Capillary Diameter Research Articles

Post-occlusive reactive hyperemia and skeletal muscle capillary hemodynamics.

In female Sprague-Dawley rats (n=6), the spinotrapezius muscle was exteriorized for evaluation of capillary hemodynamics pre-occlusion, 5min of feed artery occlusion (Occ), and 5min of reperfusion (Post-Occ). There were no differences in mean arterial pressure (MAP) or capillary diameter (Dc) between pre-occlusion and post-occlusion (P>0.05). During 30s of PORH, capillary fRBC was increased (pre: 59±4 vs. 30s-post: 77±2 cells/s; P<0.05) and VRBC was not changed (pre: 300±24 vs. 30s post: 322±25μm/s; P>0.05). Capillary hematocrit (Hctcap) was unchanged across the pre- to post-occlusion conditions (P>0.05). Following occlusion, there was a 20-30% decrease in the number of capillaries supporting RBC flow at 30s and 300s-post occlusion (pre: 92±2%; 30s-post: 66±3%; 300s-post: 72±6%; both P<0.05). Short-term feed artery occlusion (i.e. 5min) resulted in a more heterogeneous capillary flow profile with the presence of capillary no-reflow, decreasing the percentage of capillaries supporting RBC flow. A complex interaction between myogenic and metabolic mechanisms at the arteriolar level may play a role in the capillary no-reflow with PORH. Measurements at the level of the conduit artery mask significant alterations in blood flow distribution in the microcirculation.

Capillary ultrastructure anatomy and physiology: What is known, what is unknown or missing, what is wrong and what is new?

This article summarizes what is already known about the capillary ultrastructure anatomy and physiology that includes one wrong physiological law of Starling and two misconceptions on capillaries cross section area and blood speed in its lumen. It also identifies missing data such as the number and diameters of capillaries branching from the terminal arteriole, and the speed of blood at the arterial and venous ends. It also summaries the evidence on new knowledge on capillary physiology namely the hydrodynamic of the porous orifice (G) tube built on a scale to capillary ultrastructure anatomy of pre-capillary sphincter and inter-cellular slits pores. It proves Starling’s law wrong and provide its correct replacement. The Tree Branching Law (TBL) corrects both misconceptions on capillaries cross section area being larger that the aorta and capillary blood speed is “very slow”. The TBL proves that the cross-section area of all capillaries is less than that of the aorta, and the blood speed in capillary is faster than generally received. The G tube’ magnetic field like hydrodynamics is the correct replacement for Starling’s law on the capillary-ISF transfer. This new capillary-ISF transfer allows for fast efficient function that meets cells and tissue demands at rest and during strenuous exercise.

Diagnostic value of nailfold capillaroscopy changes in Henoch–Schönlein purpura children with gastrointestinal or renal involvement

Aim of the work: Capillaroscopy is known as a non-invasive, safe, and valuable diagnostic tool to assess the small vessels of the microcirculation in the nailfold. This study aimed to determine the diagnostic value of capillaroscopy in children with Henoch-Schönlein purpura (HSP) and major organ (gastrointestinal or renal) involvement which could increase the risk of complications. Patients and method: This study was conducted on 32 HSP children referred to the rheumatology ward of Children's Medical Center in Tehran, Iran, from 2019 to 2020, as well as 30 healthy controls. The vascular changes of the nailfold were assessed through capillaroscopy. Results: The mean age of the patients was 6.4 ± 2.2 years (3–11 years), and the female/male ratio was 1:1.7 (12 females and 20 males). The main finding of nail-fold capillaroscopy was abnormal morphology in 56.2 % of patients (vs. 6.7% control). A significant difference was seen between patients and control in terms of capillary morphology and architecture (p < 0.05). Abnormal capillary density and diameter tended to be more common in patients than in control (p > 0.05). Seven patients had gastrointestinal or renal involvement (21.9%). A high negative predictive value and low positive predictive value of density, architecture, and diameter of capillaries, were noted between patients with and without gastrointestinal and renal involvement. Conclusion: The role of microvascular involvement in children with HSP is emphasized. Changes in these vessels could be an image of this small vessel vasculitis to reflect major organ invovlement and thus avoid future complications of the disease.

