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Abstract
The pulmonary circulation carries deoxygenated blood from the systemic veins through the pulmonary arteries to be oxygenated in the capillaries that line the walls of the pulmonary alveoli. The pulmonary circulation carries the cardiac output with a relatively low driving pressure, and so differs considerably in structure and function from the systemic circulation to maintain a low-resistance vascular system. The pulmonary circulation is often considered to be a quasi-static system in both experimental and computational studies of pulmonary perfusion and its matching to ventilation (air flow) for exchange. However, the system is highly dynamic, with cardiac output and regional perfusion changing with posture, exercise, and over time. Here we review this dynamic system, with a focus on understanding the physiology of pulmonary vascular dynamics across spatial and temporal scales, and the changes to these dynamics that are reflective of disease. © 2019 American Physiological Society. Compr Physiol 9:1081-1100, 2019.
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Affiliation(s)
- Alys Clark
- Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand
| | - Merryn Tawhai
- Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand
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Wagner WW, Jaryszak EM, Peterson AJ, Doerschuk CM, Bohlen HG, King JAC, Tanner JA, Crockett ES, Glenny RW, Presson RG. A perpetual switching system in pulmonary capillaries. J Appl Physiol (1985) 2019; 126:494-501. [PMID: 30571293 PMCID: PMC6397411 DOI: 10.1152/japplphysiol.00507.2018] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2018] [Revised: 12/14/2018] [Accepted: 12/17/2018] [Indexed: 12/13/2022] Open
Abstract
Of the 300 billion capillaries in the human lung, a small fraction meet normal oxygen requirements at rest, with the remainder forming a large reserve. The maximum oxygen demands of the acute stress response require that the reserve capillaries are rapidly recruited. To remain primed for emergencies, the normal cardiac output must be parceled throughout the capillary bed to maintain low opening pressures. The flow-distributing system requires complex switching. Because the pulmonary microcirculation contains contractile machinery, one hypothesis posits an active switching system. The opposing hypothesis is based on passive switching that requires no regulation. Both hypotheses were tested ex vivo in canine lung lobes. The lobes were perfused first with autologous blood, and capillary switching patterns were recorded by videomicroscopy. Next, the vasculature of the lobes was saline flushed, fixed by glutaraldehyde perfusion, flushed again, and then reperfused with the original, unfixed blood. Flow patterns through the same capillaries were recorded again. The 16-min-long videos were divided into 4-s increments. Each capillary segment was recorded as being perfused if at least one red blood cell crossed the entire segment. Otherwise it was recorded as unperfused. These binary measurements were made manually for each segment during every 4 s throughout the 16-min recordings of the fresh and fixed capillaries (>60,000 measurements). Unexpectedly, the switching patterns did not change after fixation. We conclude that the pulmonary capillaries can remain primed for emergencies without requiring regulation: no detectors, no feedback loops, and no effectors-a rare system in biology. NEW & NOTEWORTHY The fluctuating flow patterns of red blood cells within the pulmonary capillary networks have been assumed to be actively controlled within the pulmonary microcirculation. Here we show that the capillary flow switching patterns in the same network are the same whether the lungs are fresh or fixed. This unexpected observation can be successfully explained by a new model of pulmonary capillary flow based on chaos theory and fractal mathematics.
