Flow of urine through the ureter: a collapsible, muscular tube undergoing peristalsis

Mechanical characteristics of the ureter and clinical implications

The ureteric wall is a complex multi-layered structure. The ureter shows variation in passive mechanical properties, histological morphology and insertion forces along the anatomical length. Ureter mechanical properties also vary depending on the direction of tensile testing and the anatomical region tested. Compliance is greatest in the proximal ureter and lower in the distal ureter, which contributes to the role of the ureter as a high-resistance sphincter. Similar to other human tissues, the ureteric wall remodels with age, resulting in changes to the mechanical properties. The passive mechanical properties of the ureter vary between species, and variation in tissue storage and testing methods limits comparison across some studies. Knowledge of the morphological and mechanical properties of the ureteric wall can aid in understanding urine transport and safety thresholds in surgical techniques. Indeed, various factors alter the forces required to insert access sheaths or scopes into the ureter, including sheath diameter, safety wires and medications. Future studies on human ureteric tissue both in vivo and ex vivo are required to understand the mechanical properties of the ureter and how forces influence these properties. Testing of instrument insertion forces in humans with a focus on defining safe upper limits and techniques to reduce trauma are also needed. Last, evaluation of dilatation limits in the mid and proximal ureter and clarification of tensile strength anisotropy in human specimens are necessary.

CFD investigation of multiple peristaltic waves in a 3D unobstructed ureter

Ureters are essential components of the urinary system and play a crucial role in the transportation of urine from the kidneys to the bladder. In the current study, a three-dimensional ureter is modelled. A series of peristaltic waves are made to travel on the ureter wall to analyse and measure parameter effects such as pressure, velocity, gradient pressure, and wall shear at different time steps. The flow dynamics in the ureters are thoroughly analysed using the commercially available ANSYS-CFX software. The maximum pressure is found in the triple wave at the ureteropelvic junction and maximum velocity is observed in the single and double wave motion due to the contraction produced by the peristalsis motion. The pressure gradient is maximum at the inlet of the ureter during the single bolus motion. The contraction produces a high jet of velocity due to neck formation and also helps in urine trapping in the form of a bolus, which leads to the formation of reverse flow. Due to the reduction in area, shear stress builds on the ureter wall. The high shear stress may rupture the junctions in the ureter.

A model for gastrointestinal tract motility in a 4D imaging phantom of human anatomy

BACKGROUND

Gastrointestinal (GI) tract motility is one of the main sources for intra/inter-fraction variability and uncertainty in radiation therapy for abdominal targets. Models for GI motility can improve the assessment of delivered dose and contribute to the development, testing, and validation of deformable image registration (DIR) and dose-accumulation algorithms.

PURPOSE

To implement GI tract motion in the 4D extended cardiac-torso (XCAT) digital phantom of human anatomy.

MATERIALS AND METHODS

Motility modes that exhibit large amplitude changes in the diameter of the GI tract and may persist over timescales comparable to online adaptive planning and radiotherapy delivery were identified based on literature research. Search criteria included amplitude changes larger than planning risk volume expansions and durations of the order of tens of minutes. The following modes were identified: peristalsis, rhythmic segmentation, high amplitude propagating contractions (HAPCs), and tonic contractions. Peristalsis and rhythmic segmentations were modeled by traveling and standing sinusoidal waves. HAPCs and tonic contractions were modeled by traveling and stationary Gaussian waves. Wave dispersion in the temporal and spatial domain was implemented by linear, exponential, and inverse power law functions. Modeling functions were applied to the control points of the nonuniform rational B-spline surfaces defined in the reference XCAT library. GI motility was combined with the cardiac and respiratory motions available in the standard 4D-XCAT phantom. Default model parameters were estimated based on the analysis of cine MRI acquisitions in 10 patients treated in a 1.5T MR-linac.

RESULTS

We demonstrate the ability to generate realistic 4D multimodal images that simulate GI motility combined with respiratory and cardiac motion. All modes of motility, except tonic contractions, were observed in the analysis of our cine MRI acquisitions. Peristalsis was the most common. Default parameters estimated from cine MRI were used as initial values for simulation experiments. It is shown that in patients undergoing stereotactic body radiotherapy for abdominal targets, the effects of GI motility can be comparable or larger than the effects of respiratory motion.

CONCLUSION

The digital phantom provides realistic models to aid in medical imaging and radiation therapy research. The addition of GI motility will further contribute to the development, testing, and validation of DIR and dose accumulation algorithms for MR-guided radiotherapy.

