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PubMed1987

Transplacental clearance and blood flows of bovine gravid uterus at several stages of gestation.

Rates of uterine and umbilical blood flows and transplacental clearance of deuterium oxide (D2O) were determined for cows on 137 +/- 1.0 (SE, n = 9), 180 +/- 0.5 (n = 8), 226 +/- 0.4 (n = 9), and 250 +/- 1.8 (n = 5) days of gestation. From days 137 to 250, rates increased 4.5-fold for uterine blood flow, 21-fold for umbilical blood flow, and 14-fold for clearance of D2O. Changes in rates of umbilical blood flow and D2O clearance paralleled increased rates of fetal growth and metabolism, which have previously been reported to occur during the last half of gestation. The regressions of D2O clearance on uterine and umbilical blood flows were significant (P less than 0.01) and explained 94-99% of the variation in placental clearance of D2O. Because the rate of D2O clearance was always less than that of uterine and umbilical blood flows, and because a relatively simple statistical model explained most of the variation in clearance, it was suggested that a concurrent or countercurrent arrangement of maternal and fetal placental microvasculatures is not adequate to explain clearance of highly diffusable substances across the bovine placenta. In addition, a placental exchange diagram of the data showed the existence of severe uneven distribution of maternal and fetal placental blood flows and/or significant shunting of maternal and fetal placental flows away from areas of exchange. Taken together, these data indicate that the placenta of the cow, like those of the sheep and goat, represents a relatively inefficient system of transplacental exchange.

Animals↗
PubMed2003

Conversion of sugarcane bagasse to carboxylic acids using a mixed culture of mesophilic microorganisms.

Using the MixAlco process, biomass can be converted into carboxylic acids, which can be chemically converted into mixed alcohol fuels. This study focused on the use of countercurrent fermentation to anaerobically convert sugarcane bagasse and chicken manure to mixed carboxylic acids using a mixed culture of mesophilic microorganisms from terrestrial and marine sources. Bagasse was pretreated with lime to increase digestibility. The continuum particle distribution model (CPDM) simulated continuous fermentors based on data collected from batch experiments. This model saves considerable time in determining optimum operating conditions. For an 80% bagasse/20% chicken manure fermentation with terrestrial inoculum at a volatile solids loading rate (VSLR) of 7.36 g/(L of liquid d) and a liquid residence time (LRT) of 8.88 d, total carboxylic acid productivity, total acid selectivity, and yield were 2.49 g/(L of liquid d), 0.581 g of total acid/ g of VS digested, and 0.338 g of total acid/g of VS fed, respectively, at a concentration of 18.7 g of total acid/L. At the same VSLR and LRT, fermentation with marine inoculum gave higher total acid productivity, total acid selectivity, and yield than fermentation with terrestrial inoculum. For an 80% bagasse/20% chicken manure fermentation with marine inoculum at a VSLR of 3.83 g/(L of liquid d) and an LRT of 12.1 d, total carboxylic acid productivity, total acid selectivity, and yield were 1.38 g/(L of liquid d), 0.667 g of total acid/g of VS digested, and 0.359 g of total acid/g of VS fed, respectively, at a concentration of 16.2 g of total acid/L.

Animals↗
PubMed1986

Distribution, progression, and recovery of acute formaldehyde-induced inhibition of nasal mucociliary function in F-344 rats.

A previous report of inhalation exposure of F-344 rats to formaldehyde gas, using a whole-body exposure system, described the induction of regional inhibition of nasal mucociliary function, with a clear concentration-response relationship. A head-only exposure system was subsequently developed in order to facilitate the present study of reversibility of acute effects of formaldehyde on the nasal mucociliary apparatus. This study also included an examination of more extensive areas of the nose than those reported in the previous work. Male F-344 rats were exposed to 2 or 15 ppm formaldehyde gas for 10, 20, 45, or 90 min or 6 hr with recovery groups examined 1 hr after the end of the 90-min and 6-hr exposures. No effects were observed in rats exposed to 2 ppm formaldehyde. In rats exposed to 15 ppm, the extent of formaldehyde-induced inhibition of mucociliary function detected in specific regions of the nose was time dependent, with increasing areas of mucostasis and ciliastasis being induced during a 6-hr exposure period. A 1-hr room-air exposure, following exposure to 15 ppm formaldehyde, resulted in marked recovery of mucociliary function, indicating the value of a head-only exposure system for rapid examination of mucociliary function following exposure. Recovery of mucociliary function occurred especially in the more posterior areas of affected regions of the nose. However, in areas of recovery mucus flow rate was reduced compared to unexposed control rates, indicating incomplete recovery of function in these areas. Regions of formaldehyde-induced inhibition of mucociliary function correlated well with the previously reported distribution of formaldehyde-induced nasal squamous cell carcinomas, with the exception of effects on the medial aspect of the maxilloturbinate. These findings were considered to provide further support for the proposal that both regional exposure and local tissue susceptibility may be responsible for the distribution of formaldehyde-induced nasal squamous cell carcinomas. It was also postulated, on the basis of mucus flow patterns derived from control animals in this study, that flow relationships between nasal mucus and inspired air form a countercurrent system which may optimize clearance of inhaled air contaminants.

