Computational modeling of RBC and neutrophil transit through the pulmonary capillaries.

A computational model of the pulmonary microcirculation is developed and used to examine blood flow from arteriole to venule through a realistically complex alveolar capillary bed. Distributions of flow, hematocrit, and pressure are presented, showing the existence of preferential pathways through the system and of large segment-to-segment differences in all parameters, confirming and extending previous work. Red blood cell (RBC) and neutrophil transit are also analyzed, the latter drawing from previous studies of leukocyte aspiration into micropipettes. Transit time distributions are in good agreement with in vivo experiments, in particular showing that neutrophils are dramatically slowed relative to the flow of RBCs because of the need to contract and elongate to fit through narrower capillaries. Predicted neutrophil transit times depend on how the effective capillary diameter is defined. Transient blockage by a neutrophil can increase the local pressure drop across a segment by 100--300%, leading to temporal variations in flow and pressure as seen by videomicroscopy. All of these effects are modulated by changes in transpulmonary pressure and arteriolar pressure, although RBCs, neutrophils, and rigid microspheres all behave differently.     

Morphometric study of the blood-carrying capillaries during the administration of a liquid into the salivary gland]

The physiological saline was injected into the excretory duct of the cat parotid gland under the pressure of 30, 70 and 120 cm H2O. It was found with the aid of transmission electronic microscopy, morphometry and statistical analysis that the liquid injection into the gland produces compression of the blood capillaries weaving the acinus. The compression of the capillary tubes is shown by a significant reduction in space given to the lumen of the capillaries and their endothelial layer. The compression of the capillary tubes is combined with a two-fold lessening suggest that the additional blood volume entering the gland in response to its perfusion by the liquid does not reach the blood capillaries and is thrown off into the vein vessels through the shunt communications and that regulation of the blood volume getting into the cat parotid gland capillaries is likely to depend on the hydraulic and osmotic state in the interstitial space of glandular lobes.     

Formation of the branching pattern of blood vessels in the wall of the avian yolk sac studied by a computer simulation

The present study was performed to provide data to support the notion previously believed but not proved experimentally or theoretically, that blood vessels are formed by the selection of capillaries in the network. In an attempt to understand the mechanism of formation of blood vessel branching structures, the transformation of a capillary network to a branching system in the wall of quail yolk sac was successively recorded by a series of photographs, and a computer simulation was carried out for the process of in vivo vascularization based on the photographs. The simulation demonstrated that a positive feedback system participated in the formation of a branching structure. That is, vessels which had been much used were enlarged, whereas less used vessels were reduced in their size and finally extinguished. The enlarged vessels became major components of the branching system. As the body of an embryo grew, it was observed that polygonal capillary networks enlarged, which led each polygon of the network to divide into a few finer polygons. Then, some of the capillary vessels were again selected and formed a branching system. This process repeated during the body growth, indicating that the vascular system developed adaptively to the body growth. A region where the growth was fast, received much blood flow and produced finer networks of capillaries. Thus, it was experimentally demonstrated for the first time that capillaries in the network are successively selected by a positive feedback mechanism and form blood vessels.     

Structural interrelations of the air-blood barrier and terminal sections of the lymphatic link in the pulmonary microcirculatory bed in chronic hypervolemia

The interalveolar septa of the human lungs are known to have no lymphatic capillaries. The topography of the pulmonary lymphatic system origin under conditions of chronic hypervolemia is still not investigated. Lungs of 24 corpses of persons, died from non-pulmonary pathology (control) and lungs of 34 corpses of persons, died from congenital and acquired heart disease accompanied with pre- and postcapillary forms of the pulmonary circulation hypertension, have been investigated. Decreased efficiency of the microcirculation, increased permeability of the blood capillary walls against the background of hypoxia result in an elevated production of lymph. Intensified collagen formation in the blood vessel walls and in the interalveolar septa is the prerequisite for reorganization of the pulmonary lymphatic bed. Lymphatic capillaries are found to grow into some sclerotic interalveolar septa and into deep structures of the blood capillary walls. This demonstrates a high plasticity of the lymphatic link terminal parts of the microcirculatory bed in pathologically changed lungs.     