3D Bioprinting of Human Hollow Organs

3D bioprinting is a rapidly evolving technique that has been found to have extensive applications in disease research, tissue engineering, and regenerative medicine. 3D bioprinting might be a solution to global organ shortages and the growing aversion to testing cell patterning for novel tissue fabrication and building superior disease models. It has the unrivaled capability of layer-by-layer deposition using different types of biomaterials, stem cells, and biomolecules with a perfectly regulated spatial distribution. The tissue regeneration of hollow organs has always been a challenge for medical science because of the complexities of their cell structures. In this mini review, we will address the status of the science behind tissue engineering and 3D bioprinting of epithelialized tubular hollow organs. This review will also cover the current challenges and prospects, as well as the application of these complicated 3D-printed organs.

Fluid mechanical modeling of the upper urinary tract

The upper urinary tract (UUT) consists of kidneys and ureters, and is an integral part of the human urogenital system. Yet malfunctioning and complications of the UUT can happen at all stages of life, attributed to reasons such as congenital anomalies, urinary tract infections, urolithiasis and urothelial cancers, all of which require urological interventions and significantly compromise patients' quality of life. Therefore, many models have been developed to address the relevant scientific and clinical challenges of the UUT. Of all approaches, fluid mechanical modeling serves a pivotal role and various methods have been employed to develop physiologically meaningful models. In this article, we provide an overview on the historical evolution of fluid mechanical models of UUT that utilize theoretical, computational, and experimental approaches. Descriptions of the physiological functionality of each component are also given and the mechanical characterizations associated with the UUT are provided. As such, it is our aim to offer a brief summary of the current knowledge of the subject, and provide a comprehensive introduction for engineers, scientists, and clinicians who are interested in the field of fluid mechanical modeling of UUT. This article is categorized under: Cancer > Biomedical Engineering Infectious Diseases > Biomedical Engineering Reproductive System Diseases > Biomedical Engineering.

Predicting oxygen tension along the ureter

Continuous measurement of bladder urine oxygen tension (Po₂) is a method to potentially detect renal medullary hypoxia in patients at risk of acute kidney injury (AKI). To assess its practicality, we developed a computational model of the peristaltic movement of a urine bolus along the ureter and the oxygen exchange between the bolus and ureter wall. This model quantifies the changes in urine Po₂ as urine transits from the renal pelvis to the bladder. The model parameters were calibrated using experimental data in rabbits, such that most of the model predictions are within ±1 SE of the reported mean in the experiment, with the average percent difference being 7.0%. Based on parametric experiments performed using a model scaled to the geometric dimensions of a human ureter, we found that bladder urine Po₂ is strongly dependent on the bolus volume (i.e., bolus volume-to-surface area ratio), especially at a volume less than its physiological (baseline) volume (<0.2 mL). For the model assumptions, changes in peristaltic frequency resulted in a minimal change in bladder urine Po₂ (<1 mmHg). The model also predicted that there exists a family of linear relationships between the bladder-urine Po₂ and pelvic urine Po₂ for different input conditions. We conclude that it may technically be possible to predict renal medullary Po₂ based on the measurement of bladder urine Po₂, provided that there are accurate real-time measurements of model input parameters.NEW & NOTEWORTHY Measurement of bladder urine oxygen tension has been proposed as a new method to potentially detect the risk of acute kidney injury in patients. A computational model of oxygen exchange between urine bolus and ureteral tissue shows that it may be technically possible to determine the risk of acute kidney injury based on the measurement of bladder urine oxygen tension, provided that the measurement data are properly interpreted via a computational model.

Computational flow analysis of a single peristaltic wave propagation in the ureter

BACKGROUND AND OBJECTIVE

The bladder receives the urine from the kidney and ureter. The series of peristaltic waves facilitate urine transport to the bladder. The peristaltic flow in the ureter is associated with fluid trapping and material reflux, which may cause an increase in bladder pressure. It is difficult to visualize the complex peristalsis phenomenon, in the ureter using image and radiography experiments. A numerical simulation will help in the understanding of urine bolus formation and its effect on the ureter wall.

METHODS

A three-dimensional computational fluid dynamic analysis is carried out to understand the flow physics associated with bolus formation and the effect of reflux on the ureter. ANSYS-CFX, a commercially available computational dynamics package is used to simulate the peristalsis. A single sinusoidal peristaltic wave traveling along a circular tube will yield the velocity, pressure, wall shear stress distributions inside the ureter.

RESULTS

The propagation of the peristaltic wave results in the backflow of urine near the inlet at the beginning of the flow. As the wave propagates towards the outlet, the flow rate decreases. It is observed that pressure distribution along the ureter axis will deteriorate towards the outlet. The contraction produces a very high-pressure gradient which causes the urine backflow. The trapping and the bolus formation cause a significant rise in bolus pressure, simultaneously developing negative pressure at the contraction neck.

CONCLUSIONS

The effect of peristalsis on the ureter biofluid dynamic behavior of the ureter is visualized in this study. It is established that the peristaltic contraction results in high-pressure formation at the bolus and negative pressure at the neck. It was found to be a maximum of 1.1 Pa at the bolus center and -1.13 Pa at the neck region. At the ureter pelvis junction, a higher wall shear of 0.095 Pa is observed as the wave starts to propagate. The velocity vectors show that the trapping of urine causes reflux and results in an adverse pressure gradient near the wall. A maximum pressure gradient of 485 Pa/meter was observed at the contraction of the ureter wall.