Animals↗
PubMed1975

[Intrarenal distribution of calcium and magnesium in the dog. Effect of an osmotic diuresis (author's transl)].

1 The determination of Na, Ca, Mg, and K concentrations was performed in four different regions of the dog kidney (cortex, outer medulla, inner medulla, and papilla) during antidiuresis and during an osmotic diuresis. 2 The results show a medullary concentration gradient for calcium. This gradient is much higher than that found for sodium. 3 An inverse concentration gradient from cortex to inner medulla for Mg and K is found. 4 An osmotic diuresis (hypertonic mannitol) decreases the corticomedullary gradient of Na, but does not alter significantly the intrarenal distribution of Ca, Mg and K. 5 These results consistent with an intracellular localization of Mg and K in the renal tissue. It is suggested that the medullary concentration gradient for Ca may be due either to a countercurrent multiplier system similar to that for Na, or to a higher tissular fixation of Ca in the inner medulla and papilla than in the outer medulla and cortex.

Animals↗
PubMed2003

Pharmacokinetics and dosage adaptation of meropenem during continuous venovenous hemodiafiltration in critically ill patients.

Meropenem, a carbapenem broad-spectrum antibiotic, is regularly used in patients undergoing continuous venovenous hemodiafiltration (CVVHDF). Its disposition was studied over one dosage interval in 15 patients under CVVHDF on a steady regimen of 500 or 1000 mg every 8 to 12 hours. Meropenem levels were measured in plasma and filtrate-dialysate by high-performance liquid chromatography (HPLC) with UV detection. The mean CVVHDF flow rates were 7.1 +/- 0.9 L/h for blood (mean +/- SD), 0.5 +/- 0.3 L/h for predilution solution, 1.2 +/- 0.3 L/h for countercurrent dialysate, and 1.8 +/- 0.5 L/h for the total filtrate-dialysate. The pharmacokinetic analysis was based both on a noncompartmental approach and on a four-compartment modeling. The mean (coefficient of variation [CV]) total body clearance, volume of distribution at steady state, and mean residence time were, respectively, 5.0 L/h (46%), 14.3 L (29%), and 4.8 h (36%). The hemodiafiltration clearances calculated from plasma data alone and plasma with filtrate-dialysate data were 1.2 L/h (26%) and 1.6 L/h (39%), respectively. The compartmental model was used to optimize the therapeutic schedule of meropenem, considering reference minimal inhibitory concentration (MIC) of sensitive strains (4 mg/L). The results indicate that two different therapeutic schedules of meropenem are equally applicable to patients receiving CVVHD: either 750 mg tid or 1500 bid.

Acute Kidney Injury↗
PubMed1980

Probabilistic modeling of oxygen transport in brain tissue.

A three-dimensional model of a subregion of the microcirculation was developed. The model employed parallel probabilistic distributions of reaction and convection of oxygen. A Green's function solution to oxygen transport in this system was derived and applied in a series of dynamic solutions on a minicomputer. Results include the transient response of oxygen tension at grid points throughout the system and a correlation of the oxygen demand and surplus supply. Results clearly represent anticipated oxygen partial pressure behavior trends of a countercurrent flow system and in pertinent ways compared favorably with the results obtained by previous investigators. The technique is conservative and employs from 2,000 to 3,600 time steps to achieve precision of seven significant figures. Excellent stability was observed as well. The simulation provides a reliable means to represent the transient response of oxygen partial pressure in a three-dimensional system. Case studies show effects of changing arterial oxygen partial pressure as well as the importance in this system of the convection of dissolved oxygen. It is concluded that the system adequately simulates the anticipated broad profiles of a probabilistically distributed system and provides a rather original insight into neuronal viability through a correlation of oxygen supply and demand. Adaptation of such a complex, dynamic, three-dimensional stimulation to a minicomputer is a step toward simulation of more sophisticated geometries.

