Capillary oxygen regulates demand-supply coupling by triggering connexin40-mediated conduction: Rethinking the metabolic hypothesis.

Coupling red blood cell (RBC) supply to O2 demand is an intricate process requiring O2 sensing, generation of a stimulus, and signal transduction that alters upstream arteriolar tone. Although actively debated, this process has been theorized to be induced by hypoxia and to involve activation of endothelial inwardly rectifying K+ channels (KIR) 2.1 by elevated extracellular K+ to trigger conducted hyperpolarization via connexin40 (Cx40) gap junctions to upstream resistors. This concept was tested in resting healthy skeletal muscle of Cx40-/- and endothelial KIR2.1-/- mice using state-of-the-art live animal imaging where the local tissue O2 environment was manipulated using a custom gas chamber. Second-by-second capillary RBC flow responses were recorded as O2 was altered. A stepwise drop in PO2 at the muscle surface increased RBC supply in capillaries of control animals while elevated O2 elicited the opposite response; capillaries were confirmed to express Cx40. The RBC flow responses were rapid and tightly coupled to O2; computer simulations did not support hypoxia as a driving factor. In contrast, RBC flow responses were significantly diminished in Cx40-/- mice. Endothelial KIR2.1-/- mice, on the other hand, reacted normally to O2 changes, even when the O2 challenge was targeted to a smaller area of tissue with fewer capillaries. Conclusively, microvascular O2 responses depend on coordinated electrical signaling via Cx40 gap junctions, and endothelial KIR2.1 channels do not initiate the event. These findings reconceptualize the paradigm of blood flow regulation in skeletal muscle and how O2 triggers this process in capillaries independent of extracellular K+.

Mechanical study of blood flow through a permeable capillary with slippery wall.

This research presents the mechanical behavior of blood flow through capillary having smooth inner surface. In this study modelling of blood flow via permeable and lubricated capillary caused by nutrients re-absorption has been done by the help of laws of momentum and mass. The nutrients re-absorption is assumed to be constant and inner walls of the capillary are smooth and slippery therefore slip condition on the velocity and constant rate in vertical direction at the wall has considered. The Kelvin Voigt model is employed to simulate blood flow via capillaries, and results for pressure, blood flow pattern, and shear force necessary for blood flow are discovered by recursive approach. Numerical results for nutrient re-absorption from the blood and impact of smooth and slippery surfaces on blood flow are shown through graphs. The novelty of the research invents that the smoothness and slickness of capillary wall is a crucial presumption to examine the blood as non-Newtonian fluid via capillary.

Self-assembled and perfusable microvasculature-on-chip for modeling leukocyte trafficking.

Leukocyte recruitment from blood to tissue is a process that occurs at the level of capillary vessels during both physiological and pathological conditions. This process is also relevant for evaluating novel adoptive cell therapies, in which the trafficking of therapeutic cells such as chimeric antigen receptor (CAR)-T cells throughout the capillaries of solid tumors is important. Local variations in blood flow, mural cell concentration, and tissue stiffness contribute to the regulation of capillary vascular permeability and leukocyte trafficking throughout the capillary microvasculature. We developed a platform to mimic a biologically functional human arteriole-venule microcirculation system consisting of pericytes (PCs) and arterial and venous primary endothelial cells (ECs) embedded within a hydrogel, which self-assembles into a perfusable, heterogeneous microvasculature. Our device shows a preferential association of PCs with arterial ECs that drives the flow-dependent formation of microvasculature networks. We show that PCs stimulate basement membrane matrix synthesis, which affects both vessel diameter and permeability in a manner correlating with the ratio of ECs to PCs. Moreover, we demonstrate that hydrogel concentration can affect capillary morphology but has no observed effect on vascular permeability. The biological function of our capillary network was demonstrated using an inflammation model, where significantly higher expression of cytokines, chemokines, and adhesion molecules was observed after tumor necrosis factor-alpha (TNF-α) treatment. Accordingly, T cell adherence and transendothelial migration were significantly increased in the immune-activated state. Taken together, our platform allows the generation of a perfusable microvasculature that recapitulates the structure and function of an in vivo capillary bed that can be used as a model for developing potential immunotherapies.

Estimation of shear stress heterogeneity along capillary segments in angiogenic rat mesenteric microvascular networks.