Neutrophil Crawling in Capillaries; A Novel Immune Response to Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA), particularly the USA300 strain, is a highly virulent pathogen responsible for an increasing number of skin and soft tissue infections globally. Furthermore, MRSA-induced soft tissue infections can rapidly progress into life-threatening conditions, such as sepsis and necrotizing fasciitis. The importance of neutrophils in these devastating soft tissue infections remains ambiguous, partly because of our incomplete understanding of their behaviour. Spinning disk confocal microscopy was used to visualize the behaviour of GR1-labelled neutrophils in subcutaneous tissue in response to GFP-expressing MRSA attached to a foreign particle (agarose bead). We observed significant directional neutrophil recruitment towards the S. aureus agarose bead but not a control agarose bead. A significant increase in neutrophil crawling within the capillaries surrounding the infectious nidus was noted, with impaired capillary perfusion in these vessels and increased parenchymal cell death. No neutrophils were able to emigrate from capillaries. The crawling within these capillaries was mediated by the β₂ and α₄ integrins and blocking these integrins 2 hours post infection eliminated neutrophil crawling, improved capillary perfusion, reduced cell death and reduced lesion size. Blocking prior to infection increased pathology. Neutrophil crawling within capillaries during MRSA soft tissue infections, while potentially contributing to walling off or preventing early dissemination of the pathogen, resulted in impaired perfusion and increased tissue injury with time.     

Vascularization of the pituitary of the Australian lungfish and Neoceratodus forsteri

The vascularization of the brain and the pituitary region of the Australian lungfish, Neoceratodus forsteri is described from serial section reconstruction. The distal lobe has no direct arterial blood supply and receives blood solely from a pituitary portal system basically similar to that of other sarcopterygians. The primary capillary plexus of the median eminence receives its arterial blood from the infundibular arteries, which on their way distribute some small branches to the prechiasmatic region. The primary plexus also receives capillaries from the adjacent pial hypothalamic plexus. The primary capillary plexus of the median eminence comprises a rostral 'uncovered' and caudal 'covered' part which are not sharply delineated. Distinct portal vessels connect the 'uncovered' rostral part of the primary plexus with the secondary capillary plexus supplying the rostral subdivision of the pars distalis. The 'covered' caudal part of the primary plexus merges into the proximal subdivision of the pars distalis, apparently without formation of distinct portal vessels. The primary plexus has some connections with the plexus intermedius via a hypophysial stem capillary plexus. The plexus intermedius has a substantial arterial supply and gives off capillaries to the parenchyma of the pars intermedia. The adenohypophysis is drained into an unpaired hypophysial vein. The significance of the vascular pathways is discussed from comparative, functional, and evolutionary viewpoints.     

Red cell distribution and the recruitment of pulmonary diffusing capacity.

The distribution of red blood cells in alveolar capillaries is typically nonuniform, as shown by intravital microscopy and in alveolar tissue fixed in situ. To determine the effects of red cell distribution on pulmonary diffusive gas transport, we computed the uptake of CO across a two-dimensional geometric capillary model containing a variable number of red blood cells. Red blood cells are spaced uniformly, randomly, or clustered without overlap within the capillary. Total CO diffusing capacity (DLCO) and membrane diffusing capacity (DmCO) are calculated by a finite-element method. Results show that distribution of red blood cells at a fixed hematocrit greatly affects capillary CO uptake. At any given average capillary red cell density, the uniform distribution of red blood cells yields the highest DmCO and DLCO, whereas the clustered distribution yields the lowest values. Random nonuniform distribution of red blood cells within a single capillary segment reduces diffusive CO uptake by up to 30%. Nonuniform distribution of red blood cells among separate capillary segments can reduce diffusive CO uptake by >50%. This analysis demonstrates that pulmonary microvascular recruitment for gas exchange does not depend solely on the number of patent capillaries or the hematocrit; simple redistribution of red blood cells within capillaries can potentially account for 50% of the observed physiological recruitment of DLCO from rest to exercise.     

Physiological neutrophil sequestration in the lung: visual evidence for localization in capillaries

Although the lung is known to be a major site of neutrophil margination, the anatomic location of these sequestered cells within the lung is controversial. To determine the site of margination and the kinetics of neutrophil transit through the pulmonary microvasculature, we infused fluorescein isothiocyanate-labeled canine neutrophils into the pulmonary arteries of 10 anesthetized normal dogs and made fluorescence videomicroscopic observations of the subpleural pulmonary microcirculation through a window inserted into the chest wall. The site of fluorescent neutrophil sequestration was exclusively in the pulmonary capillaries with a total of 951 labeled cells impeded in the capillary bed for a minimum of 2 s. No cells were delayed in the arterioles or venules. Transit times of individual neutrophils varied over a wide range from less than 2 s to greater than 20 min with an exponential distribution skewed toward rapid transit times. These observations indicate that neutrophil margination occurs in the pulmonary capillaries with neutrophils impeded for variable periods of time on each pass through the lung. The resulting wide distribution of transit times may determine the dynamic equilibrium between circulating and marginated neutrophils.     