Animals↗
PubMed1976

Estimation of blood flow with radioactive tracers.

The techniques of tracer dilution in the circulation, and of tracer uptake by and washout from an orgen, may be described using expressions that are general and are not dependent on specific models such as exponentials. The expressions have been applied to the measurement of cardiac output using impulse and constant rate injection techniques. Further expressions have been given for estimating organ blood flow from inflow/outflow concentration-time curves, and from the distribution of deposited tracer. Some problems with respect to the use of deposition techniques as they are ordinarily applied to the estimation of regional blood flow must be considered, particularly when there are capillary beds in series or where there is countercurrent diffusional shunting of diffusible tracers between inflow and outflow. This review deals with these various aspects of tracer theory as they relate to the measurement of blood flow.

Blood Circulation↗
PubMed1991

[Application of high speed countercurrent chromatography for the isolation of natural products--preparative isolation of taxane-diterpenoids and diterpene alkaloids].

High speed countercurrent chromatography (HSCCC) based on the fundamental principle of liquid-liquid partition is a newly developed technique for isolation. It utilizes a particular combination of coil orientation and planetary motion to produce a unique hydrodynamic phenomenon in the unilateral phase distribution of two immiscible solvents in a coiled column. In recent years HSCCC has gained increasing popularity in the isolation of bioactive natural products. This paper reports its application in the preparative separation of taxol (1), cephalomannine (2) and baccatin-III (3) from the alcoholic extract of Taxus yunnanensis. These 3 ingredients are first obtained from this plant.

Alkaloids↗
PubMed1981

Ultrastructural localization of Na+,K+-ATPase in rat and rabbit kidney medulla.

Na+,K+-ATPase was localized at the ultrastructural level in rat and rabbit kidney medulla. The cytochemical method for the K+-dependent phosphatase component of the enzyme, using p-nitrophenylphosphate (NPP) as substrate, was employed to demonstrate the distribution of Na+, K+-ATPase in tissue-chopped sections from kidneys perfusion-fixed with 1% paraformaldehyde-0.25% glutaraldehyde. In other outer medulla of rat kidney, ascending thick limbs (MATL) were sites of intense K+-dependent NPPase (K+-NPPase) activity, whereas descending thick limbs and collecting tubules were barely reactive. Although descending thin limbs (DTL) of short loop nephrons were unstained, DTL from long loop nephrons in outer medulla were sites of moderate K+-NPPase activity. In rat inner medulla, DTL and ascending thin limbs (ATL) were unreactive for K+-NPPase. In rabbit medulla, only MATL were sites of significant K+-NPPase activity. The specificity of the cytochemical localization of Na+,K+-ATPase at reactive sites in rat and rabbit kidney medulla was demonstrated by K+-dependence of reaction product deposition, localization of reaction product (precipitated phosphate hydrolyzed from NPP) to the cytoplasmic side of basolateral plasma membranes, insensitivity of the reaction to inhibitors of nonspecific alkaline phosphatase, and, in the glycoside-sensitive rabbit kidney, substantial inhibition of staining by ouabain. The observed pattern of distribution of the sodium transport enzyme in kidney medulla is particularly relevant to current models for urine concentration. The presence of substantial Na+,K+-ATPase in MATL is consistent with the putative role of this segment as the driving force for the countercurrent multiplication system in the outer medulla. The absence of significant activity in inner medullary ATL and DTL, however, implies that interstitial solute accumulation in this region probably occurs by passive processes. The localization of significant Na+,K+-ATPase in outer medullary DTL of long loop nephrons in the rat suggests that solute addition in this segment may occur in part by an active salt secretory mechanism that could ultimately contribute to the generation of inner medullary interstitial hypertonicity and urine concentration.

Animals↗
PubMed1975

Diffusion model for fluidized-bed drying.