A Capillary Model for Predicting Saturated Hydraulic Conductivity of Ion-Adsorption Rare Earth Ore Based on Improved Kozeny–Carman Equation

During the in-situ leaching process of ion-adsorption rare earth ore, the seepage velocity of the leaching solution is one of the core problems in studying the leaching efficiency. The determination of the saturated hydraulic conductivity is of great significance to reveal the leaching process. The classical Kozeny–Carman (KC) equation is widely employed to predict the hydraulic conductivity of sandy soils. However, in the equation, the effect of tortuosity on the hydraulic conductivity is not considered, and the specific surface area is difficult to determine in practice. In this study, the capillary model for predicting the saturated hydraulic conductivity of ion-adsorption rare earth ore was established. First, we assumed that all the pores in the ore body are a series of parallel and tortuous capillaries with equal diameters. Based on the assumption and Hagen–Poiseuille’s law, the KC equation was improved by introducing the tortuosity. Second, the constant head permeability tests were carried out to derive the seepage velocity and hydraulic head loss under the steady seepage state. According to the experimental results, the diameter of the capillary was calculated with Darcy's formula. Then we obtained a linear-fit relationship between capillary diameter and porosity to express the specific surface area variation with porosity. Third, by validating with experimental data, when the pore shape coefficient is 0.4, the saturated hydraulic conductivity calculated by the capillary model is in good agreement with the tested value. The proposed model can be considered to have a satisfactory capability to predict the saturated hydraulic conductivity of ion-adsorption rare earth ore.

Sarcopenia in the elderly versus microcirculation, inflammation status, and oxidative stress: A cross-sectional study

Age-related mechanisms of sarcopenia associated with vascular function have been recently suggested. This study compared and tested associations between muscle mass and strength, microcirculation, inflammatory biomarkers, and oxidative stress in older adults classified as sarcopenic and non-sarcopenic. Thirty-three physically inactive individuals (72±7 yrs) were assigned to age-matched sarcopenic (SG) and non-sarcopenic (NSG) groups. Between-group comparisons were performed for appendicular skeletal mass (ASM), handgrip and isokinetic strength, microvascular function and morphology, C-reactive protein, insulin-like growth factor-1, tumor necrosis factor-alpha, interleukin-6 (IL-6), soluble vascular cell adhesion molecule-1, soluble intercellular adhesion molecule-1, endothelin-1, and oxidized low-density lipoprotein. ASM and knee isokinetic strength were lower in SG than NSG (P < 0.05). No difference between groups was found for outcomes of microvascular function and morphology, but log-transformed IL-6 concentration was twice greater in SG vs. NSG (P = 0.02). Correlations between ASM index, handgrip and knee isokinetic strength vs. markers of microcirculatory function, capillary diameters, vascular reactivity, and endothelial injury were found only in SG. Decreased ASM index and strength have been associated with microcirculatory profile, indicating that microcirculation impairment may be involved somehow in Sarcopenia development. The inflammation status, particularly elevated IL-6, seems to play an important role in this process.

PH regulates the lumen diameter of tissue-engineered capillaries.

Angiogenesis is vital in tissue engineering and the size of the capillary lumen diameter directly affects vascular function. Therefore, the involvement of the pH in the regulation of the capillary lumen diameter was investigated in the present study. The cytosolic pH of different pH medium groups was measured using flow cytometry. Bromodeoxyuridine staining and wound-healing assays were performed to detect cell proliferation and migration, respectively. The expression of angiogenesis-related genes was detected using reverse transcription-quantitative PCR. In addition, cell tube formation under different pH conditions was assessed using a tube formation assay and a 3D Matrigel® model. The results indicated that a change in the pH value of the culture medium affected the cytosolic pH of the endothelial cells, which then led to a change in vascular diameter. When the medium's pH ranged from 7.4 to 7.6, the diameter of the lumen formed in the Matrigel was suitable for capillary formation in tissue engineering. The present results revealed an important role for the pH in the process of capillary formation and provided insight for pH regulation during endothelial cell tube formation and angiogenesis in tissue engineering.

Meniscus Formation in a Vertical Capillary Tube.