OBJECTIVE: Fluid shear stress is thought to be a regulator of endothelial cell behavior during angiogenesis. The link, however, requires an understanding of stress values at the capillary level in angiogenic microvascular networks. Critical questions remain. What are the stresses? Do capillaries experience similar stress magnitudes? Can variations explain vessel-specific behavior? The objective of this study was to estimate segment-specific shear stresses in angiogenic networks. METHODS: Images of angiogenic networks characterized by increased vascular density were obtained from rat mesenteric tissues stimulated by compound 48/80-induced mast cell degranulation. Vessels were identified by perfusion of a 40 kDa fixable dextran prior to harvesting and immunolabeling for PECAM. Using a network flow-based segment model with physiologically relevant parameters, stresses were computed per vessel for regions across multiple networks. RESULTS: Stresses ranged from 0.003 to 2328.1 dyne/cm2 and varied dramatically at the capillary level. For all regions, the maximum segmental shear stresses were for capillary segments. Stresses along proximal capillaries branching from arteriole inlets were increased compared to stresses along capillaries in more distal regions. CONCLUSIONS: The results highlight the variability of shear stresses along angiogenic capillaries and motivate new discussions on how endothelial cells may respond in vivo to segment-specific microenvironment during angiogenesis.

[Vascular biology and hemophilia].

By carrying a systemic circulation, hematopoietic and vascular systems coordinately govern the functional organ connections in the body. Blood vessels play an important role in the development, regeneration, and maintenance of organs by acting as conduits for environmental factors in the blood to tissues and secreting organ-specific cytokines as angiocrine signals. Recently, it has become clear that vascular endothelial cells, which are the main constituent cells of the blood vessels and play a role in homeostasis, are diverse. It has also been established that the cells of stem cell fraction exist in endothelial cells. The vascular endothelial cells in various organs are functionally different. For example, it has been discovered that sinusoidal blood vessels in the liver produce coagulation factor VIII as an organ-specific vascular function. Determining how such tissue-/organ-specific function of the endothelial cells is induced is a topic of interest in the vascular field of study.

Flow Heterogeneity and Factors Contributing to the Variability in Retinal Capillary Blood Flow.

Purpose: Capillary flow plays an important role in the nourishment and maintenance of healthy neural tissue and can be observed directly and non-invasively in the living human retina. Despite their importance, patterns of normal capillary flow are not well understood due to limitations in spatial and temporal resolution of imaging data. Methods: Capillary flow characteristics were studied in the retina of three healthy young individuals using a high-resolution adaptive optics ophthalmoscope. Imaging with frame rates of 200 to 300 frames per second was sufficient to capture details of the single-file flow of red blood cells in capillaries over the course of about 3 seconds. Results: Erythrocyte velocities were measured from 72 neighboring vessels of the parafoveal capillary network for each subject. We observed strong variability among vessels within a given subject, and even within a given imaged field, across a range of capillary flow parameters including maximum and minimum velocities, pulsatility, abruptness of the systolic peak, and phase of the cardiac cycle. The observed variability was not well explained by "local" factors such as the vessel diameter, tortuosity, length, linear cell density, or hematocrit of the vessel. Within a vessel, a moderate relation between the velocities and hematocrit was noted, suggesting a redistribution of plasma between cells with changes in flow. Conclusions: These observations advance our fundamental understanding of normal capillary physiology and raise questions regarding the potential role of network-level effects in explaining the observed flow heterogeneity.

Hematoloechus sp. attachment shifts endothelium in vivo from pro- to anti-inflammatory profile in Rana pipiens: evidence from systemic and capillary physiology.