Some considerations on capillary adjustment to changing metabolism of tissue

Local regulation of blood flow as determined by capillary diameter and the number of open capillaries in a region is considered. The local changes in capillary diameter and in the number of open capillaries are assumed to be due to concentration changes of a diffusible, nonspecified metabolite. This metabolite is produced in the tissue and carried away by the blood stream. Using these assumptions and applying pertinent data on capillaries, deductions are made concerning:

1. (a)
   the law of blood flow as a function of temperature and capillary radius for the hyperemia of high temperature,     
   
   

Pancreatic microcirculation in the common tree shrew (Tupaia glis) as revealed by scanning electron microscopy of vascular corrosion casts.

Pancreatic vascular casts of the common tree shrew (Tupaia glis) were prepared by infusion of Batson's No. 17 plastic mixture into the blood vessels and examined by scanning electron microscopy (SEM). Routine histological study of the pancreas was also performed. It was found that the A and D cells appeared to occupy the core whereas the B cells were found at the periphery of the islets of Langerhans. With SEM, the insular arteriole, a branch of the interlobular artery, was shown to penetrate deeply into the core of the islets before branching off into the glomerular capillary network supplying the islets. These capillaries reunited at the periphery of the islets to become vasa efferentia and then gave off capillaries to anastomose with those in the exocrine part of the pancreas, the insuloacinar portal system. Such an insuloacinar portal system found in the pancreas of the tree shrew was similar to that found in the horse and monkey. However, there were some intralobular arterioles which did not end in the islets but directly branched into the interacinar capillary network and periductular plexus. The capillaries in the exocrine part not only gathered into intralobular venules which confluently formed the interlobular vein but also supplied the duct system. The periductular plexus also collected blood into the intralobular venule and interlobular vein, respectively.     

A probabilistic approach for evaluating a technique for an indirect monitoring of the capillary blood flow to the gastric parietal cells

The superficial capillary network of the gastric mucosa can be monitored for red blood cell velocity measurements by a microscopic technique. This network, however, reflects the blood flow in capillaries of more physiological interest, namely those passing by the acid-producing cells and emptying into the superficial network. It is, however, not possible to study these capillaries directly and therefore the problem is to determine in what way and to what degree blood flow measurements in the superficial network reflect the capillary flow of interest. A probabilistic approach where the movements of the red blood cells have been analysed, gives indications of determinable relations between observations on the superficial network flow and the flow passing the acid-producing cells.     

Analysis of the pathways of substance transport into the acinar cells]

In the exchange link of the microcirculation system of the exocrine part of the pancreas of Rana temporaria the substances moved from the blood capillary into the pericapillary space, then into the intercellular clefts and into the acinar cells by active transport. This is confirmed by the electron microscope studies of the ATP-ase activity localization in the exchange link: there are numerous lead phosphate granules in the endothelium of blood capillaries, on the fibrillae structures of the pericapillary space interstitium, on the lateral plasmic membrane of the exocrine pancreacytes, and on the cytoplasmic plates forming pinocytotic vacuoles.     

Numerical Modeling of Interstitial Fluid Flow Coupled with Blood Flow through a Remodeled Solid Tumor Microvascular Network

Modeling of interstitial fluid flow involves processes such as fluid diffusion, convective transport in extracellular matrix, and extravasation from blood vessels. To date, majority of microvascular flow modeling has been done at different levels and scales mostly on simple tumor shapes with their capillaries. However, with our proposed numerical model, more complex and realistic tumor shapes and capillary networks can be studied. Both blood flow through a capillary network, which is induced by a solid tumor, and fluid flow in tumor’s surrounding tissue are formulated. First, governing equations of angiogenesis are implemented to specify the different domains for the network and interstitium. Then, governing equations for flow modeling are introduced for different domains. The conservation laws for mass and momentum (including continuity equation, Darcy’s law for tissue, and simplified Navier–Stokes equation for blood flow through capillaries) are used for simulating interstitial and intravascular flows and Starling’s law is used for closing this system of equations and coupling the intravascular and extravascular flows. This is the first study of flow modeling in solid tumors to naturalistically couple intravascular and extravascular flow through a network. This network is generated by sprouting angiogenesis and consisting of one parent vessel connected to the network while taking into account the non-continuous behavior of blood, adaptability of capillary diameter to hemodynamics and metabolic stimuli, non-Newtonian blood flow, and phase separation of blood flow in capillary bifurcation. The incorporation of the outlined components beyond the previous models provides a more realistic prediction of interstitial fluid flow pattern in solid tumors and surrounding tissues. Results predict higher interstitial pressure, almost two times, for realistic model compared to the simplified model.     