Endoscopic Correction of Reflux Utilizing Polyacrylate Polyalcohol Bulking Copolymer (Vantris) as a Tissue Augmenting Substance: Lessons Learned Over the 10 Years of Experience

Objectives: To prove the hypothesis that modifying the endoscopic correction of vesicoureteral reflux (VUR) technique (STING procedure) and sharpening its contraindications, lead to increased success rate and decline in the complication rate. Materials and Methods: All patients that underwent endoscopic correction of VUR utilizing Vantris were divided into two groups according to procedure date; before 2015 and 2015-2019. Indication for treatment included persistent high-grade VUR or breakthrough infections. Contraindications included voiding dysfunction, active infection and since 2015 suspicion for obstructive/refluxing ureterovesical junction (UVJ) presented by "beak" sign on voiding cystourethrography (VCUG). Follow-up regiment included ultrasound and VCUG at predetermined intervals. Results: The first group included 215 (158 girls and 57 boys) children with mean age of 4.8 ± 2.8 years who underwent endoscopic correction between 2009 and 2015 and the second group included 42 children (28 girls and 14 boys) with mean age of 3.9 ± 2.1 years who underwent surgery between 2015 and 2019. In the first group, VUR was unilateral in 74 patients and bilateral in 132 comprising 338 renal refluxing units. In the second group, VUR was unilateral in 14 patients and bilateral in 30 comprising 74 RRUs. In the first group reflux was corrected in 317 (94.9%) RRUs after a single injection, after the second in 7 (2.1%) RRUs. In seven (2.1%) RRUs, reflux downgraded to Grade I-II. Three RRUs (0.9%) failed endoscopic correction and required ureteral reimplantation. Nine (2.7%) RRUs developed UVJ obstruction. In the second group reflux was corrected in 61 (82.4%) RRUs after a single injection, after the second in 12 (16.2%) RRUs. In one (1.4%) RRU, endoscopic correction failed and required ureteral reimplantation. None of the patients developed UVJ obstruction. Reflux correction has led to the significant decrease of febrile urinary tract infection (UTI) in both groups. Conclusions: Our data indicate that endoscopic injection utilizing Vantris is safe and long durable procedure. Although utilizing the proper technique and contraindication criteria, the rates of post procedural VUJ obstruction is null.

In Situ and Intraoperative Detection of the Ureter Injury Using a Highly Sensitive Piezoresistive Sensor with a Tunable Porous Structure

Iatrogenic ureteral injury, as a commonly encountered problem in gynecologic, colorectal, and pelvic surgeries, is known to be difficult to detect in situ and in real-time. Consequently, this injury may be left untreated, thereby leading to serious complications such as infections, renal failure, or even death. Here, high-performance tubular porous pressure sensors were proposed to identify the ureter in situ intraoperatively. The electrical conductivity, mechanical compressibility, and sensor sensitivity can be tuned by changing the pore structure of porous conductive composites. A low percolation threshold of 0.33 vol % was achieved due to the segregated conductive network by pores. Pores also lead to a low effective Young's modulus and high compressibility of the composites and thus result in a high sensitivity of 448.2 kPa^-1 of sensors, which is consistent with the results of COMSOL simulation. Self-mounted on the tip of forceps, the sensors can monitor tube pressures with different frequencies and amplitudes, as demonstrated using an artificial pump system. The sensors can also differentiate ureter pulses from aorta pulses of a Bama minipig in situ and in real-time. This work provides a facile, cost-effective, and nondestructive method to identify the ureter intraoperatively, which cannot be effectively achieved by traditional methods.

Peristaltic transport in an elastic tube under the influence of dilating forcing amplitudes

We investigate flow through an elastic tube which is constrained to a prescribed external forcing consisting of a progressive travelling wave. Such a flow dynamics is closely related to that in the oesophageal tube. The mechanics of the tube is characterized by a relationship between transmural pressure difference and radial variation of the tube. Dimensionless radial variation, assumed to be small, is studied by perturbation techniques. Results demonstrate that the elasticity of the tube plays a significant role in the flow dynamics. An increment in the forcing amplitude of the inward radial force enhances pressure, time-averaged volume flow rate and hence axial and radial velocities. It is revealed that the elastic nature of the oesophageal tube favors swallowing.

