Hemodialysis (HD) is primarily a diffusion-based transfer of small solutes. Diffusion is dependent on Fick's law. Convective removal, used mainly for removal of excess plasma water by ultrafiltration, also removes larger molecular-weight solutes by the process of solvent drag.
What is the difference between convective removal and hemodialysis?
Hemodialysis (HD) is primarily a diffusion-based transfer of small solutes. Diffusion is dependent on Fick's law. Convective removal, used mainly for removal of excess plasma water by ultrafiltration, also removes larger molecular-weight solutes by the process of solvent drag.
What are the different mechanisms of renal transport?
These mechanisms include the processes of diffusion, osmosis, hydrostatic flow and endo- or exocytosis. Many are associated with specific transport proteins like channels, uniporters, symporter and antiporters. These mechanisms mediate the basic renal functions of reabsorption and secretion.
How are solutes transported in the plasma membrane?
Most organic solutes are transported via the transcellular route. This is usually accomplished by saturable transport systems (TM systems). 3. Most plasma proteins are not filtered (origin of the common statement that the glomerular filtrate is protein free). However, some small proteins, as well as amino acids, are filtered.
How are endogenous organic solutes transported in the plasma?
Many endogenous organic solutes (and many exogenous drugs) in the plasma are transported by the renal tubules. Important metabolites are almost completely reabsorbed while wastes and many exogenous substances are excreted. 2. Most organic solutes are transported via the transcellular route.
What type of transport does hemodialysis use?
diffusion4. Dialysis. Dialysis is a passive process that favors the transport of small molecules across a semipermeable membrane. Since small molecules have high diffusion coefficients, they encounter the membrane more frequently than do large molecules.
Does hemodialysis use active transport?
The dialysis membrane cannot carry out active transport like real kidneys do because it is not a living organ. To maintain homeostasis, the dialysate (liquid) in a real dialysis machine must have the same concentrations of solutes such as glucose and salts as those in normal blood plasma.
Does hemodialysis use diffusion or osmosis?
Dialysis is a process that is like osmosis. Osmosis is the process in which there is a diffusion of a solvent through a semipermeable membrane.
What is the name of the transport mechanism that drags solutes along with fluid?
In different words, convection is the transport of a solute across a membrane along with solvent (by "solvent drag"). The physicist/chemist definition is "Convection is the collective movement of molecules within fluids." This is how middle molecules are cleared during haemofiltration.
Is dialysis active or passive transport?
A final type of passive transport is filtration or dialysis. The pores that are used in biomembranes for passive transport are generally small, and therefore only allow small molecules or ions to diffuse across them.
What is mechanism used in hemodialysis?
Mechanism and technique Hemodialysis utilizes counter current flow, where the dialysate is flowing in the opposite direction to blood flow in the extracorporeal circuit. Counter-current flow maintains the concentration gradient across the membrane at a maximum and increases the efficiency of the dialysis.
What is diffusion hemodialysis?
Diffusion dialysis (DD) is an ion-exchange membrane (IEM) separation process driven by concentration gradient and has been applied for separation and recovery of acid/alkali waste solutions in a cost-effective and environmentally friendly manner.
How does diffusion work during dialysis?
During diffusion, particles in the areas of high concentration move towards the area of low concentration. Picture how a tea bag works - the leaves stay in the bag and the tea enters the hot water. In dialysis, waste in your blood moves towards dialysate, which is a drug solution that has none (or very little) waste.
What is the role of osmosis in dialysis?
Blood is pumped next to a membrane that has dialysis fluid on the other side. Because of osmosis, the water in the blood, and very small molecules of waste, move across the membrane into the dialysis fluid. Eventually the dialysis fluid will remove all of the waste materials it can from the blood.
How does osmosis and diffusion relate to dialysis?
During osmosis, fluid moves from areas of high water concentration to lower water concentration across a semi-permeable membrane until equilibrium. In dialysis, excess fluid moves from blood to the dialysate through a membrane until the fluid level is the same between blood and dialysate.
What is the predominant mechanism of small solute transport in peritoneal dialysis?
Solute transport across the peritoneal membrane occurs by either diffusion or convection. The diffusive transport rate (JD) is proportional to the difference between the solute concentration in blood (Cb) and that in the peritoneal dialysis solution (Cd).
Is convection and solvent drag?
The mechanism of convection may be described as solvent drag: if a pressure gradient exists between the two sides of a semipermeable (porous) membrane, when the molecular dimensions of a solute are such that passage through the membrane is possible, the solute is swept (“dragged”) across the membrane in association ...
Abstract
The immediate goal of renal replacement therapy is to prevent accumulation of toxic solutes in the patient’s tissues by removing them from the blood. Artificial kidneys accomplish this by dialyzing and filtering the blood across semipermeable membranes, taking advantage of the natural forces of diffusion and convection.
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What are the techniques used to remove solutes from the blood?
