How countercurrent blood flow and uneven perfusion affect the motion of inert gas

1990 ◽  
Vol 69 (1) ◽  
pp. 162-170 ◽  
Author(s):  
L. D. Homer ◽  
P. K. Weathersby ◽  
S. Survanshi
Keyword(s):  
Mean Transit Time ◽  
High Flow ◽  
Inert Gas ◽  
Low Flow ◽  
Relative Dispersion ◽  
Heterogeneous Flow ◽  
Small Vessels ◽  
Transit Times ◽  
Flow Through ◽  
Washout Curves

Monte Carlo simulations of the passage of inert gas through muscle tissue reveal that countercurrent gas exchange is more important than heterogeneity of flow in determination of the shape of inert gas washout curves. Semilog plots of inert gas washout are usually curved rather than straight. Two explanations often offered are that countercurrent flow may distort the shape and that uneven perfusion of the tissue gives rise to nonuniform washout. The curvature of the semilog plot may be summarized by the relative dispersion (RD), which is the ratio of the standard deviation of transit times to the mean transit time. For straight semilog plots, RD is 1. Semilog plots of data showing xenon washout from dog tissues are curved and have and RD of approximately 2. We have simulated the transit of gas particles through a vascular bed composed of repeating units of 100 mg of tissue perfused by three small vessels 80 microns in diameter and several levels of branching that direct flow through 190,000 capillaries. Geometric distribution of flow is important. Similar degrees of flow heterogeneity affect the curvature of the washout curve more if regions of heterogeneous flow are widely spaced than if they are close together. Diffusion blunts the effects of heterogeneous flow by mixing particles in high-flow regions with particles in low-flow regions. Because of this mixing, alternating regions of high flow and low flow spaced at intervals of less than 0.5 cm are unlikely explanations for the curved semilog plots.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of lipid on inert gas kinetics

1994 ◽  
Vol 77 (1) ◽  
pp. 303-312 ◽  
Author(s):  
J. F. Himm ◽  
L. D. Homer ◽  
J. A. Novotny
Keyword(s):  
Mean Transit Time ◽  
Lipid Fraction ◽  
Compartment Model ◽  
Inert Gas ◽  
Relative Dispersion ◽  
Volume Percentage ◽  
Muscle Lipid ◽  
High Lipid ◽  
Transit Times ◽  
Two Compartment Model

A Monte Carlo simulation of inert gas transit through skeletal muscle has been extended to include regions of increased gas solubility to simulate regions of high lipid content. Position of the regions within the simulation module was varied, as was the muscle-lipid partition coefficient (lambda). The volume percentage of the lipid regions (alpha) was varied from 0 to 25% while lambda covered the range from 1 to 50. The effects of alpha and lambda on mean transit time and on relative dispersion (RD; ratio of SD to the mean) were examined for a single lipid volume and compared with expected values under the assumption that the tissue is composed of two well-stirred compartments. Mean transit times varied from approximately 0.80 to 1.20 times the values predicted by a simple parallel two-compartment model, whereas RD varied from 0.9 to 3.6. For fixed lambda, RD as a function of lipid fraction passes through a maximum that is shifted and was also smaller than expected from a simple two-compartment model. For fixed alpha, RD approaches an asymptotic value for large lambda, but the asymptote is smaller than that expected from the two-compartment model. When lipid is distributed in only two regions, RD decreases with increasing separation of the regions and with increasing surface area of the fat regions. A model of two well-stirred compartments that allows mixing between the compartments yields results similar to those from the simulation.

How well mixed is inert gas in tissues?

1986 ◽  
Vol 60 (6) ◽  
pp. 2079-2088 ◽  
Author(s):  
L. D. Homer ◽  
P. K. Weathersby
Keyword(s):  
Skeletal Muscle ◽  
Capillary Flow ◽  
Inert Gas ◽  
Relative Dispersion ◽  
Simple Diffusion ◽  
Transit Times ◽  
Countercurrent Exchange ◽  
Gas Molecules ◽  
Washout Curves ◽  
Central Circulation

