Transport collapse in dynamically evolving networks.

Transport in complex networks can describe a variety of natural and human-engineered processes including biological, societal and technological ones. However, how the properties of the source and drain nodes can affect transport subject to random failures, attacks or maintenance optimization in the network remain unknown. In this article, the effects of both the distance between the source and drain nodes and the degree of the source node on the time of transport collapse are studied in scale-free and lattice-based transport networks. These effects are numerically evaluated for two strategies, which employ either transport-based or random link removal. Scale-free networks with small distances are found to result in larger times of collapse. In lattice-based networks, both the dimension and boundary conditions are shown to have a major effect on the time of collapse. We also show that adding a direct link between the source and the drain increases the robustness of scale-free networks when subject to random link removals. Interestingly, the distribution of the times of collapse is then similar to the one of lattice-based networks.

Modeling of Gas Exchange in the Lungs.

This overview presents the recent progress in our understanding of gas transfer by the lungs during the respiratory cycle and during breath holding. Different phenomena intervene in gas transfer, convection and diffusion in the gas, dissolution, diffusion across the alveolar-capillary membrane, diffusion across blood plasma, and finally diffusion and reaction with hemoglobin inside blood cells. The different gases, O2 , CO, and NO, have very different reaction times with hemoglobin ranging from a few microseconds to tens of milliseconds. This is leading to different outcomes. For O2 , the solutions to the coupled nonlinear gas and blood equations are obtained at the acinus level. They include the fact that the acinar internal ventilation is strongly heterogeneous due to the arborescent structure. Also, in the dynamic calculation, one takes care of the delay between the start of inhalation and arrival of fresh air in the acinus. This "dead" time is the dynamic equivalent of the dead space ventilation. The question of the dependence of Vo2 on ventilation and perfusion takes a different form. The results show that Vo2 is not only a function of the ventilation/perfusion ratio but also depends on the variables: acinar ventilation VEac and perfusion Qac . The ratio VEac /Qac roughly determines arterial O2 saturation and arterial and alveolar O2 partial pressure. The classic Roughton-Forster interpretation of DLCO (separation between independent membrane and blood resistance) was a mathematical conjecture. It was shown recently that this conjecture was violated. This article presents an alternative interpretation that uses time concepts instead of resistance. © 2021 American Physiological Society. Compr Physiol 11:1289-1314, 2021.

Pseudo-Darwinian evolution of physical flows in complex networks.

The evolution of complex transport networks is investigated under three strategies of link removal: random, intentional attack and "Pseudo-Darwinian" strategy. At each evolution step and regarding the selected strategy, one removes either a randomly chosen link, or the link carrying the strongest flux, or the link with the weakest flux, respectively. We study how the network structure and the total flux between randomly chosen source and drain nodes evolve. We discover a universal power-law decrease of the total flux, followed by an abrupt transport collapse. The time of collapse is shown to be determined by the average number of links per node in the initial network, highlighting the importance of this network property for ensuring safe and robust transport against random failures, intentional attacks and maintenance cost optimizations.

Morphological organization of point-to-point transport in complex networks.

We investigate the structural organization of the point-to-point electric, diffusive or hydraulic transport in complex scale-free networks. The random choice of two nodes, a source and a drain, to which a potential difference is applied, selects two tree-like structures, one emerging from the source and the other converging to the drain. These trees merge into a large cluster of the remaining nodes that is found to be quasi-equipotential and thus presents almost no resistance to transport. Such a global "tree-cluster-tree" structure is universal and leads to a power law decay of the currents distribution. Its exponent, -2, is determined by the multiplicative decrease of currents at successive branching points of a tree and is found to be independent of the network connectivity degree and resistance distribution.

An in silico analysis of oxygen uptake of a mild COPD patient during rest and exercise using a portable oxygen concentrator.