Extracorporeal techniques for fluid management and solute removal are a mainstay in the treatment of acute and chronic renal failure. Hemodialysis is the removal of small solutes from the blood by diffusion across a semipermeable membrane into a countercurrently flowing dialysate solution. Ultrafiltration is the removal fluid across a porous membrane by the application of a hydrostatic pressure. Solutes of small to middle size are removed by convection. The combination of ultrafiltration with replacement of fluid into the bloodstream constitutes hemofiltration. The two techniques are frequently combined as hemodiafiltration. The biophysical principles of convective and diffusive membrane transport are developed in this chapter, and their application to mathematical models is provided. The composition, performance, and biocompatibility of older and newer synthetic membranes are described. Methods for analysis of mass transfer, such as clearance, in hemodiafilters are presented. Mathematical models of urea kinetics using one and two compartmental models and their application to the dialysis prescription are also developed and presented.
How does renal replacement work?
The immediate goal of renal replacement therapy is to prevent accumulation of toxic solutes in the patient’s tissues by removing them from the blood. Artificial kidneys accomplish this by dialyzing and filtering the blood across semipermeable membranes, taking advantage of the natural forces of diffusion and convection. Native kidneys combine filtration at the glomerulus with selective reabsorption across the tubule. To allow movement of solute in the absence of a concentration gradient and to permit passage of water-soluble solutes through the lipid bilayers of tubular membranes, reabsorption of most solutes requires facilitated pathways. Reabsorption across native kidney tubules is therefore much more solute-specific than the relatively nonselective diffusion and convection across inert membranes of artificial kidneys, and the reabsorptive process works in reverse, conserving desirable solutes instead of removing unwanted solutes.
What is the life support system for end stage renal syndrome?
Hemodialysis provides the life support system for patients suffering from end stage renal syndrome (ESRD). This review presents the historical development of different polymeric materials used for the synthesis of dialysis membranes. Chronologically, cellulose acetate and its derivatives formed the bases of polymeric materials for dialysis membranes, later polyacrylonitrile, polysulfone and other materials were used. The biocompatibility issues of each of these polymers have been discussed along with the clearance of uremic toxins. The acceptability of a particular membrane material for dialysis is not based entirely on the issue of biocompatibility. The uremic toxin flux is an equally important factor when recommending a particular material for use as a dialysis membrane. An ideal material would be one with the highest flux of uremic toxins along with minimum of biological reactions during dialysis. Along with the development of biomaterials, spinning technologies employed for spinning of dialysis grade membranes have also been illustrated.
What are the indications for extracorporeal removal of drugs and toxins?
Indications for extracorporeal removal of drugs and toxins are mostly clinical and include hemodynamic instability; clinical deterioration despite supportive treatment; mental status alteration; and midbrain/brainstem dysfunction resulting in respiratory depression, hypothermia, hypotension, or bradycardia. Further indications are evidence of failure of organ systems; impaired endogenous drug clearance due to cardiac, renal, or hepatic failure; and when a drug or poison can be removed more rapidly compared with endogenous elimination. Hemodialysis and hemofiltration techniques are most effective for the elimination of small molecular size, high water soluble compounds with a low degree of protein-binding, a small volume of distribution, and rapid equilibration of drug between plasma and tissues. Peritoneal dialysis can also be employed as an acute treatment modality for intoxication with water-soluble, small-molecular-weight solutes but should probably be limited to infants, children, and hemodynamically unstable adults intolerant of a blood circuit or anticoagulation. Therapeutic plasma exchange is of clinical utility when blood purification is required for substances with very high molecular weight and/or high degree of protein binding. Hemoperfusion is an absorptive modality which effectively can clear substances that are lipid-soluble or as much as 95% protein-bound. It provides superior drug clearance and is the preferred modality for extraction of theophylline, barbiturates, organophosphates, and many hypnotics/sedatives/tranquilizers.
What is fluid management in hemodialysis?
Fluid management is one of the principal objectives of the hemodialysis (HD) treatment. During the last decade there has been a shift among nephrologists, from removal of uremic toxins to control of fluid overload and preservation of optimal fluid distribution between different body compartments as the prime targets of HD, thus putting “volume first”[1, 2].
How do mathematical models help with fluid transport?
Background Mathematical models are useful tools to predict fluid shifts between body compartments in patients undergoing hemodialysis (HD). The ability of a model to accurately describe the transport of water between cells and interstitium (Jv,ISIC), and the consequent changes in intracellular volume (ICV), is important for a complete assessment of fluid distribution and plasma refilling. In this study, we propose a model describing transport of fluid in the three main body compartments (intracellular, interstitial and vascular), complemented by transport mechanisms for proteins and small solutes. Methods The model was applied to data from 23 patients who underwent standard HD. The substances described in the baseline model were: water, proteins, Na, K, and urea. Small solutes were described with two-compartment kinetics between intracellular and extracellular compartments. Solute transport across the cell membrane took place via passive diffusion and, for Na and K, through the ATPase pump, characterized by the maximum transport rate, JpMAX. From the data we estimated JpMAX and two other parameters linked to transcapillary transport of fluid and protein: the capillary filtration coefficient Lp and its large pores fraction αLP. In an Expanded model one more generic solute was included to evaluate the impact of the number of substances appearing in the equation describing Jv,ISIC. Results In the baseline model, median values (interquartile range) of estimated parameters were: Lp: 11.63 (7.9, 14.2) mL/min/mmHg, αLP: 0.056 (0.050, 0.058), and JpMAX: 5.52 (3.75, 7.54) mmol/min. These values were significantly different from those obtained by the Expanded model: Lp: 8.14 (6.29, 10.01) mL/min/mmHg, αLP: 0.046 (0.038, 0.052), and JpMAX: 16.7 (11.9, 25.2) mmol/min. The relative RMSE (root mean squared error)averaged between all simulated quantities compared to data was 3.9 (3.1, 5.6) %. Conclusions The model was able to accurately reproduce most of the changes observed in HD by tuning only three parameters. While the drop in ICV was overestimated by the model, the difference between simulations and data was less than the measurement error. The biggest change in the estimated parameters in the Expanded model was a marked increase of JpMAX indicating that this parameter is highly sensitive to the number of species modeled, and that the value of JpMAX should be interpreted only in relation to this factor.
