Kidney Dialysis Membranes and Channels (Interactively view a molecule in this section!) Size and Shape Diffusion and Concentration Gradients (Note: this section includes an animation.) Dynamic Equilibrium Membranes and Concentration Gradients in the Kidneys Artificial Membranes and Dialysis (Interactively view a molecule in this section!) Many Different Molecules: Maintaining the Body's Chemistry In order for blood to perform its essential functions of bringing nutrients and oxygen to the cells of the body, and carrying waste materials away from those cells, the chemical composition of the blood must be carefully controlled. Blood contains particles of many different sizes and types, including cells, proteins, dissolved ions, and organic waste products. Some of these particles, such as proteins like hemoglobin , are essential for the body. Others, such as urea (a waste product from protein metabolism), must be removed from the blood or they will accumulate and interfere with normal metabolic processes. Still other particles, including many of the simple ions dissolved in the blood, are required by the body in certain concentrations that must be tightly regulated, especially when the intake of these chemicals varies. The body has many different means of controlling the chemical composition of the blood.

For instance, you learned in the " Iron Use and Storage in the Body: Ferritin and Molecular Representations " tutorial that the ferritin protein can help to control the amount of free iron in the blood. As you will discover in the tutorial entitled, " Blood, Sweat, and Buffers: pH Regulation During Exercise ", buffers dissolved in the blood can help regulate the blood's pH. But the largest responsibility for maintaining the chemistry of the blood falls to the kidneys, a pair of organs located just behind the lining of the abdominal cavity. It is the job of the kidneys to remove the harmful particles from the blood and to regulate the blood's ionic concentrations, while keeping the essential particles in the blood (Figure 1). Figure 1 This is a schematic diagram illustrating the kidneys' ability to separate particles in the blood in order to maintain optimal body chemistry. Blood enters the kidney through the renal artery. In the kidney, the blood undergoes filtration and dialysis to separate the particles that will be removed from the body (through the ureter to the bladder) from those that will be returned to the circulating blood (through the renal vein). The kidneys meet these challenges through a remarkably elegant system.

Essentially, kidneys act like dialysis units for blood, making use of the different sizes of the particles and specially-maintained concentration gradients. Blood passes through the membrane-lined tubules of the kidney, which are analogous to the dialysis bags used in this Experiment. Particles that can pass through the membrane pass out of the tubules by diffusion, thus separating the particles that remain in the blood from those that will be removed from the blood and excreted. The dialysis mechanism used by the kidneys allows them to function effectively over a very wide range of conditions. For example, sodium intake can vary from one tenth to ten times the average consumption, with only minimal fluctuations in blood-plasma sodium concentrations. Even when the kidneys are severely damaged, the kidneys can still effectively maintain the body's chemistry as long as at least ten percent of their functional units are working. Nonetheless, damage to the kidneys can cause the functional capacity to drop below this level, and fatal illness will develop unless an artificial system is employed to perform the work of the kidneys. Structure and Function of the Kidneys: Overview The kidneys have three basic mechanisms for separating the various components of the blood: filtration, reabsorption, and secretion.

These three processes occur in the nephron (Figure 2), which is the most basic functional unit of the kidney. Each kidney contains approximately one million of these functional units. The nephron contains a cluster of blood vessels known as the glomerulus, surrounded by the hollow Bowman's capsule. The glomerulus and Bowman's capsule together are known as the renal corpuscle. Bowman's capsule leads into a membrane-enclosed, U-shaped tubule that empties into a collecting duct. The collecting ducts from the various nephrons merge together, and ultimately empty into the bladder. Nephron Segment This table summarizes the essential functions of the major segments of the nephron. Renal Corpuscle Blood first enters the kidney through the renal artery (see Figure 1), which branches into a network of tiny blood vessels called arterioles. These arterioles then carry the blood into the tiny blood vessels of the glomerulus. It is here, in the renal corpuscle, where filtration occurs. The glomerulus filters proteins and cells, which are too large to pass through the membrane channels of this specialized component, from the blood.