Oxygen treatment based on intermittent-flow devices with pulse delivery modes available from portable oxygen concentrators (POCs) depends on the characteristics of the delivered pulse such as volume, pulse width (the time of the pulse to be delivered), and pulse delay (the time for the pulse to be initiated from the start of inhalation) as well as a patient's breathing characteristics, disease state, and respiratory morphology. This article presents a physiological-based analysis of the performance, in terms of blood oxygenation, of a commercial POC at different settings using an in silico model of a COPD patient at rest and during exercise. The analysis encompasses experimental measurements of pulse volume, width, and time delay of the POC at three different settings and two breathing rates related to rest and exercise. These experimental data of device performance are inputs to a physiological-based model of oxygen uptake that takes into account the real dynamic nature of gas exchange to illustrate how device- and patient-specific factors can affect patient oxygenation. This type of physiological analysis that considers the true effectiveness of oxygen transfer to the blood, as opposed to delivery to the nose (or mouth), can be instructive in applying therapies and designing new devices.

Time-based understanding of DLCO and DLNO.

Capture of CO and NO by blood requires molecules to travel by diffusion from alveolar gas to haemoglobin molecules inside RBCs and then to react. One can attach to these processes two times, a time for diffusion and a time for reaction. This reaction time is known from chemical kinetics and, therefore, constitutes a unique physical clock. This paper presents a time-based bottom-up theory that yields a simple expression for DLCO and DLNO that produces quantitative predictions which compare successfully with experiments. Specifically, when this new approach is applied to DLCO experiments, it can be used to determine the value of the characteristic diffusion time, and the value of capillary volume (Vc). The new theory also provides a simple explanation for still unexplained correlations such as the observed proportionality between the so-called membrane conductance DM and Vc of Roughton and Forster's interpretation. This new theory indicates that DLCO should be proportional to the haematocrit as found in several experiments.

A new approach to the dynamics of oxygen capture by the human lung.

Oxygen capture in the lung results from the intimate dynamic interaction between the space- and time-dependent oxygen partial pressure that results from convection-diffusion and oxygen extraction from the alveolar gas and the space and time dependence of oxygen trapping by the red blood cells flowing in the capillaries. The complexity of the problem can, however, be reduced due to the fact that the systems obey different time scales: seconds for the gas phase transport and tenths of seconds for oxygen trapping by blood. This results first from a dynamical study of gas transport in a moving acinus and second from a new theory of dynamic oxygen trapping in the capillaries. The global solution can be found only through a self-consistent iterative approach linking the two systems. This has been accomplished and used to quantify oxygen capture in various situations: at rest, during exercise, ventilation-perfusion mismatching, high altitude and pulmonary edema.

Gas-phase removal of biofilms from various surfaces using carbon dioxide aerosols.

The present study evaluated the removal of Escherichia coli XL1-blue biofilms using periodic jets of carbon dioxide aerosols (a mixture of solid and gaseous CO(2)) with nitrogen gas. The aerosols were generated by the adiabatic expansion of high-pressure CO(2) gas through a nozzle and used to remove air-dried biofilms. The areas of the biofilms were measured from scanning electron micrographs before and after applying the aerosols. The removal efficiency of the aerosol treatment was measured with various air-drying times of the biofilms before the treatment, surface materials, and durations of CO(2) aerosols in each 8-s aerosol-nitrogen cleaning cycle. Nearly 100% of the fresh biofilms were removed from the various surfaces very reliably within 90 s. This technique can be useful for removing unsaturated biofilms on solid surfaces and has potential applications for cleaning bio-contaminated surfaces.

Effect of geometric variations on pressure loss for a model bifurcation of the human lung airway.

Characteristics of pressure loss (ΔP) in human lung airways were numerically investigated using a realistic model bifurcation. Flow equations were numerically solved for the steady inspiratory condition with the tube length, the branching angle and flow velocity being varied over a wide range. In general, the ΔP coefficient K showed a power-law dependence on Reynolds number (Re) and length-to-diameter ratio with a different exponent for Re≥100 than for Re<100. The effect of different branching angles on pressure loss was very weak in the smooth-branching airways.