The washout of inert gas from tissues typically follows multiexponential curves rather than monoexponential curves as would be expected from homogeneous, well-mixed compartment. This implies that the ratio for the square root of the variance of the distribution of transit times to the mean (relative dispersion) must be greater than 1. Among the possible explanations offered for multiexponential curves are heterogeneous capillary flow, uneven capillary spacing, and countercurrent exchange in small veins and arteries. By means of computer simulations of the random walk of gas molecules across capillary beds with parameters of skeletal muscle, we find that heterogeneity involving adjacent capillaries does not suffice to give a relative dispersion greater than one. Neither heterogeneous flow, nor variations in spacing, nor countercurrent exchange between capillaries can account for the multiexponential character of experimental tissue washout curves or the large relative dispersions that have been measured. Simple diffusion calculations are used to show that many gas molecules can wander up to several millimeters away from their entry point during an average transit through a tissue bed. Analytical calculations indicate that an inert gas molecule in an arterial vessel will usually make its first vascular exit from a vessel larger than 20 micron and will wander in and out of tissue and microvessels many times before finally returning to the central circulation. The final exit from tissue will nearly always be into a vessel larger than 20 micron. We propose the hypothesis that the multiexponential character of skeletal muscle tissue inert gas washout curves must be almost entirely due to heterogeneity between tissue regions separated by 3 mm or more, or to countercurrent exchanges in vessels larger than 20 micron diam.

scholarly journals Transit times from rainfall to baseflow in headwater catchments estimated using tritium: the Ovens River, Australia

2015 ◽  
Vol 19 (9) ◽  
pp. 3771-3785 ◽  
Author(s):  
I. Cartwright ◽  
U. Morgenstern
Keyword(s):  
Transit Time ◽  
Headwater Streams ◽  
Stream Water ◽  
Mean Transit Time ◽  
Significant Proportion ◽  
Low Flow ◽  
Flow Conditions ◽  
Headwater Catchments ◽  
Flow Models ◽  
Transit Times

Abstract. Headwater streams contribute a significant proportion of the total flow to many river systems, especially during summer low-flow periods. However, despite their importance, the time taken for water to travel through headwater catchments and into the streams (the transit time) is poorly understood. Here, 3H activities of stream water are used to define transit times of water contributing to streams from the upper reaches of the Ovens River in south-east Australia at varying flow conditions. 3H activities of the stream water varied from 1.63 to 2.45 TU, which are below the average 3H activity of modern local rainfall (2.85 to 2.99 TU). The highest 3H activities were recorded following higher winter flows and the lowest 3H activities were recorded at summer low-flow conditions. Variations of major ion concentrations and 3H activities with streamflow imply that different stores of water from within the catchment (e.g. from the soil or regolith) are mobilised during rainfall events rather than there being simple dilution of an older groundwater component by event water. Mean transit times calculated using an exponential-piston flow model range from 4 to 30 years and are higher at summer low-flow conditions. Mean transit times calculated using other flow models (e.g. exponential flow or dispersion) are similar. There are broad correlations between 3H activities and the percentage of rainfall exported from each catchment and between 3H activities and Na and Cl concentrations that allow first-order estimates of mean transit times in adjacent catchments or at different times in these catchments to be made. Water from the upper Ovens River has similar mean transit times to the headwater streams implying there is no significant input of old water from the alluvial gravels. The observation that the water contributing to the headwater streams in the Ovens catchment has a mean transit time of years to decades implies that these streams are buffered against rainfall variations on timescales of a few years. However, impacts of any changes to land use in these catchments may take years to decades to manifest themselves in changes to streamflow or water quality.

Nicotine increases microvascular blood flow and flow velocity in three groups of brain areas

1993 ◽  
Vol 265 (6) ◽  
pp. H2142-H2150
Author(s):  
F. J. Hans ◽  
L. Wei ◽  
D. Bereczki ◽  
V. Acuff ◽  
J. Demaro ◽  
...  
Keyword(s):  
Blood Flow ◽  
Mean Transit Time ◽  
Microvascular Blood Flow ◽  
Brain Areas ◽  
Nicotine Treatment ◽  
Nicotine Administration ◽  
Transit Times ◽  
Flow Through ◽  
Distribution Spaces ◽  
The Brain