In this theoretical work, the energies associated with the formation of a meniscus in a small diameter capillary tube are analyzed. A mechanism for meniscus creation and an associated energy balance are proposed. Equations for work of wetting, surface energy, gravitational energy, and dissipation are derived. The relative magnitude of these quantities is compared, first to each other and then to energies from capillary rise. In capillary rise, the energy released as work of wetting is evenly split between gravitational energy stored in the liquid column and heat dissipated there. The analysis performed here suggests that meniscus formation is energetically distinct and more complex than capillary rise. In meniscus formation, most of the energy released as work of wetting is stored in the stretched air-liquid interface or dissipated in the bulk liquid; their relative distribution depends on the properties of the liquid and the tube.

Pericyte dysfunction and loss of interpericyte tunneling nanotubes promote neurovascular deficits in glaucoma

Reduced blood flow and impaired neurovascular coupling are recognized features of glaucoma, the leading cause of irreversible blindness worldwide, but the mechanisms underlying these defects are unknown. Retinal pericytes regulate microcirculatory blood flow and coordinate neurovascular coupling through interpericyte tunneling nanotubes (IP-TNTs). Using two-photon microscope live imaging of the mouse retina, we found reduced capillary diameter and impaired blood flow at pericyte locations in eyes with high intraocular pressure, the most important risk factor to develop glaucoma. We show that IP-TNTs are structurally and functionally damaged by ocular hypertension, a response that disrupted light-evoked neurovascular coupling. Pericyte-specific inhibition of excessive Ca2+ influx rescued hemodynamic responses, protected IP-TNTs and neurovascular coupling, and enhanced retinal neuronal function as well as survival in glaucomatous retinas. Our study identifies pericytes and IP-TNTs as potential therapeutic targets to counter ocular pressure-related microvascular deficits, and provides preclinical proof of concept that strategies aimed to restore intrapericyte calcium homeostasis rescue autoregulatory blood flow and prevent neuronal dysfunction.

Prototype development of a new self-aspirating liquid-fueled microcombustor

ABSTRACT This paper reports the development of a novel self-aspirating liquid fuel-based microcombustor with an embedded passive fuel pumping mechanism for fuel delivery. Most of the microcombustion concepts reported in the literature require auxiliary electric power for the fuel and combustion air supply, thus reducing the overall system efficiency and portability. The present work reports the development of a new hydrostatic pressure-driven liquid fuel pumping mechanism to maintain a steady and continuous fuel supply for long durations. The supply of oxidant is ensured through the natural convection of air to support the long-duration operation of the microcombustor. This helped to eliminate the requirement of a power-driven pump. The present design allows a much higher combustion efficiency and a simpler micro-combustion system, beneficial for miniaturization. The experiments reported clean and efficient combustion with a 6–17 W thermal output and less than 500 ppm CO emissions. Remarkably, no fouling or NOx emissions were detected in the exhaust gas emissions. Experimental investigations were conducted to understand the influence of hydrostatic pressure head on the flame characteristics of ethanol micro-diffusion flame. The effect of capillary diameter on flame stability, size, shape, and temperature were investigated experimentally, and the flame stabilization mechanism is discussed in the present work.

TMEM16A channel is a key regulator of cerebral blood flow at the capillary level

Cerebral blood flow (CBF) is increased by neuronal activity. This ensures an adequate energy supply to reverse the ion movements generating synaptic and action potentials. CBF is partly controlled at the capillary level, by pericytes contracting or relaxing to alter capillary diameter. Pericytes are also important in pathology; during ischaemia, they contract and die in rigor, hampering restoration of blood flow. Thus, explaining pericyte contraction is key for understanding regulation of energy supply to the brain and its pathology, and for developing new therapies.

Enhanced mass transfer between matrix and filled fracture in dual-porosity media during spontaneous imbibition based on low-field nuclear magnetic resonance

Spontaneous imbibition (SI) of wetting fluids is an important process for many hydrological and geological applications. In this study, we investigated experimentally and theoretically the SI process in a dual-porosity medium consisting of the matrix and the filled fracture. Low-field nuclear magnetic resonance (LF NMR) technology was used to dynamically monitor the distribution of the imbibed water in the dual-porosity media during the SI experiments. Based on the LF NMR theory, a cut-off method was proposed to quantitatively distinguish the imbibed water from the matrix and the filled fracture. The results showed that the rate of matrix imbibition was greater in the dual-porosity media than in single-porosity media. This inconsistent rate of matrix imbibition between the single-porosity media and the dual-porosity media was caused by the enhanced mass transfer between the matrix and the filled fracture. An analytical model was then proposed to characterize this enhanced mass transfer between the matrix and the filled fracture. It was found that the rate of matrix imbibition was not only affected by the presence of the filled fractures but also depended on the size of the particles filled in the fracture. A clear non-monotonic relationship was found between the rate of matrix imbibition and the size of the particles filled in the fracture. The rate of matrix imbibition initially increased and then decreased as the size of the particles filled in the fracture increased. The proposed analytical model not only highlighted the mechanism of enhanced mass transfer between the matrix and the filled fracture, but was also able to determine the optimum equivalent capillary diameter of the filled particles in the fracture for enhanced rate of matrix imbibition in dual-porosity media.