A sucrose-lactose-starch granulation was used to study particulate motion and attrition in a fluid bed dryer. There is some classification of material in the dryer as drying proceeds; fine particles are dried faster and become less dense, and the less dry but denser large particles show some (although not great) accumulation tendencies in the lower central area. Unlike countercurrent rotary drying, fluid bed drying cannot be accounted for by water diffusion inside the granule as the rate-limiting step. In its place, a model of external water vapor diffusion is proposed and is supported by vapor-concentration curves and by the linear dependence of the rate constants on the linear air velocities. The dried granulation exhibits the same trend as does countercurrent dried material in that larger particles have higher moisture contents than do smaller particles. Quantitative relationships between content of moisture and size were developed and are supported by experimental data. The granulation, upon storage, does not equilibrate, indicating that this type of water distribution is a problem in batch process granulations as well as in the earlier reported case of granulations for continuous production.

Diffusion↗
PubMed1999

Reduced water permeability and altered ultrastructure in thin descending limb of Henle in aquaporin-1 null mice.

It has been controversial whether high water permeability in the thin descending limb of Henle (TDLH) is required for formation of a concentrated urine by the kidney. Freeze-fracture electron microscopy (FFEM) of rat TDLH has shown an exceptionally high density of intramembrane particles (IMPs), which were proposed to consist of tetramers of aquaporin-1 (AQP1) water channels. In this study, transepithelial osmotic water permeability (Pf) was measured in isolated perfused segments (0.5-1 mm) of TDLH in wild-type (+/+), AQP1 heterozygous (+/-), and AQP1 null (-/-) mice. Pf was measured at 37 degrees C using a 100 mM bath-to-lumen osmotic gradient of raffinose, and fluorescein isothiocyanate (FITC)-dextran as the luminal volume marker. Pf was (in cm/s): 0.26 +/- 0.02 ([+/+]; SE, n = 9 tubules), 0.21 +/- 0.01 ([+/-]; n = 12), and 0.031 +/- 0.007 ([-/-]; n = 6) (P < 0.02, [+/+] vs. [+/-]; P < 0.0001, [+/+] vs. [-/-]). FFEM of kidney medulla showed remarkably fewer IMPs in TDLH from (-/-) vs. (+/+) and (+/-) mice. IMP densities were (in microm-2, SD, 5-12 micrographs): 5,880 +/- 238 (+/+); 5,780 +/- 450 (+/-); and 877 +/- 420 (-/-). IMP size distribution analysis revealed mean IMP diameters of 8.4 nm ([+/+] and [+/-]) and 5.2 nm ([-/-]). These results demonstrate that AQP1 is the principal water channel in TDLH and support the view that osmotic equilibration along TDLH by water transport plays a key role in the renal countercurrent concentrating mechanism. The similar Pf and AQP1 expression in TDLH of (+/+) and (+/-) mice was an unexpected finding that probably accounts for the unimpaired urinary concentrating ability in (+/-) mice.

Animals↗
PubMed1987

Capillary grouping in hamster tibials anterior muscles: flow patterns, and physiological significance.

We have used fluorescein-labeled albumin and epifluorescence videomicroscopy to visualize the organization and flow patterns in the capillaries of a postural muscle, the tibialis anterior, in the pentobarbital-anesthetized hamster. Video tape records were made of overlapping microscope fields comprising portions of the muscle up to two millimeters in length and as much as 180 micron in depth. Three-dimensional reconstructions made from these tapes incorporated observations on flow patterns and geometry of the microvessels. The microcirculation in the tibialis was found to be arranged in repeating modules or 'units', consisting of approximately 15 capillaries supplied by a common arteriole and drained by a common venule. The mean distance from arteriole to venule in a unit was 855 +/- 233 micron (X +/- SD) and the mean capillary length was 805 +/- 330 micron. Flow in the capillaries within a unit was almost entirely concurrent, and there was little interchange of blood flow between the capillaries of adjacent units. The flow in capillaries of adjacent units was both countercurrent and concurrent. For each capillary, a count of the adjacent capillaries with concurrent and countercurrent flow was made. The ratio of concurrent to countercurrent flow varied from 2.4 to 4.3, being greatest at the midpoint of the unit and least at the arterial and venous ends. Our observations suggest that the capillaries of striated muscle are arranged in modules which may function as the fundamental control and distribution elements in the microcirculation.

Animals↗
PubMed1996

Immunocytochemical studies of the Na-K-Cl cotransporter of shark kidney.

