Oxygen transport across vasa recta in the renal medulla

2002 ◽  
Vol 283 (3) ◽  
pp. H1042-H1055 ◽  
Author(s):  
Wensheng Zhang ◽  
Aurélie Edwards
Keyword(s):  
Oxygen Transport ◽  
Renal Medulla ◽  
Blood Flow Rate ◽  
Sodium Reabsorption ◽  
Corticomedullary Junction ◽  
Vasa Recta ◽  
Oxygen Requirements ◽  
Experimental Findings ◽  
Parameter Values ◽  
Baseline Parameter

In this model of oxygen transport in the renal medullary microcirculation, we predicted that the net amount of oxygen reabsorbed from vasa recta into the interstitium is on the order of 10−6 mmol/s, i.e., significantly lower than estimated medullary oxygen requirements based on active sodium reabsorption. Our simulations confirmed a number of experimental findings. Low medullary Po 2results from the countercurrent arrangement of vessels and an elevated vasa recta permeability to oxygen, as well as high metabolic needs. Diffusional shunting of oxygen between descending vasa recta (DVR) and ascending vasa recta also explains why a 20-mmHg decrease in initial Po 2 at the corticomedullary junction only leads to a small drop in papillary tip Po 2 (<2 mmHg with baseline parameter values). Conversely, small changes in the consumption rate of DVR-supplied oxygen, in blood flow rate, in hematocrit, or in capillary permeability to oxygen, beyond certain values sharply reduce interstitial Po 2. Without erythrocytes, papillary tip Po 2 cannot be maintained above 10 mmHg, even when oxygen consumption is zero.

Analysis of microvascular water and solute exchanges in the renal medulla

1984 ◽  
Vol 247 (2) ◽  
pp. F303-F315 ◽  
Author(s):  
T. L. Pallone ◽  
T. I. Morgenthaler ◽  
W. M. Deen
Keyword(s):  
Renal Medulla ◽  
Blood Flow Rate ◽  
Reflection Coefficients ◽  
Hydraulic Permeability ◽  
Mass Balances ◽  
Flow Rates ◽  
Corticomedullary Junction ◽  
Vasa Recta ◽  
Capillary Plexus

A theoretical model has been developed to simulate solute and water transport in the medullary microcirculation of the normal hydropenic rat. The model is formulated in terms of a countercurrent vascular unit consisting of one descending (DVR) and several ascending vasa recta (AVR) extending from the corticomedullary junction to the tip of the papilla. Steady-state mass balances relate gradients in NaCl, urea, and plasma protein concentrations and variations in the flow rates of plasma and red blood cells to permeability properties of the vasa recta and erythrocytes. In contrast to previous models, transmural volume fluxes are assumed to be present in both DVR and AVR. Available micropuncture measurements suggesting net volume removal from DVR within the inner medulla are found to be consistent with NaCl reflection coefficients in DVR between 0.10 and 0.80. The hydraulic permeability in the DVR is estimated to be greater than 0.18 X 10(-6) cm X s-1 X mmHg-1. Based on currently available data, reliable bounds cannot yet be placed on the hydraulic permeability of the AVR. The vascular unit is predicted to accomplish substantial net removal of NaCl and water from the inner medullary interstitium but relatively little removal of urea. Red cells leaving the inner medulla in the AVR are found to be slightly dehydrated. It is calculated that at a given blood flow rate, the lower the initial medullary hematocrit, the more effective the vascular unit is at removing water. Several unresolved issues are discussed, including the role of the capillary plexus that joins DVR with AVR. To the extent that the volume uptake observed in the exposed papilla in structures beyond the DVR occurs in the capillary plexus and not in the AVR, estimated values of AVR hydraulic permeability are reduced, as is predicted overall volume uptake by the vascular unit in the inner medulla.

scholarly journals A multiunit model of solute and water removal by inner medullary vasa recta

1998 ◽  
Vol 274 (4) ◽  
pp. H1202-H1210 ◽  
Author(s):  
Aurélie Edwards ◽  
Thomas L. Pallone
Keyword(s):  
Pressure Difference ◽  
Renal Medulla ◽  
Blood Flow Rate ◽  
Maximum Pressure ◽  
Hydraulic Pressure ◽  
Pressure Gradients ◽  
Vasa Recta ◽  
Rate Ratios ◽  
Limiting Case ◽  
Small Solute