This prospective, descriptive study focused on lung flukes (Hematoloechus sp., H) and their impact on systemic and individual capillary variables measured in pithed Rana pipiens, a long-standing model for studies of capillary physiology. Three groups were identified based on Hematoloechus attachment: no Hematoloechus (No H), Hematoloechus not attached (H Not Att), and Hematoloechus attached (H Att). Among 38 descriptive, cardiovascular, and immunological variables, 18 changed significantly with H. Symptoms of H included weight loss, elevated immune cells, heart rate variability, faster coagulation, lower hematocrit, and fluid accumulation. Important capillary function discoveries included median baselines for hydraulic conductivity (Lp) of 7.0 (No H), 12.4 (H Not Att), and 4.2 (H Att) × 10-7 cm·s-1·cmH2O-1 (P < 0.0001) plus seasonal adaptation of sigma delta pi [σ(πc-πi), P = 0.03]. Pro- and anti-inflammatory phases were revealed for Lp and plasma nitrite/nitrate concentration ([NOx]) in both H Not Att and H Att, whereas capillary wall tensile strength increased in the H Att. H attachment was advantageous for the host due to lower edema and for the parasite via a sustained food source illustrating an excellent example of natural symbiosis. However, H attachment also resulted in host weight loss: in time, a conundrum for the highly dependent parasite. The study increases overall knowledge of Rana pipiens by revealing intriguing effects of H and previously unknown, naturally occurring seasonal changes in many variables. The data improve Rana pipiens as a general scientific and capillary physiology model. Diseases of inflammation and stroke are among the clinical applications.

Blood flow characterization in nailfold capillary using optical flow-assisted two-stream network and spatial-temporal image.

The blood flow velocity in the nailfold capillary is an important indicator of the status of microcirculation. The conventional manual processing method is both laborious and prone to human artifacts. A feasible way to solve this problem is to use machine learning to assist in image processing and diagnosis. Inspired by the Two-Stream Convolutional Networks, this study proposes an optical flow-assisted two-stream network to segment nailfold blood vessels. Firstly, we use U-Net as the spatial flow network and the dense optical flow as the temporal stream. The results show that the optical flow information can effectively improve the integrity of the segmentation of blood vessels. The overall accuracy is 94.01 %, the Dice score is 0.8099, the IoU score is 0.6806, and the VOE score is 0.3194. Secondly, The flow velocity of the segmented blood vessel is determined by constructing the spatial-temporal (ST) image. The blood flow velocity evaluated is consistent with the typical blood flow speed reported. This study proposes a novel two-stream network for blood vessel segmentation of nailfold capillary images. Combined with ST image and line detection method, it provides an effective workflow for measuring the blood flow velocity of nailfold capillaries.

Tissue Oxygenation Around Capillaries: Effects of Hematocrit and Arteriole Oxygen Condition.

Oxygen transfer in the microvasculature is a complex phenomenon that involves multiple physical and chemical processes and multiple media. Hematocrit, the volume fraction of red blood cells (RBCs) in blood, has direct influences on the blood flow as well as the oxygen supply in the microcirculation. On the one hand, a higher hematocrit means that more RBCs present in capillaries, and thus, more oxygen is available at the source end. On the other hand, the flow resistance increases with hematocrit, and therefore, the RBC motion becomes slower, which in turn reduces the influx of oxygen-rich RBCs entering capillaries. Such double roles of hematocrit have not been investigated adequately. Moreover, the oxygen-hemoglobin dissociation rate depends on the oxygen tension and hemoglobin saturation of the cytoplasm inside RBCs, and the dissociation kinetics exhibits a nonlinear fashion at different oxygen tensions. To understand how these factors and mechanisms interplay in the oxygen transport process, computational modeling and simulations are favorite since we have a good control of the system parameters and also we can access to the detailed information during the transport process. In this study, we conduct numerical simulations for the blood flow and RBC deformation along a capillary and the oxygen transfer from RBCs to the surrounding tissue. Different values for the hematocrit, arteriole oxygen tension, tissue metabolism rate and hemoglobin concentration and affinity are considered, and the simulated spatial and temporal variations of oxygen concentration are analyzed in conjunction with the nonlinear oxygen-hemoglobin reaction kinetics. Our results show that there are two competing mechanisms for the tissue oxygenation response to a hematocrit increases: the favorite effect of the higher RBC density and the negative effect of the slower RBC motion. Moreover, in the low oxygen situations with RBC oxygen tension less than 50 mmHg at capillary inlet, the reduced RBC velocity effect dominates, resulting in a decrease in tissue oxygenation at higher hematocrit. On the opposite, for RBC oxygen tension higher than 50 mmHg when entering the capillary, a higher hematocrit is beneficial to the tissue oxygenation. More interestingly, the pivoting arteriole oxygen tension at which the two competing mechanisms switch dominance on tissue oxygenation becomes lower for higher oxygen-hemoglobin affinity and lower hemoglobin concentration. This observation has also been analyzed based on the oxygen supply from RBCs and the oxygen-hemoglobin reaction kinetics. The results and discussions presented in this article could be helpful for a better understanding of oxygen transport in microcirculation.