To examine the mechanism of local cerebral blood flow (LCBF) elevation, nicotine (1.75 mg/kg sc) was administered to rats, and LCBF plus the distribution spaces of radiolabeled albumin (RISA) and red blood cells (RBC) in parenchymal microvessels were measured throughout the brain. Microvascular blood spaces and transit times were calculated from the data. From 1.5 to 3 min after nicotine administration, LCBF was raised by 40–150% in 16 of the brain areas and unaltered in the remaining 28 areas. The affected structures included parts of the visual-auditory, sensorimotor-cortical, and interpeduncular systems. RBC spaces were not changed by nicotine treatment. RISA and blood spaces were increased slightly but not significantly in some of the LCBF-affected areas but nowhere else. Nicotine seemingly elevates LCBF in the affected areas mainly by increasing linear velocity of flow through the microvascular beds. In agreement with this, mean transit time, which is inversely related to velocity, was decreased from 0.3-0.5 to approximately 0.2 s in the microvascular systems of the nicotine-affected areas.

scholarly journals Transit times from rainfall to baseflow in headwater catchments estimated using tritium: the Ovens River, Australia

2015 ◽  
Vol 12 (6) ◽  
pp. 5427-5463 ◽  
Author(s):  
I. Cartwright ◽  
U. Morgenstern
Keyword(s):  
Transit Time ◽  
Headwater Streams ◽  
Stream Water ◽  
Mean Transit Time ◽  
Significant Proportion ◽  
Low Flow ◽  
Flow Conditions ◽  
Headwater Catchments ◽  
Flow Models ◽  
Transit Times

Abstract. Headwater streams contribute a significant proportion of the total flow to many river systems, especially during summer low-flow periods. However, despite their importance, the time taken for water to travel through headwater catchments and into the streams (the transit time) is poorly constrained. Here, 3H activities of stream water are used to define transit times of water contributing to streams from the upper reaches of the Ovens River in southeast Australia at varying flow conditions. 3H activities of the stream water varied from 1.63 to 2.45 TU, which are below the average 3H activity of modern local rainfall (~3 TU). The highest 3H activities were recorded following higher winter flows and the lowest 3H activities were recorded at summer low-flow conditions. Variations of major ion concentrations and 3H activities with streamflow imply that different stores of water from within the catchment (e.g. from the soil or regolith) are mobilised during rainfall events rather than there being simple dilution of an older groundwater component by event water. Mean transit times calculated using an exponential-piston flow model range between 5 and 31 years and are higher at summer low-flow conditions. Mean transit times calculated using other flow models (e.g. exponential flow or dispersion) are similar. There are broad correlations between 3H activities and the percentage of rainfall exported from each catchment and between 3H activities and Na and Cl concentrations that allow first-order estimates of mean transit times in adjacent catchments or at different times in these catchments to be made. Water from the upper Ovens River has similar mean transit times to the headwater streams implying there is no significant input of old water from the alluvial gravels. The observation that the water contributing to the headwater streams in the Ovens catchment has a mean transit time of years to decades implies that these streams are buffered against rainfall variations on timescales of a few years. However, impacts of any changes to landuse in these catchments may take years to decades to manifest itself in changes to streamflow or water quality.

scholarly journals Dating of streamwater using tritium in a post-bomb world: continuous variation of mean transit time with streamflow

2010 ◽  
Vol 7 (4) ◽  
pp. 4731-4760 ◽  
Author(s):  
U. Morgenstern ◽  
M. K. Stewart ◽  
R. Stenger
Keyword(s):  
New Zealand ◽  
Transit Time ◽  
Time Series Data ◽  
Stream Water ◽  
Mean Transit Time ◽  
Low Flow ◽  
Series Data ◽  
Transit Times ◽  
Bomb Test ◽  
The Mean

Abstract. Tritium measurements of streamwater draining the Toenepi catchment, a small dairy farming area in Waikato, New Zealand, have shown that the mean transit time of the water varies with the flow of the stream. Mean transit times through the catchment are 2–5 years during high baseflow conditions (in winter), becoming older as streamflow decreases (in summer), and then quite dramatically older during drought conditions, with ages of more than 100 years. Older water seems to be gained in the lower reaches of the stream, compared to younger water in the headwater catchment. The groundwater store supplying baseflow was estimated from the mean transit time and average baseflow to be 15.4×106 m3 of water, about 1 m water equivalent over the catchment and 2.3 times total annual streamflow. Nitrate from recent intensified land use is relatively high at normal streamflow, but is low at times of low flow with old water. This reflects both lower nitrate loading in the catchment several decades ago, and active denitrification processes in older groundwater. Silica, leached from the aquifer material and accumulating in the water in proportion to contact time, is high at times of low streamflow. There was a good correlation between silica and streamwater age, which potentially allows silica concentrations to be used as a proxy for age when calibrated by tritium measurements. This study shows that tritium dating of stream water is possible with single tritium measurements now that bomb-test tritium has effectively disappeared from hydrological systems in New Zealand, without the need for time-series data.