These large particles remain in the blood vessels of the glomerulus, which join with other blood vessels so that the proteins remain circulating in the blood throughout the body. The small particles (e.g., ions, sugars, and ammonia) pass through the membranes of the glomerulus into Bowman's capsule. These smaller components then enter the membrane-enclosed tubule in essentially the same concentrations as they have in the blood. Hence, the fluid entering the tubule is identical to the blood, except that it contains no proteins or cells. Tubule The tubule functions as a dialysis unit, in which the fluid inside the tubule is the internal solution and the blood (in capillaries surrounding the tubule) acts as the external solution. Particles may pass through the membrane and return to the blood stream in the process known as reabsorption, which is analogous to the movement of particles from the internal to the external solution in the dialysis experiment you performed in lab. The reabsorption of many blood components is regulated physiologically, as discussed below.

Alternatively, particles may pass through the membrane from the blood into this tubule in the process known as secretion, which is analogous to the movement of particles from the external solution into the dialysis bag in the experiment you performed in lab. The most important particles that are secreted from the blood back into the tubules are H+ and K+ ions, as well as organic ions from foreign chemicals or the natural by-products of the body's metabolism. Collecting Duct The blood components that remain in the nephron when the fluid reaches the collecting duct are excreted from the body.The collecting duct from one nephron meets up with many others to feed into the ureter. The ureters (one from each kidney) enter the bladder, which leads to the urethra, where the liquid waste is excreted from the body. Hence, the material that is filtered and secreted from the blood into the tubule, less the amount that is reabsorbed into the blood, is ultimately excreted from the body. The localization of each of these processes within specific components of the nephron is summarized in Table 1, above.

Membrane Channels From the overview of kidney function above, it is clear that blood components (e.g., water, ions, sugars) must be able to pass between the nephron tubules and the blood-filled capillaries surrounding them. But recall from the Introduction to this experiment (in the lab manual) that phospholipid-bilayer membranes are not permeable to polar molecules, because the interior lipid region of the membrane is nonpolar. Thus, the polar components of blood could not cross the membranes surrounding the tubules (Figure 3), unless these membranes contained special channels to allow the passage of polar species. Figure 3 This is a schematic diagram of a segment of a nephron tubule with no protein channels (unlike a real tubule segment, which contains channels) in the phospholipid-bilayer membrane surrounding the tubule, shown as a lengthwise slice through the tubule segment. Polar molecules (green) cannot travel out of the tubule to the blood in the capillaries, because they are insoluble in the hydrophobic (nonpolar) lipid interior of the membrane.

To permit passage of polar and charged species between the capillaries and the nephron, the membrane must have protein channels embedded in it, as discussed below. Phospholipid-bilayer membranes are discussed further in the Introduction to the experiment in the lab manual. Note: For simplicity, the tubule is depicted here as being enclosed by a single membrane. In fact, the tubule and capillaries are lined with cells that are surrounded by membranes. Thus, a particle must travel across several membranes in order to move between the interior of the tubule and the blood-containing capillaries. This figure is not drawn to scale. The channels required to allow the passage of polar blood components are formed by proteins embedded in the phospholipid-bilayer membrane (Figure 4). Proteins that form channels in the membrane typically have membrane-spanning cylindrical shapes: there is a hydrophobic surface that can interact with the tail region of the phospholipid-bilayer membrane and a hollow internal core that forms the pore. These proteins form a "tunnel" from the aqueous phase on one side of the membrane to the aqueous phase on the other side of the membrane.

The size of the tunnel determines the size of the particles that will be able to pass through the channel. Figure 4 This is a CPK representation of a potassium channel embedded in a schematic phospholipid-bilayer membrane. (The pale yellow circles represent the polar head groups and the gray lines represent the nonpolar tails.) This channel is composed of four polypeptide chains (shown in different colors) that span the width of the membrane, with a hollow space through which potassium ions may pass (like a tunnel). Some of the amino acids have been removed to reveal the space occupied by the potassium ion as it crosses the membrane from the aqueous phase on one side to the aqueous phase on the other side. Note: The coordinates for this protein were determined by x-ray crystallography, and the protein component of this image was rendered using SwissPDB Viewer and POV-Ray (see References ). If the internal core of the protein channel is lined with hydrophilic amino-acid residues, then the channel allows passage of polar or charged particles between the two aqueous sides of the membrane.