E. coli "super-contaminates" narrow ducts fostered by broad run-time distribution

One notable feature of bacterial motion is their ability to swim upstream along corners and crevices, by leveraging hydrodynamic interactions. This motion through anatomic ducts or medical devices might be at the origin of serious infections. However, it remains unclear how bacteria can maintain persistent upstream motion while exhibiting run-and-tumble dynamics. Here, we demonstrate that Escherichia coli can travel upstream in microfluidic devices over distances of 15 mm in times as short as 15 min. Using a stochastic model relating the run times to the time that bacteria spend on surfaces, we quantitatively reproduce the evolution of the contamination profiles when considering a broad distribution of run times. The experimental data cannot be reproduced using the usually accepted exponential distribution of run times. Our study demonstrates that the run-and-tumble statistics determine macroscopic bacterial transport properties. This effect, which we name "super-contamination," could explain the fast onset of some life-threatening medical emergencies.

3D bioprinting for lungs and hollow organs

Three-dimensional bioprinting has been gaining attention as a potential method for creating biological tissues, supplementing the current arsenal of tissue engineering techniques. 3D bioprinting raises the possibility of reproducibly creating complex macro- and microscale architectures using multiple different cell types. This is promising for creation of multilayered hollow organs, which has been challenging using more traditional tissue engineering techniques. In this review, the state of the field in bioprinting of epithelialized hollow and tubular organs is discussed. Most of the progress for the pulmonary system has been restricted to the trachea. Due to the gross structural similarities and common engineering challenges when creating any epithelialized hollow organ, this review also covers current progress in printing within the gastrointestinal and genitourinary systems, as well as applications of traditional plastic printing in engineering these tissues.

Freeze-casting porous chitosan ureteral stents for improved drainage

As a new strategy for improved urinary drainage, in parallel to the potential for additional functions such as drug release and self-removal, highly porous chitosan stents are manufactured by radial, bi-directional freeze-casting. Inserting the porous stent in to a silicone tube to emulate its placement in the ureter shows that it is shape conforming and remains safely positioned in place, also during flow tests, including those performed in a peristaltic pump. Cyclic compression tests on fully-hydrated porous stents reveal high stent resilience and close to full elastic recovery upon unloading. The drainage performance of the chitosan stent is evaluated, using effective viscosity in addition to volumetric flow and flux; the porous stent's performance is compared to that of the straight portion of a commercial 8 Fr double-J stent which possesses, in its otherwise solid tube wall, regularly spaced holes along its length. Both the porous and the 8 Fr stent show higher effective viscosities, when tested in the silicone tube. The performance of the porous stent improves considerably more (47.5%) than that of the 8 Fr stent (30.6%) upon removal from the tube, illustrating the effectiveness of the radially aligned porosity for drainage. We conclude that the newly-developed porous chitosan ureteral stent merits further in vitro and in vivo assessment of its promise as an alternative and complement to currently available medical devices. STATEMENT OF SIGNIFICANCE: No papers, to date, report on porous ureteral stents, which we propose as a new strategy for improved urinary drainage. The highly porous chitosan stents of our study are manufactured by radial, bi-directional freeze casting. Cyclic compression tests on fully-hydrated porous stents revealed high stent resilience and close to full recovery upon unloading. The drainage performance of the chitosan is evaluated, using effective viscosity in addition to volumetric flow and flux, and compared to that of the straight portion of a commercial 8 Fr double-J stent. The performance of the porous stent improves considerably more (47.5%) than that of the 8 Fr stent (30.6%) upon removal from the tube, illustrating the effectiveness of the radially aligned porosity for drainage. While further studies are required to explore other potential benefits of the porous stent design such as antimicrobial behavior, drug release, and biodegradability, we conclude that the newly-developed porous chitosan ureteral stent has considerable potential as a medical device.

What are the predictive factors leading to ureteral obstruction following endoscopic correction of VUR in the pediatric population?

BACKGROUND

It is extremely important to not only address the short-term success following endoscopic correction of vesicoureteral reflux (VUR) but also the long-term efficacy and safety of the tissue augmenting substance utilized for endoscopic correction.

OBJECTIVE

This study retrospectively evaluated all cases of ureterovesical junction (UVJ) obstruction following endoscopic treatment of VUR over the last 5 years utilizing two tissue augmenting substances, with special emphasis on the safety of Vantris^®, and performed clinical and histological review of these patients.

METHODS

The study population comprised 2495 patients who underwent endoscopic correction of VUR utilizing Deflux^® (1790) and Vantris^® (705). Tissue sections were stained with hematoxylin & eosin and trichrome, and examined under a light microscope. Nine primary obstructive megaureters after ureteral re-implantation served as controls.