Capillary-Mitochondrial Oxygen Transport in Muscle: Paradigm Shifts.

When exercising humans increase their oxygen uptake (V̇O2) 20-fold above rest the numbers are staggering: Each minute the O2 transport system - lungs, cardiovascular, active muscles - transports and utilizes 161 sextillion (10 21) O2 molecules. Leg extension exercise increases the quadriceps muscles' blood flow 100-times; transporting 17 sextillion O2 molecules per kilogram per minute from microcirculation (capillaries) to mitochondria powering their cellular energetics. Within these muscles, the capillary network constitutes a prodigious blood-tissue interface essential to exchange O2 and carbon dioxide requisite for muscle function. In disease, microcirculatory dysfunction underlies the pathophysiology of heart failure, diabetes, hypertension, pulmonary disease, sepsis, stroke and senile dementia. Effective therapeutic countermeasure design demands knowledge of microvascular/capillary function in health to recognize and combat pathological dysfunction. Dated concepts of skeletal muscle capillary (from the Latin capillus meaning 'hair') function prevail despite rigorous data-supported contemporary models; hindering progress in the field for future and current students, researchers and clinicians. Following closely the 100th anniversary of August Krogh's 1920 Nobel Prize for capillary function this Evidence Review presents an anatomical and physiological development of this dynamic field: Constructing a scientifically defensible platform for our current understanding of microcirculatory physiological function in supporting blood-mitochondrial O2 transport. New developments include: 1. Putative roles of red blood cell aquaporin and rhesus channels in determining tissue O2 diffusion. 2. Recent discoveries regarding intramyocyte O2 transport. 3. Developing a comprehensive capillary functional model for muscle O2 delivery-to-V̇O2 matching. 4. Use of kinetics analysis to discriminate control mechanisms from collateral or pathological phenomena.

Manufacturing the multiscale vascular hierarchy: progress toward solving the grand challenge of tissue engineering.

In human vascular anatomy, blood flows from the heart to organs and tissues through a hierarchical vascular tree, comprising large arteries that branch into arterioles and further into capillaries, where gas and nutrient exchange occur. Engineering a complete, integrated vascular hierarchy with vessels large enough to suture, strong enough to withstand hemodynamic forces, and a branching structure to permit immediate perfusion of a fluidic circuit across scales would be transformative for regenerative medicine (RM), enabling the translation of engineered tissues of clinically relevant size, and perhaps whole organs. How close are we to solving this biological plumbing problem? In this review, we highlight advances in engineered vasculature at individual scales and focus on recent strategies to integrate across scales.

Skeletal muscle adaptation to indirect electrical stimulation: divergence between microvascular and metabolic adaptations.

NEW FINDINGS: What is the central question of this study? Can we manipulate muscle recruitment to differentially enhance skeletal muscle fatigue resistance? What is the main finding and its importance? Through manipulation of muscle activation patterns, it is possible to promote distinct microvascular growth. Enhancement of fatigue resistance is closely associated with the distribution of the capillaries within the muscle, not necessarily with quantity. Additionally, at the acute stages of remodelling in response to indirect electrical stimulation, the improvement in fatigue resistance appears to be primarily driven by vascular remodelling, with metabolic adaptation of secondary importance. ABSTRACT: Exercise involves a complex interaction of factors influencing muscle performance, where variations in recruitment pattern (e.g., endurance vs. resistance training) may differentially modulate the local tissue environment (i.e., oxygenation, blood flow, fuel utilization). These exercise stimuli are potent drivers of vascular and metabolic change. However, their relative contribution to adaptive remodelling of skeletal muscle and subsequent performance is unclear. Using implantable devices, indirect electrical stimulation (ES) of locomotor muscles of rat at different pacing frequencies (4, 10 and 40 Hz) was used to differentially recruit hindlimb blood flow and modulate fuel utilization. After 7 days, ES promoted significant remodelling of microvascular composition, increasing capillary density in the cortex of the tibialis anterior by 73%, 110% and 55% for the 4 Hz, 10 and 40 Hz groups, respectively. Additionally, there was remodelling of the whole muscle metabolome, including significantly elevated amino acid turnover, with muscle kynurenic acid levels doubled by pacing at 10 Hz (P < 0.05). Interestingly, the fatigue index of skeletal muscle was only significantly elevated in 10 Hz (58% increase) and 40 Hz (73% increase) ES groups, apparently linked to improved capillary distribution. These data demonstrate that manipulation of muscle recruitment pattern may be used to differentially expand the capillary network prior to altering the metabolome, emphasising the importance of local capillary supply in promoting exercise tolerance.