The role of the blood-brain barrier in perfusion fixation of the brain for electron microscopy

Synopsis Regions of the brain vascularized by capillaries of the blood-brain barrier (BBB) type require a different fixative from regions which have capillaries of the endocrine type. Fixative with isotonic buffer gives excellent ultrastructural preservation in the BBB regions, but causes severe shrinkage of cells in the endocrine regions. This is evidently due to the difference in the permeability of the capillary walls to solutes in the fixative. In the BBB regions the less perimeable capillaries do not allow outflow of osmotically active particles to a harmful extent, whereas in the endocrine regions osmotic imbalances are created between the intra-and extracellular compartments.The diffusion rate of the fixative and the final volume of the fixed brain depend on the balance between the intravascular and interstitial hydrostatic and oncotic pressures across the capillary wall during the perfusion fixation, as those pressures regulate the amount of perfusate that will enter the parenchyma. Generally, as high a perfusion pressure as possible is recommended to obtain effective wash-out of blood and rapid diffusion of dixative into the tissue. Addition of macromolecules (2% PVP, mol. wt. 40000) into the fixative slightly improved the ultrastructural preservation in the BBB regions of the centrel nervous system.     

Der O2-Austausch zwischen Capillaren und Gewebe

Summary The distribution of oxygen concentration or oxygen partial pressure (=po₂) in models with different capillary arrangements is calculated. The capillaries form homogeneously perfused concurrent and countercurrent systems or inhomogeneously perfused capillary mesh systems with different velocities of blood flow on the inside of the capillaries. The question arises as to what extent the distribution of the po₂ in the tissue and the capillaries depends on the differences in streaming velocities and directions of the blood. The results show, that the greatest gradients in the po₂ distribution are to be found in the capillary mesh system. A typical example with a set of constants after Thews (1960) shows that the differences between the venous po₂ and the lowest po₂ in the tissue is 32 mm Hg in the case of the capillary network, 9 mm Hg in the case of the concurrent and 3 mm Hg in that of the countercurrent system.     

Total and shunting circulation in human central hemodynamics

The goal of the study was to develop a method for the separate measurement of capillary and metarteriolar circulation. Data on the cardiovascular system of 301 male patients (1–49 years of age) and 344 female patients (1–50 years of age) with a diagnosis of functional murmur were used. In the process of heart and major vessel diagnostic catheterization, the diagnosis of heart defect was excluded. The cardiac output (Q) was estimated. The calculated oxygen consumption was converted using the Hüfner’s coefficient to the equivalent quantity of hemoglobin (Hb) delivering this oxygen into exchange capillaries. The Hb content per milliliter of blood is known; therefore, by dividing the total quantity of Hb that passed through capillaries by its content per milliliter of blood, one can obtain the blood volume (in milliliters) that passed through the capillary bed (Q _cap). A shunt in the microvasculature was found as the difference between Q and Q _cap). Thus, there exist in the microvascular module two parallel bloodstreams: a slow one, which goes through true capillaries, where the exchange happens, and a fast shunting stream through metarterioles, direct channels, and arteriolevenous anastomoses. The latter not only takes part in the tissue thermal exchange, but are also channels that ensure the free transfer of white blood cells through the microcirculatory module, especially of those whose characteristic sizes exceed the diameter of the metabolic capillaries. The contribution of these two parallel streams in the microcirculatory module into Q is different. According to other results of this study, the slow capillary stream makes up approximately 20%, whereas the fast shunting bloodstream, 80% of Q.     

