison with published data. Results for RBCs moving at speeds varying from $1.0mm∕s$ to $10mm∕s$, and for capillary hematocrit (Ht) from 5% to 55%, revealed an increase of up to 60% in $DL,$ as compared to the stationary blood case. The increase in $DL$ is more pronounced at low Ht (less than 25%) and high RBC speed and it seems to be caused primarily by the presence of plasma. The results also indicate that capillary blood convection affects $DL$ not only by improving the plasma mixing in the capillary bed but also by replenishing the capillary with fresh (zero concentration) plasma, providing an additional reservoir for the consumption of CO. Our findings cast doubt on the current belief that an increase in the lung diffusing capacity of humans (for instance, during exercising), with fixed hematocrit, can only be accomplished by an increase in the lung volume effectively active in the respiration process.

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## Figures

Figure 1

Scanning electron micrograph of a dog lung helps identify (a) the complex internal overall structure of the alveolar region and (b) the constituents of the capillary bed, namely, gas region, membrane, and blood (plasma and red blood cell). (Courtesy of Professor Connie Hsia, UT Southwest Medical Center and Prof. Ewald R. Weibel, University of Bern, Switzerland.)

Figure 2

Two-dimensional schematic of blood flowing from right to left through a straight capillary as proposed by Aroesty and Gross (17). White arrows show direction and strength of gas transport from membrane to plasma to RBCs.

Figure 3

Schematic of the two-dimensional numerical domain: The plasma moves by the viscous interaction with the RBCs, which move as solid bodies with speed U from right to left with respect to the stationary solid membrane

Figure 4

Schematic showing the equivalent resistance method incorporating the resistance to CO uptake by the RBC as a result of the competition between O2 and CO

Figure 5

Pure diffusion (no blood flow, U=0) results: comparison with the results of Hsia (14). [θCO in μm3∕(sTorrRBC) and DL in μm3∕(sTorr)]

Figure 6

Percentage increase ε, as defined by Eq. 24, in the lung diffusing capacity as result of blood speed versus hematocrit Ht

Figure 7

Three sets of isolines for a single RBC flowing in the capillary with U=1mm∕s (a), U=5mm∕s (b), and U=10mm∕s (c). In each set, isobars are shown in the top, and the corresponding streamlines in the bottom.

Figure 8

Isolines for 11 RBCs flowing in the capillary with U=10mm∕s: (a) isobars, (b) streamlines, and (c) velocity vectors. [Note: (a) and (b) show only a section of the entire capillary, and (c) shows only the region between two consecutive RBCs.]

Figure 9

Variation of a and b coefficients with U for use in Eq. 25

## Errata

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