Impaired dynamics of precapillary sphincters and pericytes at first-order capillaries predict reduced neurovascular function in the aging mouse brain.

The microvascular inflow tract, comprising the penetrating arterioles, precapillary sphincters and first-order capillaries, is the bottleneck for brain blood flow and energy supply. Exactly how aging alters the structure and function of the microvascular inflow tract remains unclear. By in vivo four-dimensional two-photon imaging, we reveal an age-dependent decrease in vaso-responsivity accompanied by a decrease in vessel density close to the arterioles and loss of vascular mural cell processes, although the number of mural cell somas and their alpha smooth muscle actin density were preserved. The age-related reduction in vascular reactivity was mostly pronounced at precapillary sphincters, highlighting their crucial role in capillary blood flow regulation. Mathematical modeling revealed impaired pressure and flow control in aged mice during vasoconstriction. Interventions that preserve dynamics of cerebral blood vessels may ameliorate age-related decreases in blood flow and prevent brain frailty.

A tight squeeze: how do we make sense of small changes in microvascular diameter?

The brain is an energetically demanding tissue which, to function adequately, requires constant fine tuning of its supporting blood flow, and hence energy supply. Whilst blood flow was traditionally believed to be regulated only by vascular smooth muscle cells on arteries and arterioles supplying the brain, recent work has suggested a critical role for capillary pericytes, which are also contractile. This concept has evoked some controversy, especially over the relative contributions of arterioles and capillaries to the control of cerebral blood flow. Here we outline why pericytes are in a privileged position to control cerebral blood flow. First we discuss the evidence, and fundamental equations, which describe how the small starting diameter of capillaries, compared to upstream arterioles, confers a potentially greater control by capillary pericytes than by arterioles over total cerebral vascular resistance. Then we suggest that the faster time frame over which low branch order capillary pericytes dilate in response to local energy demands provides a niche role for pericytes to regulate blood flow compared to slower responding arterioles. Finally, we discuss the role of pericytes in capillary stalling, whereby pericyte contraction appears to facilitate a transient stall of circulating blood cells, exacerbating the effect of pericytes upon cerebral blood flow.

The post-arteriole transitional zone: a specialized capillary region that regulates blood flow within the CNS microvasculature.

The brain is an energy hog, consuming available energy supplies at a rate out of all proportion to its relatively small size. This outsized demand, largely reflecting the unique computational activity of the brain, is met by an ensemble of neurovascular coupling mechanisms that link neuronal activity with local increases in blood delivery. This just-in-time replenishment strategy, made necessary by the limited energy-storage capacity of neurons, complicates the nutrient-delivery task of the cerebral vasculature, layering on a temporo-spatial requirement that invites - and challenges - mechanistic interpretation. The centre of gravity of research efforts to disentangle these mechanisms has shifted from an initial emphasis on astrocyte-arteriole-level processes to mechanisms that operate on the capillary level, a shift that has brought into sharp focus questions regarding the fine control of blood distribution to active neurons. As these investigations have drilled down into finer reaches of the microvasculature, they have revealed an arteriole-proximate subregion of CNS capillary networks that serves a regulatory function in directing blood flow into and within downstream capillaries. They have also illuminated differences in researchers' perspectives on the vascular structures and identity of mural cells in this region that impart the vasomodulatory effects that control blood distribution. In this review, we highlight the regulatory role of a variably named region of the microvasculature, referred to here as the post-arteriole transition zone, in channeling blood flow within CNS capillary networks, and underscore the contribution of dynamically contractile perivascular mural cell - generally, but not universally, recognized as pericytes - to this function.

Capillary responses to functional and pathological activations rely on the capillary states at rest.