The Na-K-Cl cotransporter (NKCC or BSC) has been described in numerous secretory and reabsorptive epithelia and is an important part of the mechanism of NaCl reabsorption in both the mammalian and elasmobranch kidneys. We have recently developed a panel of four monoclonal antibodies (MAbs) raised to the 195-kDa Na-K-Cl cotransport protein of the shark rectal gland (sNKCC1), which is expressed along the basolateral plasma membrane of secretory cells in this tissue (29). Here, we report immunologic studies of the Na-K-Cl cotransporter in the kidney of the dogfish shark Squalus acanthias. Western blot analysis of shark renal microsomes using MAbs J3, J7, and J25 identified proteins of approximately 195 and 150 kDa, whereas MAb J4 was not reactive. To define the cellular and subcellular distribution of the cotransport protein, immunofluorescence and immunoelectron microscopy studies were performed on fixed kidneys. Immunofluorescence microscopy on semithin (0.5-micron) cryosections demonstrated that MAbs J3, J7, and J25 intensely stained the apical plasma membrane of all distal tubule segments. Weak staining was also seen along the basolateral membrane of most distal nephrons. Immunoelectron microscopy confirmed this observation and showed that some of these segments were morphologically similar to diluting segments from other species. MAbs also reacted with the brush border and, to a lesser extent, the basolateral membrane of proximal tubules. This study supports the hypothesis that the lateral bundle zone of the elasmobranch kidney functions as a countercurrent exchanger and is consistent with the presence of multiple isoforms of the Na-K-Cl cotransporter in the shark kidney.

Animals↗
PubMed1984

Microvasculature of the avian eye: studies on the eye of the duckling with microcorrosion casting, scanning electron microscopy, and stereology.

The microvasculature of the eye of the duckling was studied with microcorrosion casting, scanning electron microscopy, and stereology. Most blood to the eyeball first passes through the arterial ophthalmic rete mirabile, a complex of small arteries which intermixes with a similar complex of veins (venous ophthalmic rete mirabile) at the ventrotemporal angle of the eye. The present study reveals the ultrastructural anatomy and the compact, three-dimensional arrangement of vessels in this rete, which had been shown by previous investigators to function as a countercurrent heat exchanger. Vessels from this rete include the supraorbital and infraorbital arteries, which supply the eyeball anteriorly, and the ophthalmotemporal artery, which supplies the eyeball posteriorly. The internal ophthalmic and ethmoidal arteries, branches of the cerebral carotid artery, anastomose with the ophthalmotemporal artery posteriorly. Blood is distributed to the eyeball anteriorly by two ring arteries: the iridial ring artery, which circumscribes the iris and which receives blood from the long ciliary and infraorbital arteries; and the more peripheral, ciliary ring artery, which receives blood mostly from the infraorbital and ethmoidal arteries. Within the iris is a dense, freely anastomosing bed of capillaries which extends to the edge of the pupil and then loops back beneath the ciliary body. The vasculature of the ciliary body consists of radially arranged plates of anastomosing capillaries of irregular bore which mimic the contours of that organ, but permit changes in pupil diameter. The present study demonstrates the three-dimensional anatomy of the very dense capillary net of the choriocapillaris deep to the retina and the capillary mass of the pecten, and thus supports the finding of earlier investigators that nutrients diffusing from these structures nourish the avascular retina. The pecten consists of a pleated sheet of freely anastomosing capillaries which protrudes into the vitreous body from near the optic nerve. The choriocapillaris and the pecten are supplied by branches of the ophthalmotemporal artery: the former by numerous short posterior ciliary arteries, the latter by two or three arteries which further divide into one or two smaller vessels for each of its folds. Veins of the choroid layer at the periphery of the anterior surface of the eyeball, and to some extent on its lateral walls, are revealed by the corrosion-casting technique as unusual, flattened vessels of large caliber which lie in closely spaced parallel arrays. The large surface area thus created may function in heat dissipation.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

Determinants of axial osmotic gradients in the differentiating countercurrent system.

The renal medullary countercurrent system differentiates into its final segmental nephron function and geometry during perinatal development. The influence of these changes on the medullary longitudinal osmotic gradient cannot be evaluated by experimental studies. Therefore, a computation analysis using a differential equation model of the renal countercurrent system was applied to quantitate the effect of medullary architecture and solute transport on the concentration profiles for salt and urea in tubules (loop of Henle and collecting duct) and in the central core along the entire medulla during ontogeny. The results indicate that both the changing distribution of loop segments within the medulla and the increase in active salt transport of the individual thick ascending loop determine the magnitude and slope of the axial medullary solute gradients.