A recent model of volume and solute microcirculatory exchange in the renal medulla based on a single descending vasa rectum (DVR) was extended to account for the varying number of vessels along the corticomedullary axis. The assumption that concentration polarization at the walls of ascending vasa recta (AVR) during volume uptake eliminates transmural oncotic pressure gradients was examined. In this limiting case, small hydrostatic pressure gradients can drive AVR volume uptake if the pressure in the interstitium exceeds that in the AVR lumen. The calculated hydraulic pressure difference across AVR yielding agreement between predicted and measured values of AVR-to-DVR blood flow rate ratios was found to be smaller than the reported maximum pressure difference AVR can sustain. Simulations also confirmed previous conclusions suggesting that the presence of urea transporters in DVR counterbalances that of water channels that would otherwise decrease the efficiency of small solute trapping in the renal medulla.

A model of glucose transport and conversion to lactate in the renal medullary microcirculation

2006 ◽  
Vol 290 (1) ◽  
pp. F87-F102 ◽  
Author(s):  
Wensheng Zhang ◽  
Aurélie Edwards
Keyword(s):  
Lactate Concentration ◽  
Anaerobic Glycolysis ◽  
Kinetic Rate Constant ◽  
Outer Medulla ◽  
Kinetic Rate ◽  
Corticomedullary Junction ◽  
Vasa Recta ◽  
Countercurrent Exchange ◽  
Parameter Values ◽  
Good Agreement

In this study, we modeled mathematically the transport of glucose across renal medullary vasa recta and its conversion to lactate by anaerobic glycolysis. Uncertain parameter values were determined by seeking good agreement between predictions and experimental measurements of lactate generation rates, as well as glucose and lactate concentration ratios between the papilla and the corticomedullary junction; plausible kinetic rate constant and permeability values are summarized in tabular form. Our simulations indicate that countercurrent exchange of glucose from descending (DVR) to ascending vasa recta (AVR) in the outer medulla (OM) and upper inner medulla (IM) severely limits delivery to the deep inner medulla, thereby limiting medullary lactate generation. If the permeability to glucose of OMDVR and IMDVR is taken to be the same and equal to 4 × 10−4 cm/s, the fraction of glucose that bypasses the IM is calculated as 54%; it is predicted as 37% if the presence of pericytes in OMDVR reduces the glucose permeability of these vessels by a factor of 2 relative to that of IMDVR. Our results also suggest that red blood cells (RBCs) act as a reservoir that reduces the bypass of glucose from DVR to AVR. The rate of lactate generation by anaerobic glycolysis of glucose supplied by blood from glomerular efferent arterioles is predicted to range from 2 to 8 nmol/s, in good agreement with lower estimates obtained from the literature (Bernanke D and Epstein FH. Am J Physiol 208: 541–545, 1965; Bartlett S, Espinal J, Janssens P, and Ross BD. Biochem J 219: 73–78, 1984).

Vasoconstriction of outer medullary vasa recta by angiotensin II is modulated by prostaglandin E2

1994 ◽  
Vol 266 (6) ◽  
pp. F850-F857 ◽  
Author(s):  
T. L. Pallone
Keyword(s):  
Angiotensin Ii ◽  
Prostaglandin E2 ◽  
Regional Blood Flow ◽  
Renal Medulla ◽  
Hydraulic Pressure ◽  
Vascular Bundles ◽  
Ang Ii ◽  
Vasa Recta ◽  
Anatomical Arrangement

Vasa recta were dissected from outer medullary vascular bundles in the rat and perfused in vitro. Examination by transmission electron microscopy reveals them to be only outer medullary descending vasa recta (OM-DVR). To establish a method for systematic examination of vasoconstriction, OMDVR were perfused at 5 nl/min with collection pressure increased to 5 mmHg. Under these conditions, transmembrane volume flux was found to be near zero, and the transmural hydraulic pressure gradient was found to be < 15 mmHg. Over a concentration range of 10(-12) to 10(-8) M, abluminal application of angiotensin II (ANG II) caused graded focal vasoconstriction of OMDVR that is blocked by saralasin. Luminal application of ANG II over the same concentration range was much less effective. Abluminal application of prostaglandin E2 (PGE2) shifted the vasoconstrictor response of OMDVR to higher ANG II concentrations. PGE2 reversibly dilated OMDVR that had been preconstricted by ANG II. These results demonstrate that OMDVR are vasoactive segments. Their anatomical arrangement suggests that they play a key role in the regulation of total and regional blood flow to the renal medulla.