Spontaneous Imbibition of Capillaries under the End Effect and Wetting Hysteresis.

The phenomenon of spontaneous imbibition is widely present in the development process of tight oil/gas reservoirs. To further explore the spontaneous imbibition behavior of capillary tubes to provide theoretical and methodological references for the study of microscopic porous media imbibition phenomena, the capillaries that can be observed with the naked eye on the order of 10–100 μm were selected as research objects. Based on the theory of interface chemistry, the capillary end effect, and wetting hysteresis, the influence of the additional pressures generated by the two-phase interface on the spontaneous absorption of the horizontal capillary was studied. Some of the capillaries were processed for wettability, and then the water wettability of different capillaries was measured by the introduced concept, which is the conversion height of the self-absorption phase in the capillary. The capillaries were horizontally placed in the liquid for a spontaneous imbibition experiment, and the air–liquid two-phase menisci behavior was observed at the same time, and then the influence of water wettability, surfactant, and capillary diameter on spontaneous imbibition was discussed. It was found that in the equal diameter capillaries, the spontaneous air–liquid imbibition behavior of capillary tubes with different water wetting properties is different in sensitivity to surfactants and tube diameters; when surfactants are used to improve capillary water wettability to increase spontaneous imbibition efficiency, the initial water wettability of the capillary and the comprehensive changes in the capillary pressure caused by interfacial tension should be considered.

Microfluidic elaboration of polymer microfibers from miscible phases: Effect of operating and material parameters on fiber diameter

Backgroundfiber diameter is one of the most important morphological parameters which drives the applications of microfibers. This creates a need for the development of processes capable of producing a large variety of microfibers with a given diameter. To this regards, microfluidic spinning has recently emerged as an outstanding and simple technique for the production of micro- and nanofibers with controllable size and morphology. Methodsherein, microfibers were produced from (macro)monomers or prepolymers (core phase) by in situ photoirradiation using a capillary-based microfluidic device and a miscible sheath phase of various viscosity. The effects of the flow rate of both phases as well as the viscosity of the sheath fluid, the capillary dimensions and the monomer volume fraction in the core phase were thoroughly studied. Significant findingsby calculating the capillary number ratio from the ratios of sheath to core flow rate and viscosity, an empirical relationship which perfectly predicts the microfiber diameter as a function of monomer volume fraction, the capillary number ratio and capillary inner diameter but independent of its outer diameter is extracted. This result paves the way to the continuous-flow production of microfibers with well-controlled morphological characteristics.

VEGF signalling causes stalls in brain capillaries and reduces cerebral blood flow in Alzheimer's mice.

Increased incidence of stalled capillary blood flow caused by adhesion of leucocytes to the brain microvascular endothelium leads to a 17% reduction of cerebral blood flow and exacerbates short-term memory loss in multiple mouse models of Alzheimer’s disease.Here, we report that vascular endothelial growth factor (VEGF) signalling at the luminal side of the brain microvasculature plays an integral role in the capillary stalling phenomenon of the APP/PS1 mouse model.Administration of the anti-mouse VEGF-A164 antibody, an isoform that inhibits blood–brain barrier hyperpermeability, reduced the number of stalled capillaries within an hour of injection, leading to an immediate increase in average capillary blood flow but not capillary diameter. VEGF-A inhibition also reduced the overall endothelial nitric oxide synthase protein concentrations, increased occludin levels and decreased the penetration of circulating Evans Blue dye across the blood–brain barrier into the brain parenchyma, suggesting increased blood–brain barrier integrity. Capillaries prone to neutrophil adhesion after anti-VEGF-A treatment also had lower occludin concentrations than flowing capillaries.Taken together, our findings demonstrate that VEGF-A signalling in APP/PS1 mice contributes to aberrant endothelial nitric oxide synthase /occludin-associated blood–brain barrier permeability, increases the incidence of capillary stalls, and leads to reductions in cerebral blood flow. Reducing leucocyte adhesion by inhibiting luminal VEGF signalling may provide a novel and well-tolerated strategy for improving brain microvascular blood flow in Alzheimer’s disease patients.