Animals↗
PubMed1988

Structural and functional aspects of porcine placental microvasculature.

The microvascular architecture of the pig placenta was studied by serial semithin histological sections for light microscopy, which were compared with scanning electron microscopy of artificially exposed materno-fetal contact surfaces as well as of vessel casts prepared from the maternal, fetal, and combined maternal and fetal sides. The superficial reliefs from the exposed surfaces as well as from the casts are almost identical with the complementary maternal and fetal sides. In order to meet the physiological needs of materno-fetal exchange for the rapidly growing fetuses, these reliefs develop from a simple to a more complex system during pregnancy and can be described as follows: (1) The degree of interlocking increases between the fetal ridges or bulbous protrusions and maternal ridges of different orders separated by maternal troughs of variable depth, most clearly seen on vessel casts. It creates a three-dimensional notch-arrangement, giving strength to the materno-fetal contact area. (2) The structure of precapillary vessels as well as of the meshwork, and the diameter of capillaries of the maternal and fetal sides, adapt during gestation giving a good distribution of oxygenated blood into the maternal capillaries; these, with the development of large prevenous connecting capillaries on the fetal side, favour a high arterio-venous difference of fetal blood O2 pressure. (3) The vascular architecture of endometrial and fetal ridges and troughs develop into a crosscurrent to countercurrent materno-fetal blood interrelationship. Our demonstration of the materno-fetal capillary inter-relationship in the porcine placenta thus shows that the latter is a much more efficient organ for exchange than hitherto assumed.

Animals↗
PubMed2006

Readdressing the issue of thermally significant blood vessels using a countercurrent vessel network.

A physiologically realistic arterio-venous countercurrent vessel network model consisting of ten branching vessel generations, where the diameter of each generation of vessels is smaller than the previous ones, has been created and used to determine the thermal significance of different vessel generations by investigating their ability to exchange thermal energy with the tissue. The temperature distribution in the 3D network (8178 vessels; diameters from 10 to 1000 microm) is obtained by solving the conduction equation in the tissue and the convective energy equation with a specified Nusselt number in the vessels. The sensitivity of the exchange of energy between the vessels and the tissue to changes in the network parameters is studied for two cases; a high temperature thermal therapy case when tissue is heated by a uniformly distributed source term and the network cools the tissue, and a hypothermia related case, when tissue is cooled from the surface and the blood heats the tissue. Results show that first, the relative roles of vessels of different diameters are strongly determined by the inlet temperatures to those vessels (e.g., as affected by changing mass flow rates), and the surrounding tissue temperature, but not by their diameter. Second, changes in the following do not significantly affect the heat transfer rates between tissue and vessels; (a) the ratio of arterial to venous vessel diameter, (b) the diameter reduction coefficient (the ratio of diameters of successive vessel generations), and (c) the Nusselt number. Third, both arteries and veins play significant roles in the exchange of energy between tissue and vessels, with arteries playing a more significant role. These results suggest that the determination of which diameter vessels are thermally important should be performed on a case-by-case, problem dependent basis. And, that in the development of site-specific vessel network models, reasonable predictions of the relative roles of different vessel diameters can be obtained by using any physiologically realistic values of Nusselt number and the diameter reduction coefficient.

Animals↗
PubMed2000

Modeling exchange of plasma proteins between microcirculation and interstitium of the renal medulla.

In the absence of evidence for lymphatics in the inner medulla of the kidney, it has been proposed that plasma proteins are cleared by convection out of the medullary interstitial fluid (ISF) directly into the ascending vasa recta (AVR). To clarify this hypothesis we have developed a mathematical model of the microvascular exchange of fluid, plasma proteins, and small solutes among the descending vasa recta (DVR), the AVR, and the ISF. The model represents the DVR and AVR as limbs of a countercurrent exchange loop separated and surrounded by the ISF. Steady-state exchange of fluid and solute are considered by using conservation and exchange equations. We have used values for parameters based on experimental measurements and investigated the effects of the properties of the vasa recta, the flow, and the gradient of small solutes on the distribution of plasma proteins. Results from the model agree reasonably well with experimental measurements, suggesting that convection may account for the clearance of plasma proteins from the renal medulla maintaining their concentration below that of the AVR.

Animals↗