Role of the renal medulla in volume and arterial pressure regulation

1997 ◽  
Vol 273 (1) ◽  
pp. R1-R15 ◽  
Author(s):  
A. W. Cowley
Keyword(s):  
Blood Flow ◽  
Arterial Pressure ◽  
Renal Blood Flow ◽  
Fluid Pressure ◽  
Regulatory System ◽  
Renal Medulla ◽  
Water Excretion ◽  
Vasa Recta ◽  
Pressure Natriuresis ◽  
Total Renal Blood Flow

The original fascination with the medullary circulation of the kidney was driven by the unique structure of vasa recta capillary circulation, which Berliner and colleagues (Berliner, R. W., N. G. Levinsky, D. G. Davidson, and M. Eden. Am. J. Med. 24: 730-744, 1958) demonstrated could provide the economy of countercurrent exchange to concentrate large volumes of blood filtrate and produce small volumes of concentrated urine. We now believe we have found another equally important function of the renal medullary circulation. The data show that it is indeed the forces defined by Starling 100 years ago that are responsible for the pressure-natriuresis mechanisms through the transmission of changes of renal perfusion pressure to the vasa recta circulation. Despite receiving only 5-10% of the total renal blood flow, increases of blood flow to this region of the kidney cause a washout of the medullary urea gradient and a rise of the renal interstitial fluid pressure. These forces reduce tubular reabsorption of sodium and water, leading to a natriuresis and diuresis. Many of Starling's intrinsic chemicals, which he named "hormones," importantly modulate this pressure-natriuresis response by altering both the sensitivity and range of arterial pressure around which these responses occur. The vasculature of the renal medulla is uniquely sensitive to many of these vasoactive agents. Finally, we have found that the renal medullary circulation can play an important role in determining the level of arterial pressure required to achieve long-term fluid and electrolyte homeostasis by establishing the slope and set point of the pressure-natriuresis relationship. Measurable decreases of blood flow to the renal medulla with imperceptible changes of total renal blood flow can lead to the development of hypertension. Many questions remain, and it is now evident that this is a very complex regulatory system. It appears, however, that the medullary blood flow is a potent determinant of both sodium and water excretion and signals changes in blood volume and arterial pressure to the tubules via the physical forces that Professor Starling so clearly defined 100 years ago.

Diffusion of oxygen from arterial to venous segments of renal capillaries

1959 ◽  
Vol 196 (6) ◽  
pp. 1336-1339 ◽  
Author(s):  
Matthew N. Levy ◽  
Gerardo Sauceda
Keyword(s):  
Renal Artery ◽  
Oxygen Saturation ◽  
Renal Vein ◽  
Venous Blood ◽  
Renal Medulla ◽  
Appearance Time ◽  
Arterial Blood ◽  
Vasa Recta ◽  
Initial Appearance ◽  
Predominant Effect

Injections of three types of blood preparations were made into the renal arteries of dogs, namely, a) blood equilibrated with 95% O2, 5% CO2, b) arterial blood containing some methemoglobin-labeled erythrocytes and c) blood containing methemoglobinemic cells, but equilibrated with 95% O2, 5% CO2. The initial appearance time in the renal vein was 1.25 ± 0.97 second earlier for oxygen than for the methemoglobinemic red cells. When preparation c was introduced into the renal artery, a diphasic curve was consistently registered from the renal venous blood. The initial deflection was uniformly upright, indicating a preponderant effect due to increased oxygen saturation. This was followed by an inverted deflection, resulting from the predominant effect of methemoglobin. These findings are interpreted to indicate diffusion of some of the oxygen from arterial to venous limbs of capillary loops, probably the vasa recta located in the renal medulla.

Influence of the renal medullary circulation on the control of sodium excretion

1993 ◽  
Vol 265 (5) ◽  
pp. R963-R973 ◽  
Author(s):  
R. J. Roman ◽  
A. P. Zou
Keyword(s):  
Blood Flow ◽  
Renal Perfusion ◽  
Sodium Reabsorption ◽  
Doppler Flowmetry ◽  
Medullary Blood Flow ◽  
Tubular Sodium Reabsorption ◽  
Vasa Recta ◽  
Urinary Concentrating Ability ◽  
Reliable Methods

Although the role of the renal medullary circulation in the control of urinary concentrating ability is well established, its potential influence on tubular sodium reabsorption is not generally recognized. Nearly 30 years ago, changes in the intrarenal distribution of blood flow were first proposed to contribute to the natriuretic response to volume expansion. However, the lack of reliable methods for studying medullary blood flow limited progress in this area. The recent development of laser-Doppler flowmetry and videomicroscopic techniques for the study of the vasa recta circulation has renewed interest in the role of medullary hemodynamics in the control of sodium reabsorption. Results of these studies indicate that changes in renal medullary hemodynamics alter renal interstitial pressure and the medullary solute gradient and play an important role in the natriuretic response to elevations in renal perfusion pressure, intravenous infusion of saline, and changes in tubular sodium reabsorption produced by vasoactive compounds. What is emerging from these studies is the view that changes in renal medullary hemodynamics represent an important but misunderstood and long-ignored factor in the control of tubular sodium reabsorption.