Brain capillaries play a crucial role in maintaining cellular viability and thus preventing neurodegeneration. The aim of this study was to characterize the brain capillary morphology at rest and during neural activation based on a big data analysis from three-dimensional microangiography. Neurovascular responses were measured using a genetic calcium sensor expressed in neurons and microangiography with two-photon microscopy, while neural acivity was modulated by stimulation of contralateral whiskers or by a seizure evoked by kainic acid. For whisker stimulation, 84% of the capillary sites showed no detectable diameter change. The remaining 10% and 6% were dilated and constricted, respectively. Significant differences were observed for capillaries in the diameter at rest between the locations of dilation and constriction. Even the seizures resulted in 44% of the capillaries having no detectable change in diameter, while 56% of the capillaries dilated. The extent of dilation was dependent on the diameter at rest. In conclusion, big data analysis on brain capillary morphology has identified at least two types of capillary states: capillaries with diameters that are relatively large at rest and stable over time regardless of neural activity and capillaries whose diameters are relatively small at rest and vary according to neural activity.

Noradrenaline released from locus coeruleus axons contracts cerebral capillary pericytes via α2 adrenergic receptors.

Noradrenaline (NA) release from locus coeruleus axons generates vascular contractile tone in arteriolar smooth muscle and contractile capillary pericytes. This tone allows neuronal activity to evoke vasodilation that increases local cerebral blood flow (CBF). Much of the vascular resistance within the brain is located in capillaries and locus coeruleus axons have NA release sites closer to pericytes than to arterioles. In acute brain slices, NA contracted pericytes but did not raise the pericyte cytoplasmic Ca2+ concentration, while the α1 agonist phenylephrine did not evoke contraction. Blocking α2 adrenergic receptors (α2Rs, which induce contraction by inhibiting cAMP production), greatly reduced the NA-evoked pericyte contraction, whereas stimulating α2Rs using xylazine (a sedative) or clonidine (an anti-hypertensive drug) evoked pericyte contraction. Noradrenaline-evoked pericyte contraction and capillary constriction are thus mediated via α2Rs. Consequently, α2Rs may not only modulate CBF in health and pathological conditions, but also contribute to CBF changes evoked by α2R ligands administered in research, veterinary and clinical settings.

Mathematical analysis of oxygen and carbon dioxide exchange in the human capillary and tissue system.

Human body has a great ability to maintain homeostasis and the respiratory system plays a pivotal role in physiological processes. In this paper, a mathematical model of oxygen and carbon dioxide transport in the human body through capillary and tissue system has been formulated. The model is given by four ordinary differential equations for the oxygen and carbon dioxide transport, two equations for the capillary and other two for the tissue. An analytic approach based on Taylor's series method has been presented in this paper to obtain a computable approximate solution of the differential equation to model the oxygen and carbon dioxide diffusion in a spherical tissue. The concentration profiles at the capillary and tissue regions has been estimated in relation with partial pressure as the main driving force. The results are in agreement with the literature data those arrived at by Whiteley et al. (2005). The results obtained may help bio-medical sciences to deal with hypoxia and other respiratory ailments faced by the people living at high altitudes. Moreover, facilitated diffusion due to haemoglobin has been presented.

Measuring capillary flow dynamics using interlaced two-photon volumetric scanning.

Two photon microscopy and optical coherence tomography (OCT) are two standard methods for measuring flow speeds of red blood cells in microvessels, particularly in animal models. However, traditional two photon microscopy lacks the depth of field to adequately capture the full volumetric complexity of the cerebral microvasculature and OCT lacks the specificity offered by fluorescent labeling. In addition, the traditional raster scanning technique utilized in both modalities requires a balance of image frame rate and field of view, which severely limits the study of RBC velocities in the microvascular network. Here, we overcome this by using a custom two photon system with an axicon based Bessel beam to obtain volumetric images of the microvascular network with fluorescent specificity. We combine this with a novel scan pattern that generates pairs of frames with short time delay sufficient for tracking red blood cell flow in capillaries. We track RBC flow speeds in 10 or more capillaries simultaneously at 1 Hz in a 237 µm × 237 µm × 120 µm volume and quantified both their spatial and temporal variability in speed. We also demonstrate the ability to track flow speed changes around stalls in capillary flow and measure to 300 µm in depth.