scholarly journals Dating of streamwater using tritium in a post nuclear bomb pulse world: continuous variation of mean transit time with streamflow

2010 ◽  
Vol 14 (11) ◽  
pp. 2289-2301 ◽  
Author(s):  
U. Morgenstern ◽  
M. K. Stewart ◽  
R. Stenger
Keyword(s):  
New Zealand ◽  
Transit Time ◽  
Time Series Data ◽  
Stream Water ◽  
Mean Transit Time ◽  
Dairy Farming ◽  
Low Flow ◽  
Series Data ◽  
Transit Times ◽  
The Mean

Abstract. Tritium measurements of streamwater draining the Toenepi catchment, a small dairy farming area in Waikato, New Zealand, have shown that the mean transit time of the water varies with the flow rate of the stream. Mean transit times through the catchment are 2–5 years during high baseflow conditions in winter, increasing to 30–40 years as baseflow decreases in summer, and then dramatically older water during drought conditions with mean transit time of more than 100 years. Older water is gained in the lower reaches of the stream, compared to younger water in the headwater catchment. The groundwater store supplying baseflow was estimated from the mean transit time and average baseflow to be 15.4 × 106 m3 of water, about 1 m water equivalent over the catchment and 2.3 times total annual streamflow. Nitrate is relatively high at higher flow rates in winter, but is low at times of low flow with old water. This reflects both lower nitrate loading in the catchment several decades ago as compared to current intensive dairy farming, and denitrification processes occurring in the older groundwater. Silica, leached from the aquifer material and accumulating in the water in proportion to contact time, is high at times of low streamflow with old water. There was a good correlation between silica concentration and streamwater age, which potentially allows silica concentrations to be used as a proxy for age when calibrated by tritium measurements. This study shows that tritium dating of stream water is possible with single tritium measurements now that bomb-test tritium has effectively disappeared from hydrological systems in New Zealand, without the need for time-series data.

Pulmonary arterial transit times

1987 ◽  
Vol 63 (2) ◽  
pp. 770-777 ◽  
Author(s):  
C. A. Dawson ◽  
R. L. Capen ◽  
L. P. Latham ◽  
W. L. Hanson ◽  
S. E. Hofmeister ◽  
...  
Keyword(s):  
Standard Deviation ◽  
Pulmonary Artery ◽  
Transit Time ◽  
Mean Transit Time ◽  
Main Pulmonary Artery ◽  
Relative Dispersion ◽  
Transport Function ◽  
Arterial Tree ◽  
Transit Times ◽  
Pulmonary Arterial

To begin to characterize the pulmonary arterial transport function we rapidly injected a bolus containing a radiopaque dye and a fluorescence dye into the right atrium of anesthetized dogs. The concentrations of the dye indicators were measured in the main pulmonary artery (fluoroscopically) and in a subpleural pulmonary arteriole (by fluorescence microscopy). The resulting concentration vs. time curves were subjected to numerical deconvolution and moment analysis to determine how the bolus was dispersed as it traveled through the arteriole stream tube from the main pulmonary artery to the arteriole. The mean transit time and standard deviation of the transport function from the main pulmonary artery to the arterioles studied averaged 1.94 and 1.23 s, respectively, and the relative dispersion (ratio of standard deviation to mean transit time) was approximately 64%. This relative dispersion is at least as large as those reported for the whole dog lung, indicating that relative to their respective mean transit times the dispersion upstream from the arterioles is comparable to that taking place in capillaries and/or veins. The standard deviations of the transport functions were proportional to their mean transit times. Thus the relative dispersion from the main pulmonary artery to the various arterioles studied was fairly consistent. However, there were variations in mean transit time even between closely adjacent arterioles, suggesting that variations in mean transit times between arteriole stream tubes also contribute to the dispersion in the pulmonary arterial tree.