Non-invasive Assessment of Microvascular and Endothelial Function

The authors have utilized capillaroscopy and forearm blood flow techniques to investigate the role of microvascular dysfunction in pathogenesis of cardiovascular disease. Capillaroscopy is a non-invasive, relatively inexpensive methodology for directly visualizing the microcirculation. Percent capillary recruitment is assessed by dividing the increase in capillary density induced by postocclusive reactive hyperemia (postocclusive reactive hyperemia capillary density minus baseline capillary density), by the maximal capillary density (observed during passive venous occlusion). Percent perfused capillaries represents the proportion of all capillaries present that are perfused (functionally active), and is calculated by dividing postocclusive reactive hyperemia capillary density by the maximal capillary density. Both percent capillary recruitment and percent perfused capillaries reflect the number of functional capillaries. The forearm blood flow (FBF) technique provides accepted non-invasive measures of endothelial function: The ratio FBF_max/FBF_base is computed as an estimate of vasodilation, by dividing the mean of the four FBF_max values by the mean of the four FBF_base values. Forearm vascular resistance at maximal vasodilation (FVR_max) is calculated as the mean arterial pressure (MAP) divided by FBF_max. Both the capillaroscopy and forearm techniques are readily acceptable to patients and can be learned quickly.The microvascular and endothelial function measures obtained using the methodologies described in this paper may have future utility in clinical patient cardiovascular risk-reduction strategies. As we have published reports demonstrating that microvascular and endothelial dysfunction are found in initial stages of hypertension including prehypertension, microvascular and endothelial function measures may eventually aid in early identification, risk-stratification and prevention of end-stage vascular pathology, with its potentially fatal consequences.     

A Model for Fluid Drainage by the Lymphatic System

This study investigates the fluid flow through tissues where lymphatic drainage occurs. Lymphatic drainage requires the use of two valve systems, primary and secondary. Primary valves are located in the initial lymphatics. Overlapping endothelial cells around the circumferential lining of lymphatic capillaries are presumed to act as a unidirectional valve system. Secondary valves are located in the lumen of the collecting lymphatics and act as another unidirectional valve system; these are well studied in contrast to primary valves. We propose a model for the drainage of fluid by the lymphatic system that includes the primary valve system. The analysis in this work incorporates the mechanics of the primary lymphatic valves as well as the fluid flow through the interstitium and that through the walls of the blood capillaries. The model predicts a piecewise linear relation between the drainage flux and the pressure difference between the blood and lymphatic capillaries. The model describes a permeable membrane around a blood capillary, an elastic primary lymphatic valve and the interstitium lying between the two.     

ELECTRON MICROSCOPY OF THE AVIAN RENAL GLOMERULUS

Electron microscopy of sections of chicken glomeruli shows them to possess a large central cell mass, occupying the hilum and the centre of the glomerulus, and continuous with the adventitia of the afferent and efferent arterioles. The glomerular capillaries form a much simpler system than in mammals and are spread over the surface of the central cell mass. Between the capillaries the mass is limited externally by the major component of the glomerular capillary basement membrane, which continues over the surface of the mass from one capillary to the next. Projections of the central cell mass characteristically form the support for glomerular capillaries, and smaller knobs of the central mass may project actually into the lumen of the capillaries, but always carry a layer of endothelial cytoplasm before them. They are never in direct contact with blood. The basement membrane of the glomerular capillary loop has a central dense layer and two lateral less dense layers as in mammals. The central dense layer is continuous with similar appearing dense material in the intercellular spaces of the adventitiae of the arterioles, and also with that of the central cell mass. The two less dense layers can also be traced into direct continuity with the less dense regions of this intercellular substance. The endothelial cytoplasm is spread as a thin sheet over the inner surface of the capillary basement membrane, and shows scattered "pores" resembling those described in mammals. Epithelial cells with interlacing pedicels are at least as prominent as those in mammals. Bowman's capsular membrane also possesses three layers similar to but less wide than those of the capillary basement membrane, and all three layers can be traced into continuity with the dark and light regions of the intercellular material of the adventitial cells of the arterioles, and beyond them with that of the central cell mass. At the hilum Bowman's capsular membrane also fuses with the capillary basement membrane.     

Microangioarchitecture of the islets of Langerhans in the snakes,Naja naja,Vipera russelli,andEchis carinatus

Summary Due to a decided lack in the literature of reports on the microangioarchitecture of the pancreas of snakes, an analysis was performed in three different species of two different ophidian families with the use of cast preparations and complementary scanning electron microscopy. The vascular architecture reflects the lobular substructure of the pancreas; the organ is supplied by branches of the superior mesentric artery. Coiled lobular arteries and arterioles continue into a meshwork of capillaries. Dilated capillaries of the endocrine portion of the pancreas are an integral component of this fine capillary network. A few very small capillaries establish a connection between the endocrine and the exocrine pancreas. The other capillaries drain into venules, which ultimately join their respective veins. No interspecific differences were noted in the vascularization of the pancreas of the three ophidian species examined. The present results suggest the existence of arterio-venous anastomoses and speak against the presence of a portal system, but establish a feed-forward capillary connection from the endocrine to the exocrine component of the ophidian pancreas.