RESULTS

Nine (0.5%) children (three female and six male) in the Deflux group and nine (1.3%) (five females and four males) in the Vantris group developed UVJ obstruction and required ureteral re-implantation. Obstruction developed during the period ranging 2-49 months (average 16 months) following endoscopic correction. The primary reflux grade was III in seven, IV in six, and V in six children. The mean volume of the injected material in all obstructed patients was 1.2 ± 0.6 cc (mean ± SD). Histopathological analysis revealed a pseudocapsule composed of fibrous tissue and foreign-body giant cells surrounding the Vantris implant in all patients. The distal part of the ureters demonstrated significant ureteral dilatation without ureteral fibrosis. In all patients, additional biopsies from the muscularis propria adjacent to the injection site were examined and showed no significant abnormalities. There was an increased collagen deposition in the juxtavesical segment of the obstructive ureters following Deflux and Vantris injections, and of primary obstructive megaureter. No significant difference was found in the tissue response between Deflux and Vantris patients and controls. Statistical analysis of the nonhomogeneous population demonstrated higher obstruction rates in patients from the Vantris group. However, no statistical difference was demonstrated regarding the obstruction rate in the homogenous group with relation to gender, age and reflux grade group of patients. Moreover, univariate analysis revealed that Grade V reflux, the presence of beak sign on the reviewed pretreatment, and inflamed bladder mucosa upon injection were significant independent risk factors leading to obstruction.

DISCUSSION

This study suggested that the underlining ureteral pathology lead to UVJ obstruction following Vantris injection. There was increased collagen deposition in the juxtavesical segment of the obstructive ureters following Vantris injection. Furthermore, these findings were similar to those discovered in patients who underwent endoscopic correction with Deflux, and in patients who required ureteral reimplantation due to primary obstructive megaureter. Additional biopsies from the muscularis propria adjacent to the injection site showed no significant abnormalities, ironing out the fact that Vantris did not led to adverse tissue reaction following injection. Univariate analysis further ironed out the hypothesis that underlying ureteral pathology was responsible for the increased incidence of UVJ obstruction and demonstrated that Grade V reflux, the presence of beak sign on the reviewed pretreatment VCUG, and inflamed bladder mucosa upon injection were significant independent risk factors leading to obstruction.

CONCLUSION

Data showed that Vantris injection did not lead to any different ureteral fibrosis or inflammatory changes to the tissue augmenting substances utilized in past and present clinical practice, and therefore did not seem to increase the incidence of UVJ obstruction. High reflux grade, presence of obstructive/refluxing megaureter and inflamed bladder mucosa were the only statistically significant and independent predictive factors for UVJ obstruction following endoscopic correction of VUR.

A two way fully coupled fluid structure simulation of human ureter peristalsis

Numerical simulations of ureter peristalsis have been carried out in the past to understand both the flow field and ureter wall mechanics. The main objective of the current investigations is to have a better understanding of the urine transport due to the peristalsis in the ureter, thus making the information helpful for a better treatment and diagnosis of ureteral complications like urine reflux. In the current study, a numerical simulation is performed using a finite-element-based solver with a two-way fully coupled fluid structure interaction approach between the ureter wall and urine. For the first time, the ureter wall is modeled as an anisotropic hyper-elastic material based on experiments performed in previous literature on the human ureter. Peristalsis in the ureter is modeled as a series of isolated boluses. By observing the flow field it is clear that the peristalsis mechanism has a natural tendency to create a backflow as the isolated bolus moves forward. As a result, the urine can flow back from the bladder to the ureter at the ureterovesical (ureter-bladder) junctions, if the one-way valve starts to malfunction.

Fluid-structure interaction simulation of ureter with vesicoureteral reflux and primary obstructed megaureter

Two common abnormalities in ureters include primary refluxing megaureter (PRM) and primary obstructed megaureter (POM). The aim of this study was to represent the numerical simulation of the urine flow at the end of the ureter with vesicoureteral reflux (VUR) and POM during peristalsis. Methodologically, the peristalsis in the ureter wall was created using Gaussian distribution. Fluid-structure interaction (FSI) was applied to simulate urine-elastic wall interactions; and governing equations were solved using the arbitrary Lagrangian-Eulerian method. Theories such as wall elasticity, Newtonian fluid, and incompressible Navier-Stokes equations were used. Velocity fields, viscous stresses and volumetric outflow rate profiles were obtained through the simulation of the ureter with VUR and POM during peristalsis. In addition, the effect of urine viscosity on flow rate was investigated. When the bladder pressure increased, VUR occurred because of the ureterovesical junction (UVJ) dysfunction, leading to high stresses on the wall. In the POM, the outflow rate was ultimately zero, and stresses on the wall were severe in the obstructed section. Comparing the results demonstrated that the peristalsis leads to even further dilation of the prestenosis portion. It was also observed that the reflux occurs in the ureter with VUR when the bladder pressure is high. Additionally, the urine velocity during the peristalsis was higher than the non-peristaltic ureter.