Renal medullary blood flow: its measurement and physiology

1986 ◽  
Vol 64 (7) ◽  
pp. 873-880 ◽  
Author(s):  
W. A. Cupples
Keyword(s):  
Blood Flow ◽  
Renal Medulla ◽  
Vascular Bundles ◽  
Medullary Blood Flow ◽  
Vasa Recta ◽  
Countercurrent Exchange ◽  
Renal Medullary Blood Flow ◽  
Urinary Concentrating Ability ◽  
Velocity Tracking ◽  
Uptake Method

The vasculature of the mammalian renal medulla is complex, having neither discrete input nor output. There is also efficient countercurrent exchange between ascending and descending vasa recta in the vascular bundles. These considerations have hampered measurement of medullary blood flow since they impose pronounced constraints on methods used to assess flow. Three main strategies have been used: (i) indicator extraction; (ii) erythrocyte velocity tracking; and (iii) indicator dilution. These are discussed with respect to their assumptions, requirements, and limitations. There is a consensus that medullary blood flow is autoregulated, albeit over a narrower pressure range than is total renal blood flow. When normalized to gram tissue weight, medullary blood flow in the dog is similar to that in the rat, on the order of 1 to 1.5 mL∙min−1∙g−1. This is considerably greater than estimated by the radioiodinated albumin uptake method which has severe conceptual and practical problems. From both theoretical and experimental evidence it ssems that urinary concentrating ability is considerably less sensitive to changes in medullary blood flow than is often assumed.

scholarly journals Two-compartment model of inner medullary vasculature supports dual modes of vasopressin-regulated inner medullary blood flow

2010 ◽  
Vol 299 (1) ◽  
pp. F273-F279 ◽  
Author(s):  
Julie Kim ◽  
Thomas L. Pannabecker
Keyword(s):  
Blood Flow ◽  
Blood Flow Rate ◽  
Outer Zone ◽  
Compartment Model ◽  
Transverse Dimension ◽  
Vessel Diameter ◽  
Functional Coupling ◽  
Vascular Bundles ◽  
Transport Pathways ◽  
Vasa Recta

The outer zone of the renal inner medulla (IM) is spatially partitioned into two distinct interstitial compartments in the transverse dimension. In one compartment (the intercluster region), collecting ducts (CDs) are absent and vascular bundles are present. Ascending vasa recta (AVR) that lie within and ascend through the intercluster region (intercluster AVR are designated AVR2) participate with descending vasa recta (DVR) in classic countercurrent exchange. Direct evidence from former studies suggests that vasopressin binds to V1 receptors on smooth muscle-like pericytes that regulate vessel diameter and blood flow rate in DVR in this compartment. In a second transverse compartment (the intracluster region), DVR are absent and CDs and AVR are present. Many AVR of the intracluster compartment exhibit multiple branching, with formation of many short interconnecting segments (intracluster AVR are designated AVR1). AVR1 are linked together and connect intercluster DVR to AVR2 by way of sparse networks. Vasopressin V2 receptors regulate multiple fluid and solute transport pathways in CDs in the intracluster compartment. Reabsorbate from IMCDs, ascending thin limbs, and prebend segments passes into AVR1 and is conveyed either upward toward DVR and AVR2 of the intercluster region, or is retained within the intracluster region and is conveyed toward higher levels of the intracluster region. Thus variable rates of fluid reabsorption by CDs potentially lead to variable blood flow rates in either compartment. Net flow between the two transverse compartments would be dependent on the degree of structural and functional coupling between intracluster vessels and intercluster vessels. In the outermost IM, AVR1 pass directly from the IM to the outer medulla, bypassing vascular bundles, the primary blood outflow route. Therefore, two defined vascular pathways exist for fluid outflow from the IM. Compartmental partitioning of V1 and V2 receptors may underlie vasopressin-regulated functional compartmentation of IM blood flow.

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