Flow regulation of ecNOS and Cu/Zn SOD mRNA expression in porcine coronary arterioles

1999 ◽  
Vol 276 (3) ◽  
pp. H1058-H1063 ◽  
Author(s):  
Christopher R. Woodman ◽  
Judy M. Muller ◽  
James W. E. Rush ◽  
M. Harold Laughlin ◽  
Elmer M. Price
Keyword(s):  
Shear Stress ◽  
Mrna Expression ◽  
Threshold Level ◽  
Flow Regulation ◽  
High Flow ◽  
Low Flow ◽  
Rt Pcr ◽  
Flow Through ◽  
Oxide Synthase ◽  
Coronary Arterioles

The purpose of this study was to test the hypothesis that increased flow through coronary arterioles increases endothelial cell nitric oxide synthase (ecNOS) and Cu/Zn superoxide dismutase (SOD) mRNA expression. Single porcine coronary arterioles (ID 100–160 μm; pressurized) were cannulated, perfused, and exposed to intraluminal flow sufficient to produce maximal flow-induced dilation of coronary arterioles (high flow; 7.52 ± 0.22 μl/min), low flow (0.84 ± 0.05 μl/min), or no flow for 2 or 4 h. Mean shear stress was calculated to be 5.7 ± 1.0 dyn/cm2 for high-flow arterioles and 1.6 ± 1.0 dyn/cm2 for low-flow arterioles. At the end of the treatment period, mRNA was isolated from each vessel, and ecNOS and SOD mRNA expression was assessed using a semiquantitative RT-PCR. All data were standardized by coamplifying ecNOS or SOD with glyceraldehyde-3-phosphate dehydrogenase. The results indicate that ecNOS mRNA expression is increased in arterioles exposed to 2 or 4 h of high flow. In contrast, SOD mRNA expression was increased only after 4 h of high flow. Neither gene is induced by exposure to low flow. On the basis of these data, we concluded that ecNOS and SOD mRNA expression is regulated by flow in porcine coronary arterioles. In addition, we concluded that a threshold level of flow and shear stress must be sustained to elicit the upregulation of ecNOS and SOD mRNA expression.

Distribution function of transit times in the human pulmonary circulation

1994 ◽  
Vol 76 (3) ◽  
pp. 1363-1371 ◽  
Author(s):  
M. L. Lewis ◽  
R. De Caterina ◽  
C. Giuntini
Keyword(s):  
Distribution Function ◽  
Mean Transit Time ◽  
Disease Process ◽  
Left Ventricular ◽  
Relative Dispersion ◽  
Cube Root ◽  
Metabolic Functions ◽  
Transit Times ◽  
Volume Relation ◽  
Linear Regressions

The distribution function of pulmonary transit times (fPTT) defines contact time between blood and vascular bed, which affects gas exchange and endothelial metabolic functions. This study was undertaken to assess effects of abnormal pulmonary inflow (PPA) and outflow pressures (PLA) on fPTT. Three groups were studied: five patients with elevated PLA and passive pulmonary hypertension (LVD-Ab), eight with normal PLA (LVD-Nl), and six with pulmonary disease and various levels of PPA (PD). Empirical complex exponential functions were convoluted on right and left ventricular indicator-dilution curves to derive fPTT; mean transit time (Mo1), standard deviation (Sm2), and cube root of the third moment about Mo1 (Sm3) were calculated by standard equations. A single linear regression of Sm3 and Sm2 on Mo1 was observed for all patients, regardless of disease process. Inverse relations between Mo1, Sm2, and Sm3 and blood flow were highly significant, but dispersion volumes (DV = Mo1 x flow) were higher in patients with elevated PPA. Significant linear regressions of fPTT parameters on PPA, derived in LVD-Nl and LVD-Ab patients, failed to predict Mo1, Sm2, and Sm3 for the PD group, whereas linear regressions on PLA accurately predicted Mo1, Sm2, and Sm3 in the PD group. Relations between fPTT parameters and PLA were equally well fit by exponential equations in all 19 patients, consistent with an asymptotic pressure-volume relation of distensible vessels. Microvascular pressure (PMV), combining PPA and PLA, was not a better predictor of fPTT parameters in LVD-NL and LVD-Ab patients but provided a slightly closer estimate of relative dispersion and skewness in PD patients.(ABSTRACT TRUNCATED AT 250 WORDS)

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