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 1.0mms to 10mms, 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.

1.
Krogh
,
A.
, and
Krogh
,
M.
, 1909, “
Rate of Diffusion into Lungs of Men
,”
Skand. Arch. Physiol.
,
23
, pp.
236
247
.
2.
American Thoracic Society
, 1995, “
Single Breath Carbon Monoxide Diffusing Capacity (Transfer Factor): Recommendations for a Standard Technique
,
Am. J. Respir. Crit. Care Med.
1073-449X,
152
,
2185
2198
.
3.
Hughes
,
J. M. B.
, and
Bates
,
D. V.
, 2003,
Historical Review: The Carbon Monoxide Diffusing Capacity (DLCO) and its Membrane (DM) and Red Cell (θVC) Components
,
Resp. Phys. Neurobiology
138
,
155
-
142
.
4.
Krogh
,
M.
, 1915,
The diffusion of gases through the lungs of man
,
J. Physiol. (London)
0022-3751,
49
,
271
-
296
.
5.
Comroe
,
J. H.
, Jr.
,
Forster
,
R. E.
,
Dubois
,
A. B.
,
Briscoe
,
W. A.
, and
Carlsen
,
E.
, 1962,
The Lung: Clinical Physiology and Pulmonary Function Tests
,
2nd ed.
,
Year Book Medical Publishers
, Chicago, (reprint, 1974).
6.
Roughton
,
F. J. W.
, and
Forster
,
R. E.
, 1957,
Relative importance of diffusion and chemical reaction rates in determining the rate of exchange of gases in human lung, with special reference to true diffusion capacity of the pulmonary membrane and volume of blood in lung capillaries
,
J. Appl. Physiol.
0021-8987,
11
,
290
302
.
7.
Johnson
,
R. L.
, Jr.
,
Taylor
,
H. F.
, and
DeGraff
,
A. C.
, 1965,
Functional Significance of a Low Pulmonary Diffusing Capacity for Carbon Monoxide
,
J. Clin. Invest.
0021-9738,
44
,
789
-
800
.
8.
Weibel
,
E. R.
, 1970,
Morphometric estimation of pulmonary diffusion capacity, I. Model and method
,
Respir. Physiol.
0034-5687,
11
,
54
75
.
9.
Lage
,
J. L.
,
Merrikh
,
A. A.
, and
Kulish
,
V. V.
, 2004, “
A Porous Medium Model to Investigate the Red Cell Distribution Effect on Alveolar Respiration: Numerical Simulations to CO Diffusion in the Alveolar Region of the Lungs
,”
Emerging Technologies and Techniques in Porous Media
, vol.
1
, chap. 25, pp.
381
407
,
Kluwer Academic
, Dordrecht, The Netherlands.
10.
Kulish
,
V. V.
,
Lage
,
J. L.
,
Hsia
,
C. C. W.
, and
Johnson
,
R. L.
, Jr.
, 2002, “
Three-dimensional, unsteady simulation of alveolar respiration
,
ASME J. Biomech. Eng.
0148-0731,
124
,
609
616
.
11.
Hellumus
,
J. D.
, 1977
The Resistance to Oxygen Transport in the Capillaries Relative to That in the Surrounding Ttissue
.
Microvasc. Res.
0026-2862,
13
,
131
136
.
12.
Federspiel
,
W. J.
, and
Popel
,
A. S.
, 1986,
A Theoretical Analysis of the Effect of the Particulate Nature of Blood on Oxygen Release in Capillaries
,
Microvasc. Res.
0026-2862,
32
,
164
169
.
13.
Fedrespiel
,
W. J.
, 1989
Pulmonary diffusing capacity: implications of two-phase blood flow in capillaries
,
Respir. Physiol.
0034-5687,
77
,
119
134
.
14.
Hsia
,
C. C. W.
,
Chuong
,
C. J. C.
, and
Johnson
,
R. L.
, Jr.
, 1995,
Critique of conceptual basis of diffusion capacity estimates: A finite element analysis
,
J. Appl. Physiol.
8750-7587,
79
,
1039
1047
.
15.
Hsia
,
C. C. W.
,
Chuong
,
C. J. C.
, and
Johnson
, Jr.,
R. L.
, 1997
Red cell distortion and conceptual basis of diffusion capacity estimates: a finite element analysis
,
J. Appl. Physiol.
8750-7587,
83
,
1397
1404
.
16.
Frank
,
A. O.
,
Chuong
,
C. J. C.
, and
Johnson
, Jr.,
R. L.
, 1997
A finite element model of oxygen diffusion in the pulmonary capillaries
,
J. Appl. Physiol.
8750-7587,
82
,
2036
2044
.
17.
Aroesty
,
J.
, and
Gross
,
J. F.
, 1970,
Convection and diffusion in the microcirculation
,
Microvasc. Res.
0026-2862,
2
,
247
267
.
18.
Groebe
,
K.
, and
Thews
,
G.
, 1989
Effect of red cell spacing and red cell movement upon oxygen release under conditions of maximally working skeletal muscle
,
Adv. Exp. Med. Biol.
0065-2598,
248
,
175
185
.
19.
Patel
,
S.
, 2002
Evaluation of the resistance of membrane and erythrocytes to oxygen transport in pulmonary capillaries
,
Resp Physiol & Neorobio
,
130
,
181
187
.
20.
Honig
,
C. R.
, and
Odoroff
,
C. L.
, 1981,
Calculated dispersion of capillary transit times: Significance for oxygen exchange
,
Am. J. Physiol.
0002-9513,
240
,
199
208
, 1981.
21.
Popel
,
A.
, 1989
Theory of oxygen transport to tissue
,
Crit. Rev. Biomed. Eng.
0278-940X,
17–3
,
257
321
.
22.
Bloch
,
E. H.
, 1962
A quantitative study of the hemodynamics in the living microvascular system
,
Am. J. Anat.
0002-9106,
110
,
125
145
.
23.
Patankar
,
S.
, 1980,
Numerical Heat Transfer and Fluid Flow
,
Hemisphere
, Washington DC.
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