A fluid-structure interaction (FSI)-based numerical investigation of peristalsis in an obstructed human ureter

Urine moves from the kidney to the bladder through the ureter. A series of compression waves facilitates this transport. Due to the highly concentrated mineral deposits in urine, stones are formed in the kidney and travel down through the urinary tract. While passing, a larger stone can get stuck and cause severe damage to ureter wall. Also, stones in the ureter obstructing the urine flow can cause pain and backflow of urine which in turn might require surgical intervention. The current study develops a 2D axisymmetric numerical model to gain an understanding of the ureter obstruction and its effects on the flow, which are critical in assessing the different treatment options. Transient computational analysis involving a two-way fully coupled fluid-structure interaction with the arbitrary Lagrangian-Eulerian method between the ureteral wall and urine flow is conducted with an obstruction in the ureter. The ureter wall is modeled as an anisotropic hyperelastic material, data of which, is based on biaxial tests on human ureter from previous literature, while the incompressible Navier-Stokes equations are solved to calculate urine flow. A finite element-based monolithic solver is used for the simulations here. The obstruction is placed in the fluid domain as a circular stone at the proximal part of the ureter. One of the objectives of this study is to quantify the effect of the ureteral obstruction. A sharp jump in pressure gradient and wall shear stress, as well as retrograde urine flow, is observed as a result of the obstruction.

A two-dimensional numerical study of peristaltic contractions in obstructed ureter flows

The flow of urine from the kidneys to the bladder is accomplished via peristaltic contractions in the ureters. The peristalsis of urine through the ureter can sometimes be accompanied, more specifically, obstructed to a certain degree, by entities such as kidney stones. In this paper, 2D axisymmetric computational fluid dynamics simulations are carried out using the commercial code ANSYS FLUENT[Formula: see text], to model the peristaltic movement of the ureter with and without stone. The peristaltic movement was assumed to be a sinusoidal wave on the boundary of the ureter with a specific physiological velocity. While the first part of the study considers flow in the ureter with prescribed peristaltic contractions in absence of any obstruction, the second part compares the effect of varying obstructions (0, 5, 15, and 35%) in terms of spherical stones of different sizes. Pressure contours, velocity vectors, and profiles of pressure gradient magnitudes and wall shear stresses are presented along one bolus of the ureter, during contraction and expansion of the ureteral wall, in order to understand backflow, trapping and reflux phenomena, as well as monitor the health of the ureteral wall in the presence of any obstruction. The 35% ureteral obstruction case resulted in a significant backflow at the inlet in comparison to the other cases, and also a wall shear stress that was up to 20x larger than the case without any obstruction.

Holding Water: Congenital Anomalies of the Kidney and Urinary Tract, CKD, and the Ongoing Role of Excellence in Plumbing

Congenital anomalies of the kidneys and urinary tracts can result in diminished natal kidney function, possibly through common embryologic pathway disruption or as a result of development taking place in the face of disordered 'post-renal' drainage. Impaired conduit and reservoir function present potential for an ongoing assault leading to further deterioration and progression of chronic kidney disease, a risk that extends to adults with these conditions, even after "correction". The drainage and storage aspects of the urinary system that can impact kidney function are reviewed with attention to correctable or manageable problems including: Bladder dysfunction wherein the low pressure storage of urine is compromised requiring the kidney to work against a pressure gradient, the classic post renal failure problem. The kidney in the aftermath of obstruction which may have lost concentrating capacity leading to a tendency to dehydration ('pre-renal' failure) and through polyuria which exacerbates bladder pressure problems. Further there is an added challenge in evaluation for ongoing or reemergent obstruction in a significantly dilated system where the capacious system leads to slow turnover of urine often requiring a ureteral stent or nephrostomy to clearly establish clinical significance of delayed drainage. Stasis where slow urine flow leads to buildup of debris (stone) or potentiates infection. Vessicoureteral reflux which allows for introduction of lower urinary tract bacteria to the kidney and can lead to pyelonephritis. Conditions which combine problems such as posterior urethral valves where the bladder outlet obstruction compromises kidney function potentially impairing concentrating ability, creates bladder compromise often reducing emptying efficiency or elevating bladder storage pressures, as well as dilating the system potentially promoting stasis. Cognizance of the potential for plumbing problems to further kidney deterioration as patients with congenital urinary tract anomalies, even after they have been repaired is incumbent on those caring for these patients as they age. Thoughtful evaluation of those patients in whom kidney compromise maybe aggravated by drainage and storage disorder will optimize native renal function.

Emergence and development of gut motility in the chicken embryo

The gastrointestinal tract transports the food bolus by peristalsis. Gut motility starts at an early age in the developing embryo, well before it is required for nutrition of the organism. We present a comprehensive kinematic study of the emergence and physiological development of gut motility in all regions of the lower digestive tract of the chicken embryo from embryonic days E5 through E9. We characterized motility emergence time, propagation patterns, speed, frequency and amplitude of peristalsis waves. We found that the emergence of an uninterrupted circular ring of smooth muscle correlated with the appearance of propagative contractile waves, at E6 in the hindgut and midgut, and at E9 in the caecal appendix. We show that peristalsis at these stages is critically dependent on calcium and is not mediated by neurons as gut motility is insensitive to tetrodotoxin and takes place in the hindgut in the absence of neurons. We further demonstrate that motility also matures in ex-vivo organ culture. We compare our results to existing literature on zebrafish, mouse and human motility development, and discuss their chronological relationship with other major developmental events occurring in the chicken embryonic gut at these stages. Our work sets a baseline for further investigations of motility development in this important animal model.