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Affiliation(s)
- Wiltz W Wagner
- Department of Anesthesiology, Indiana University School of Medicine , Indianapolis, Indiana
- Department of Cellular and Integrative Physiology, Indiana University School of Medicine , Indianapolis, Indiana
- Department of Molecular and Cellular Pharmacology, Department of Physiology and Cell Biology, Center for Lung Biology, University of South Alabama , Mobile, Alabama
| | - Eric M Jaryszak
- Department of Cellular and Integrative Physiology, Indiana University School of Medicine , Indianapolis, Indiana
| | - Amanda J Peterson
- Department of Anesthesiology, Indiana University School of Medicine , Indianapolis, Indiana
| | - Claire M Doerschuk
- Center for Airways Disease, Department of Medicine, University of North Carolina , Chapel Hill, North Carolina
| | - H Glenn Bohlen
- Department of Cellular and Integrative Physiology, Indiana University School of Medicine , Indianapolis, Indiana
| | - Judy A C King
- Department of Molecular and Cellular Pharmacology, Department of Physiology and Cell Biology, Center for Lung Biology, University of South Alabama , Mobile, Alabama
| | - Judith A Tanner
- Department of Cellular and Integrative Physiology, Indiana University School of Medicine , Indianapolis, Indiana
| | - Edward S Crockett
- Department of Molecular and Cellular Pharmacology, Department of Physiology and Cell Biology, Center for Lung Biology, University of South Alabama , Mobile, Alabama
| | - Robb W Glenny
- Departments of Medicine and of Physiology and Biophysics, University of Washington , Seattle, Washington
| | - Robert G Presson
- Department of Anesthesiology, Indiana University School of Medicine , Indianapolis, Indiana
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3
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Abstract
Computational model analysis has been used widely to understand and interpret complexity of interactions in the pulmonary system. Pulmonary blood transport is a multi-scale phenomenon that involves scale-dependent structure and function, therefore requiring different model assumptions for the microcirculation and the arterial or venous flows. The blood transport systems interact with the surrounding lung tissue, and are dependent on hydrostatic pressure gradients, control of vasoconstriction, and the topology and material composition of the vascular trees. This review focuses on computational models that have been developed to study the different mechanisms contributing to regional perfusion of the lung. Different models for the microcirculation and the pulmonary arteries are considered, including fractal approaches and anatomically-based methods. The studies that are reviewed illustrate the different complementary approaches that can be used to address the same physiological question of flow heterogeneity.
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Affiliation(s)
- Merryn H Tawhai
- Auckland Bioengineering Institute, University of Auckland, Private Bag 92019, Auckland 1010, New Zealand.
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Won C, Chon D, Tajik J, Tran BQ, Robinswood GB, Beck KC, Hoffman EA. CT-based assessment of regional pulmonary microvascular blood flow parameters. J Appl Physiol (1985) 2003; 94:2483-93. [PMID: 12588787 DOI: 10.1152/japplphysiol.00688.2002] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
To determine regional pulmonary microvascular mean transit times (MTTs), we used electrocardiogram-gated X-ray computed tomographic imaging to follow bolus radiopaque contrast material through the lungs in anesthetized animals (7 dogs and 1 pig, prone and supine). By deconvolution/reconvolution of regional time-attenuation curves obtained from parenchyma and large lobar arteries, we estimated the microvascular residue function and reconstituted the regional microvascular time-attenuation curves and, thus, regional microvascular MTTs. The mean microvascular MTTs in the supine and prone postures were 3.94 +/- 1.0 and 3.40 +/- 0.84 (mean +/- SD), respectively. The dependent-nondependent vertical gradient of MTT was greater in the supine [slope = 0.25 +/- 0.10 (SD), P < 0.001 by t-test] than in the prone (-0.03 +/- 0.06 in 6 of 8 animals; 2 outliers had positive slopes) posture. In both postures, there was a trend toward faster transit times in the dorsal-basal lung region in six of the eight animals, suggesting gravity-independent higher vascular conductance dorsocaudally. We conclude that deconvolution methods, in association with electrocardiogram-gated high-speed X-ray computed tomography, can provide insights into regional heterogeneity of pulmonary microvascular MTT in vivo.
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Affiliation(s)
- Chulho Won
- Department of Radiology, University of Iowa, Iowa City, Iowa 52241, USA
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5
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Abstract
To estimate the functional diameter of alveolar microvessels, we perfused isolated rat lungs with fluorescent latex particles (1 diameter/lung) at inflation, pulmonary arterial, and left atrial pressures of 25, 30, and 0 cmH2O, respectively. We used confocal microscopy to count latex particles within septal microvessels and flow cytometry to count particle concentrations in venous outflow. We found 1-, 2-, and 4-micron-diameter particles within septal vessels of 45 +/- 12, 31 +/- 12, and 25 +/- 9%, respectively, of examined alveoli. Particles of 5-micron diameter were absent from septal vessels but were present within a small percentage of corner vessels. Particle concentrations in the venous outflow for 1-, 2-, 4-, and 5-micron-diameter particles were 54 +/- 28, 67 +/- 32, 2.2 +/- 0.3, and 0.4 +/- 0.3%, respectively, of the arterial inflow. Particles with diameters of 6 or 10 micron were absent from venous outflow. Our results suggest that, under these conditions, the functional diameter of the septal microvessels is approximately 4 micron and that the diameter of the adjacent corner vessels is slightly larger but <6 micron.
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Affiliation(s)
- R L Conhaim
- Department of Surgery, University of Wisconsin-Madison, Madison, Wisconsin 53705, USA.
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