Figure 5 shows a representative ion channel, with hydrophilic residues lining the internal core and hydrophobic residues lining the regions of the protein that contact the lipid tails in the interior of the membrane. Figure 5 This is a view through the opening of the same potassium channel shown in Figure 4. Notice that the inner core is lined with hydrophilic amino-acid residues (blue) that interact favorably with the charge on the ion (yellow). The outer areas of the channel contain hydrophobic amino-acid residues (plum), which interact favorably with the hydrophobic lipids in the membrane. Note: The coordinates for this protein were determined by x-ray crystallography, and the image was rendered using SwissPDB Viewer and POV-Ray (see References ). Note: To view the molecule interactively, please use Jmol , and click on the button to the left. These channels may be left open continuously, or they may be opened and closed by elaborate cellular gating mechanisms, as we will see below for three representative cases in the kidneys. In either case, passage of particles through the membrane is dictated by the size, shape, and polarity of the channel.

Questions on Membrane Channels A certain channel has been isolated from a cell membrane, that allows cyanide (CN-) ions, but not nitrate (NO3-) ions, to cross a phospholipid-bilayer membrane. Briefly, explain how the structure of the channel might allow the channel to discriminate between these two anions. HINT: You may find it helpful to consult the three-dimensional CPK representations in the on-line table of polyatomic ions . Which, if any, of the following ions would you expect to be able to pass through this channel: bicarbonate (HCO3-), hydroxide (OH-), or permanganate (MnO4-)? Briefly, explain your reasoning. HINT: You may find it helpful to consult the three-dimensional CPK representations in the on-line table of polyatomic ions . Neglecting size considerations, tell whether you expect methane (CH4) to pass through the K+ channel depicted in Figure 4. Briefly, explain your reasoning. Diffusion and Concentration Gradients The direction of the passage of particles through the channel is also dependent on concentration gradients. A concentration gradient exists whenever a concentrated solution is in contact with a less concentrated solution.

Because the solutions are in contact, particles may flow between the two solutions (or between two regions of the same solution) by the process known as diffusion. Diffusion is a term used to describe the mixing of two different substances that are placed in contact. The substances may be gases, liquids, or solids. Diffusion is the migrating by random motion of these different particles.Although particles move in every direction, there is a net flow from the more concentrated solution to the less concentrated solution ("down the concentration gradient"). As the number of particles in the more concentrated solution diminishes and the number of particles in the less concentrated increases, the difference in concentration between the two solutions decreases. Hence, the concentration gradient is said to get smaller (Movie 1). All else being equal, the concentrations of the solutions change more rapidly when the difference in their concentrations is greater. This diffusion process continues until the concentrations of the two solutions are equal. This state is known as dynamic equilibrium. When the two solutions are in dynamic equilibrium, particles continue to move between the two solutions, but there is no net flow in any one direction, i.e., the concentrations do not change.

Figure 6 The graph at the top of this figure plots the time course of the changes in concentration that occur after a solution (A) with a 1.0 M concentration of some particle is placed in contact (via a semipermeable membrane) with another solution with a 0.0 M concentration of the particle. The blue line represents the concentration of the particle in solution A, and the magenta line represents the concentration of the particle in solution B. Over time, the concentrations become equal and no longer change; at this point, the solutions are said to be in dynamic equilibrium. The schematic at the bottom shows the two solutions approximately 2 seconds after the solutions are placed in contact with one another. To view a QuickTime movie showing the movement of the particles by diffusion between these two solutions, please click on the pink button below. Click the blue button below to download QuickTime 4.0 to view the movie. In biological systems such as the kidney, the two solutions are often separated by a membrane. Protein channels in the membrane allow particles to cross the membrane, flowing "down the concentration gradient" until equilibrium is reached.

Sometimes these channels may be closed, so that particles will not travel across the membrane, even if there is a strong concentration gradient. (In effect, the two solutions are no longer in contact when the channels are closed.) In other cases, the proteins in the membranes act like "pumps," using energy to move particles "against the concentration gradient" (i.e., so the more concentrated solution becomes even more concentrated); examples are the light-driven proton pump that occurs in the photosynthetic thylakoid membrane discussed in the introduction to the Experiment, the proton pumps used in the synthesis of ATP, the body's energy currency (which you will encounter in the tutorial entitled " Energy for the Body: Oxidative Phosphorylation "), and the sodium pumps discussed below. Questions on Diffusion and Concentration Gradients Look at Figure 6, and the movie showing the diffusion of particles between two solutions, separated by a membrane. At what point is the rate of change in the concentrations of the two solutions greatest? Briefly, explain why the rate of concentration change is greatest at this point.