Mathematical modelling of peristaltic propulsion of viscoplastic bio-fluids

This article studies theoretically the transportation of rheological viscoplastic fluids through physiological vessels by continuous muscle contraction and relaxation, that is, peristalsis. Both cases of planar and cylindrical physiological vessels are considered. A mathematical model is developed under long wavelength and low Reynolds number approximations. Expressions for axial velocity in core region, axial velocity in plug flow region, volume flow rate and pressure gradient in non-dimensional form are obtained. A comparative study of velocity profiles, pressure distribution, friction force and mechanical efficiency for different viscoplastic liquids is conducted. The influence of width of plug flow region, shear rate strain index and yield stress index on the pressure distribution, friction force and mechanical efficiency is elaborated. The study is relevant to gastric fluid mechanics and also non-Newtonian biomimetic pump hazardous waste systems exploiting peristaltic mechanisms.

A biomechanical simulation of ureteral flow during peristalsis using intraluminal morphometric data

Reflux nephropathy and vesicoureteral reflux are two of the most important abnormalities in the upper urinary system in which toxins and bacteria from the bladder infect the ureter and the kidney and initiate renal scar formation. A quantitative analysis that characterizes urine flow will further help our understanding of the ureter and also assist in the design of flow aided devices such as valves and stents to correct reflux situations. Here, A numerical simulation with fluid-structure interactions (FSI) using arbitrary Lagrangian-Eulerian (ALE) formulation and adaptive mesh procedure was introduced and solved to perform ureteral flow analysis. Incompressible Navier-Stokes equations were utilized as the governing equations of fluid domain. Ureteral in-vivo morphometric data during peristalsis were used to construct the presented model. A nonlinear material model was used to exhibit ureteral wall mechanical properties. Direct coupling method was used to solve the solid, fluid and interface equations simultaneously. Results showed that recirculation regions formed against the jet flow, neighboring the bolus peak. Through wave propagation, separation occurred behind the moving bolus on the wall and ureteropelvic reflux began from that location and extended upstream to the ureteral inlet. The maximum luminal pressure consistently occurred behind the urine bolus during peristalsis. The measured magnitude of maximum volumetric flow rate resulted from isolated bolus transportation was 0.92 ml/min. In conclusion; due to presence of fluid inertial forces during peristalsis, the function of ureteropelvic junction in prevention of reflux is significant, especially at the beginning of peristaltic wave propagation. Moreover, modeling of ureteral function using imaging data will be valuable and it may help physicians to diagnose and cure the abnormalities.

New valve-mechanical model of urinary tract function: the theory of biological dual valves

INTRODUCTION

Until now, peristalsis has been the only known method of urine transport. The main objective of this paper was to study urinary tract function - especially the upper urinary tract, the ureter - from a new mechanical point of view. The physical (physical dual valves) and biological basis (biological dual valves) of a new functional model is presented based on previous observations and knowledge.

METHODS

A review of the literature was performed, with special emphasis on ureter motility.

RESULTS

After analyzing the anatomy and physiology of the urinary tract, complemented by basic physical observations, the authors have developed a new valve-mechanical model of urinary tract function. A comprehensive mechanical hypothesis is also presented, integrating the role of peristalsis.

CONCLUSIONS

The authors believe that the new theory enhances previous knowledge. From a structural point of view, the urinary tract may be considered to consist of dualvalves. The dual-valve mechanism combined with peristalsis allows better explanation of the function of the upper urinary tract in particular. The main conclusion is that the flow in the urinary tract must be studied integrally within the body. This new theory does not contradict well-known and acknowledged theories, and moreover, it may help solve certain medical problems.

Particle motion in unsteady two-dimensional peristaltic flow with application to the ureter

Particle motion in an unsteady peristaltic fluid flow is analyzed. The fluid is incompressible and Newtonian in a two-dimensional planar geometry. A perturbation method based on a small ratio of wave height to wavelength is used to obtain a closed-form solution for the fluid velocity field. This analytical solution is used in conjunction with an equation of motion for a small rigid sphere in nonuniform flow taking Stokes drag, virtual mass, Faxén, Basset, and gravity forces into account. Fluid streamlines and velocity profiles are calculated. Theoretical values for pumping rates are compared with available experimental data. An application to ureteral peristaltic flow is considered since fluid flow in the ureter is sometimes accompanied by particles such as stones or bacteriuria. Particle trajectories for parameters that correspond to calcium oxalates for calculosis and Escherichia coli type for bacteria are analyzed. The findings show that retrograde or reflux motion of the particles is possible and bacterial transport can occur in the upper urinary tract when there is a partial occlusion of the wave. Dilute particle mixing is also investigated, and it is found that some of the particles participate in the formation of a recirculating bolus, and some of them are delayed in transit and eventually reach the walls. This can explain the failure of clearing residuals from the upper urinary tract calculi after successful extracorporeal shock wave lithotripsy. The results may also be relevant to the transport of other physiological fluids and industrial applications in which peristaltic pumping is used.