What are the substances in the HD model?
The model was applied to data from 23 patients who underwent standard HD. The substances described in the baseline model were: water, proteins, Na, K, and urea. Small solutes were described with two-compartment kinetics between intracellular and extracellular compartments. Solute transport across the cell membrane took place via passive diffusion and, for Na and K, through the ATPase pump, characterized by the maximum transport rate, JpMAX. From the data we estimated JpMAXand two other parameters linked to transcapillary transport of fluid and protein: the capillary filtration coefficient Lpand its large pores fraction αLP. In an Expanded model one more generic solute was included to evaluate the impact of the number of substances appearing in the equation describing Jv,ISIC.
What is the transmembrane water transfer coefficient?
The transmembrane water transfer coefficient, kf, incorporates the cell membrane hydraulic filtration coefficient and the factor R·T, and its value, kf= 0.024 L2·min-1·mmol-1, was adapted from the value used in [16, 17] (see Discussion). In Eq 7, it is also assumed that the reflection coefficient of the cell membrane for all solutes is equal to 1 [36].
What are the three compartments of the vascular system?
The model describes the distribution of fluid across three compartments, vascular (plasma), interstitial, and intracellular (Fig 1). Proteins (represented by a molecule of the size of albumin) exchange only between interstitium and plasma, while the kinetics of ionic solutes (sodium, potassium) and urea—collectively referred to as small solutes–is described with intracellular and extracellular compartments. The underlying assumption is that small solutes traverse the capillary membrane unimpeded, and diffuse so quickly that almost no delay exists between changes of solute’s concentration in plasma and interstitial fluid [14, 16, 17, 25, 26]. The volume of the extracellular space is equal to the sum of plasma and interstitial fluid. The transport of sodium and potassium in the model was based only on mass conservation considerations without taking into account electrodiffusion and the effect of membrane potentials.
Where does transcapillary transport take place?
The transport of water and solutes, including proteins, between plasma and interstitium takes place across the pores of the capillary endothelial membrane: large pores (LP), small pores (SP) and ultrasmall pores (UP), according to the 3-pore model [27, 28].
How are dialysances calculated?
Dialysances were calculated from the concentrations at the inlet and outlet of the dialysate circuit, for each patients [24].
What is N#A model of solute transfer through a functionalized membrane?
•#N#A model of solute transfer through a functionalized membrane is developed.#N#•#N#The model demonstrates the solute concentration profile across the membran e.#N#•#N#The model quantifies the transfer improvement induced by the adsorption layer inside the membran e.#N#•#N#The developed model is validated by comparison with experimental results.
What is hemodialyzer treatment?
Hemodialysis is a life-sustaining treatment that patients undergo when their kidneys malfunction. Even though this technique is constantly being improved for more than four decades it is still one of the major healthcare problems with high mortality and morbidity of the patients. High mortality rates are usually attributed to incomplete removal of the blood toxins during the dialysis treatment ( Dobre et al., 2013, Meyer et al., 2011, Vanholder et al., 2015 ). The treatment provides adequate removal of only the small water soluble molecules, such as urea or creatinine. However, larger solutes, referred to as middle molecules and the protein-bound toxins, have inadequate clearance even after the development of more permeable high-flux hemodialyzers ( Eloot et al., 2012, Luo et al., 2009 ).
What is double layer membrane?
The concept of double layer membranes aiming the improvement of removal of blood toxins is a promising advancement of dialysis treatment. In the present study, we presented the model which allows more in-depth analysis of interplay of three solute removal mechanisms: diffusion, convection, and adsorption. The model demonstrated the solute concentration profile across the membrane and quantified the transfer improvement induced by the adsorption layer inside the membrane. The model was validated via comparison of model predictions with outcome of experimental testing of home-made double layer mixed matrix membranes. The developed model provides an accurate agreement with diffusive and convective removals obtained experimentally for both small and middle sized uremic toxins. Moreover, even a rather simplistic approach in the modelling of adsorptive removal of toxins (first-order reaction) provides a possibility to evaluate the amount of adsorbed species at the early stages of treatment process. The developed model may be further applied in the optimization of double layer membrane properties and the process conditions.