Experimental Investigation on the Effect of Pore Size on Spontaneous Imbibition Recovery in Oil-Wet Reservoirs

Spontaneous imbibition has been considered as a significant method to enhanced oil recovery (EOR) in unconventional reservoirs. The main impediment to comprehending the variation characteristics of spontaneous imbibition at different pore scales was the reservoir’s opacity. To this end, a series of spontaneous imbibition experiments of visualized oil-wet microtubes with six different diameters (10 μm~60 μm) were performed, as were the corresponding macroscopic core imbibition tests with six different permeabilities under the same conditions. The results showed that formation brine mostly advanced along the wall surface of a capillary during the imbibition process, and some were even isolated within a capillary. The imbibition recovery in the capillary with a diameter of 10 μm was the highest (27.81%), which was more than three times that of the capillary with a diameter of 60 μm (8.3%). There was a good power function decline relationship between capillary diameter and imbibition recovery, and 30 μm appeared to be a critical inflection point in both capillary tube and macroscopic core imbibition tests. In addition, the majority of the detained residual oil clusters not only cut off the continuity of formation brine but also increased the imbibition flow resistance, accelerating the imbibition to balance. This research provides a new perspective for comprehending the imbibition characteristics at different pore scales.

The influence of the optical properties on the determination of capillary diameters

Various clinically applicable scores and indices are available to help identify the state of a microcirculatory disorder in a patient. Several of these methods, however, leave room for interpretation and only provide clues for diagnosis. Thus, a measurement method that allows a reliable detection of impending or manifest circulatory malfunctions would be of great value. In this context, the optical and non-invasive method of shifted position-diffuse reflectance imaging (SP-DRI) was developed. It allows to determine the capillary diameter and thus to assess the state of the microcirculation. The aim of the present study is to investigate how the quantification of capillary diameters by SP-DRI behaves in different individuals, i.e. for a wide range of optical properties. For this, within Monte-Carlo simulations all optical properties (seven skin layers, hemoglobin) were randomly varied following a Gaussian distribution. An important finding from the present investigation is that SP-DRI works when the optical properties are chosen randomly. Furthermore, it is shown that appropriate data analysis allows calibration-free absolute quantification of the capillary diameter across individuals using SP-DRI. This underpins the potential of SP-DRI to serve as an early alert system for the onset of microcirculatory associated diseases.

A State-of-the-Art Review on Two-Phase Flow-Induced Noise in Expansion Devices

This paper presents a state-of-the-art review of the literature concerning flow-induced noise in expansion devices. The topics covered are theoretical natural frequency models of bubbles with spherical and cylindrical shapes and flow-induced noise in capillary tubes, such as orifice, thermal and electric expansion valve. Several parametric conditions affect the flow-induced noise in capillary tubes, such as outlet capillary geometry, capillary inner diameter and length, flow orientation, outlet vapor quality, mass velocity, pressure drop, in/outlet flow pattern, evaporation temperature, and refrigeration cycle operational conditions. Despite the broad use of orifice, there is a lack of knowledge in the literature of flow-induced noise in these devices. The flow-induced noise seems primarily a function of mass velocity, vapor quality, and flow pattern. It seems that outlet intermittent flow patterns lead to significant noise in capillary tubes and expansion valves. In general, for the noise evaluated outside the expansion device, electric expansion valves exhibited higher sound pressure levels than capillary tubes. Furthermore, the present literature review indicates that efforts should be made to perform tests for a wide range of conditions evaluating the noise within and outside the expansion device, device wall acceleration and flow topology for current and new refrigerants, simultaneously.