A case of acute kidney injury due to complex, partial, multifocal ureteral strictures

BACKGROUND

An 89-year-old man with a history of prostate cancer who had undergone radical prostatectomy 15 years ago presented with hyperkalemia (serum potassium level 6.9 mmol/l) and kidney failure (serum creatinine level 937 micromol/l [10.6 mg/dl]). Ultrasound scan of his kidneys showed mild bilateral hydronephrosis. Although placement of a bladder catheter led to an initial increase in glomerular filtration rate, the improvement was delayed and incomplete. Subsequently, the patient's glomerular filtration rate decreased acutely. This unusual biphasic course of kidney injury begged explanation.

INVESTIGATIONS

Physical examination, measurements of serum creatinine level and electrolytes, imaging of the urinary tract (ultrasound and CT scans), and nephrostograms.

DIAGNOSIS

Acute kidney injury due to upper (multiple ureteral strictures bilaterally) and lower (urethral) urinary tract obstruction.

MANAGEMENT

Placement of bladder catheter and percutaneous nephrostomy tubes followed by bilateral internal ureteral stents.

The effects of flow rate, length and external pressure upon the pressure required for fluid to flow through a ureter

OBJECTIVE

To determine in vitro the effects of increments of external pressure on the pressure required to conduct fluid through ureters of various lengths at different flow rates, as the flow of a fluid through a collapsible tube is influenced by various factors (e.g. external pressure, the pressure gradient between the ends, the length and diameter of the tube, and the viscosity of fluid).

MATERIALS AND METHODS

Two in vitro systems were designed, composed of three parts, i.e. a perfusion line, an exit line and a container of two different widths in which short or long ureteric segments, obtained from cattle, could be placed; the ureter was connected to the perfusion and exit lines. Physiological saline was added to the container until the desired external pressure was applied to the ureter. The flow pressure (height of the perfusion line) was recorded when producing flows through ureters of varying length at 1.5 and 6 mL/min, and determined under various external pressures. The intra-ureteric pressure during flow was also monitored by a pressure transducer. The four combinations of long and short ureters with high and low flow rates were compared using analysis of variance, with the Pearson correlation coefficient used to evaluate the relationships between the various pressures.

RESULTS

There were close relationships between flow pressure and external pressure (r = 0.727), intra-ureteric and external pressure (r = 0.766), and the flow pressure and intra-ureteric pressure (r = 0.940, all P < 0.001). Increments in external pressure resulted in greater flow and intra-ureteric pressure (P < 0.05). Increases in flow pressure were more pronounced than increases in intra-ureteric pressure at the same external pressure (P < 0.05) at high flow rates. A longer ureter and higher flow rates caused greater intra-ureteric pressure (P < 0.05).

CONCLUSION

External pressure increases the pressure required to conduct fluid through a ureter and the effect is more pronounced at high flow rates. The length of the ureter also affects the flow pressure at high flow rates. Therefore, flow through the ureter follows the Poiseuille equation only at high flow rates. Thus, increases in intra-abdominal pressure may cause greater intrapelvic pressure and induce ureteric obstruction, contributing to the pathogenesis of hydronephrosis.

An active membrane model for peristaltic pumping: Part I--Periodic activation waves in an infinite tube

A model for the coupled problem of wall deformation and fluid flow, based on thin-shell and lubrication theories, and driven by a propagating wave of smooth muscle activation, is proposed for peristaltic pumping in the ureter. The model makes use of the available experimental data on the mechanical properties of smooth muscle and accounts for the soft material between the muscle layer and the vessel lumen. The main input is the activation wave of muscular contraction. Equations for the time-dependent problem in tubes of arbitrary length are derived and applied to the particular case of periodic activation waves in an infinite tube. Mathematical (small amplitude) and numerical analyses of this case are presented. Predictions on phase-lag in wall constriction with respect to peak activation wave, lumen occlusion due to thickening lumen material with contracting smooth muscle, and the general bolus shape are in qualitative agreement with observation. Some modifications to the mechanical, elastic, and hydrodynamic properties of the ureter that will make peristalsis less efficient, due for example to disease, are identified. In particular, the flow rate-pressure rise relationship in linear for weak to moderate activation waves, but as the lumen is squeezed shut, it is seen to be nonlinear in a way that increases pumping efficiency. In every case a ureter whose lumen can theoretically be squeezed shut is the one for which pumping is most efficient.