The potential effect of intradermal Botulinum Toxin Type-A (BTA) injection to increase extended random skin flap survival

Background: Extended random skin flap failure often occurs due to insufficient vascularization. Several attempts have been tried to increase the viability of the flap, but none of these attempts have achieved maximal success. Previous research has shown that Botulinum toxin type A (BTA) increases the viability of cutaneous flaps by inhibiting muscle contraction along with increasing vasodilation and angiogenesis. This study examines the effect of intradermal BTA injection on extended random skin flaps viability.Methods: In thirty-six Wistar rats, a rectangular random cutaneous flap (6 cm × 1.5 cm) was made on each dorsal rat area and then elevated the flap. The treatment group was intradermally injected evenly with BTA 8 IU and saline solution for the control group. Each rat flap viability was observed after five days. Primary outcome measures are survival flap area using Visitrak, microscopic assessment of the number and diameter of capillaries and Vascular Endothelial Growth Factor (VEGF) expression was analyzed using immunohistochemistry semi-quantitative scoring system. Data were analyzed using SPSS version 21 for Windows.Results: The treatment group has better results in all observed parameters than the control group. However, Only the VEGF expression of proximal edge showed a statistically significant increase in the group treated with BTA injection (p=0.004).Conclusion: This study showed that the BTA treatment has a positive effect on random skin flap survival and the reduction of distal necrosis in rats, so it has been proven that BTA has the potential to increase flap survival rates in small animal models. Further studies are needed to determine whether BTA treatment can produce similar findings in humans.

Study on bubble penetrating solution/frother interface in the presence of ions

The drainage and rupture of solution/frother interface is a prerequisite to predict the adsorption of frother onto the bubble surface. Using a high-speed motion acquisition system, we investigated the dynamic penetration of bubble at the solution/frother interface under four factors that height, capillary inner diameter, ion concentration and ionic valence. Since the kinetic energy of bubble was proportional to the radius and rising velocity, the kinetic energy increased with increasing the height and capillary inner diameter while decreased with increasing the ion concentration and ionic valence. The variation of kinetic energy followed the order: capillary inner diameter > ion concentration > height > ionic valence. The drainage time of the bubble at the solution/frother interface was found to decrease as the kinetic energy increased independently of these factors, indicating that the rupture of the hydration film was significantly influenced by the kinetic energy. A solution column caused by the bubble penetrating the solution/frother interface was observed. The column height increased as the kinetic energy increased for the factors of height and capillary while an opposite trend was presented by the factors of ion concentration and ionic valence. IFT was discussed to be the essential reason for the difference in the penetration. IFT of the solution/air interface increased as the ion concentration increased independently of ion species, and the IFT of different interfaces followed the order: solution/air > frother/air > solution/frother. Our results can provide valuable insight into the development of technology for surfactant adsorption and mineral flotation.

Влияние преждевременного рождения на васкуляризацию миокарда левого желудочка крыс в постнатальном периоде онтогенеза

Introduction. Preterm birth disrupts important processes of fetal growth and development and can serve as a trigger for maladaptive organ remodeling in the postnatal ontogenesis. The aim of the study was to evaluate the effect of preterm birth on the angiogenesis of the left ventricular myocardium of rats from days 1 to 21 of the postnatal ontogenesis. Materials and methods. We performed immunohistochemical (CD31) and morphometric (specific volume of blood capillaries, specific volume of cardiomyocytes, diameter of blood capillaries, trophic index and pericapillary diffusion zone) study of the left ventricular myocardium of full-term and prematurely born (12 and 24 hours preterm) Wistar rats (n=88) of both sexes on days 1, 7, 14, and 21 of the postnatal ontogenesis. Results. The birth of rats 12 and 24 hours preterm does not result in change in the specific volume of blood capillaries and the trophic index in the left ventricular myocardium on days 1, 7, 14, and 21 of the postnatal period. Preterm birth leads to a decrease in the specific volume of cardiomyocytes of the left ventricular myocardium of rats, compared to the parameters of animals in the control group, on days 7–21 of the postnatal period. Preterm birth results in a decrease in the diameter of the blood capillaries of the left ventricular myocardium in female rats on day 21 of the postnatal period. Preterm birth (24 hours preterm) is accompanied by an increase in the size of the zone of pericapillary diffusion in the left ventricular myocardium of rats on day 14 of the postnatal ontogenesis, in comparison with that of animals in the control group. Conclusion. The shorter the duration of the intrauterine period of development of rats, the greater the severity of the observed changes in the left ventricular myocardium are. It seems promising to study the functional state of endotheliocytes of myocardial blood vessels in preterm animals in the postnatal period. Keywords: preterm birth, prematurity, myocardium, angiogenesis, vascular indexes