A solution of 0.10 M NaCl is separated from another solution of 0.10 M NaCl by a membrane that is permeable to Na+ and Cl- ions. Does diffusion occur across the membrane? Briefly, explain your answer. Does the concentration of Na+ or Cl- change in neither, either, or both of the two solutions? Briefly, explain your answer. Membranes and Concentration Gradients in the Kidneys How do the kidneys actually filter the blood to remove the necessary particles in the proper amounts? Each component of the nephron contains specialized semipermeable membranes that filter molecules and maintain tightly-regulated concentration gradients. The selectivity of the membrane acts in conjunction with the concentration gradients to perform the dialysis functions of the kidney. Lipid-soluble substances can easily pass through the phospholipid membrane, and so these substances tend to be readily reabsorbed into the blood, even without protein channels. This can be a problem, because many drugs and toxins, such as the pesticide DDT, are lipid-soluble, and hence are reabsorbed into the blood. Thus, it is very difficult to remove these toxins.

Most of the components of the blood, however, are polar or charged and hence require protein channels to cross the membrane (i.e., through the hydrophobic membrane interior). The channels in the nephrons are specialized to allow only the passage of particular types of particles, based on size, shape, and charge interactions with the amino acids lining the channel interior. The number and regulation of these specialized channels allow the kidneys to control the amount of each polar (or charged) species in the blood that is excreted. Most waste products undergo only partial reabsorption, so that large amounts of the substance remain in the tubule and are thus removed from the body in the urine. In contrast, useful plasma components, such as water, nutrients, and inorganic ions, are reabsorbed completely or nearly completely. Three Representative Cases To demonstrate how the specialized membranes of the kidneys work to maintain the blood's chemistry properly, we shall consider three different blood-plasma components (Na+, H2O, and urea), and how the flow of each component between the nephron tubule and the surrounding blood-containing capillaries is controlled.

Sodium Reabsorption (Using "Pumps"; Movement of Particles "Against" a Concentration Gradient) Certain segments of the nephron tubule contain proteins that act as pumps for sodium ions. These pumps use energy from the body to pump sodium ions out of the tubule into the blood (Figure 7). In this manner, Na+ is actively reabsorbed into the blood. As more and more ions are pumped out of the tubule, a concentration gradient builds up, with greater Na+ concentration outside the tubule than inside the tubule. However, because this reabsorption is achieved by active pumping, rather than passive diffusion, sodium ions continue to leave the tubule. The amount of sodium ions that are reabsorbed can be controlled by the hormone aldosterone. When large quantities of aldosterone are present, sodium reabsorption into the blood is enhanced, and so very little sodium is excreted from the body. When aldosterone levels are low, the pumps are less active, so more sodium remains in the tubules and is excreted.

Hence, the body can maintain the optimal blood concentrations of sodium ions by secreting aldosterone in response to low sodium levels or decreasing aldosterone secretion in response to high sodium levels. Figure 7 This schematic diagram shows the reabsorption of Na+ ions (pink) and water (blue). Na+ crosses the tubular membrane into the blood outside the tubule by a pump, against the concentration gradient. H2O crosses the tubular membrane into the blood outside the tubule by passive diffusion through a channel, down the concentration gradient. Water Reabsorption (Using "Channels"; Movement of Particles "Down" (With) a Concentration Gradient) The active pumping of sodium ions out of the tubule and into the blood causes the water concentration inside the tubule to increase (since the number of water molecules is the same, but the total number of particles (Na+ and H2O) is smaller), while the water concentration in the blood decreases. Hence, a water-concentration gradient is established. Portions of the tubular membrane are impermeable to water, but other portions contain hydrophilic channels through which water can flow.

Water exits the tubule and enters the blood through these hydrophilic (polar) channels by passive diffusion down the concentration gradient (Figure 7). In the collecting duct (see Figure 2), the permeability of the membrane is subject to being altered in response to the hormone vasopresin, also known as antidiuretic hormone (ADH). When the body needs to retain water, as in dehydration situations, the concentration of ADH increases, and the high ADH level causes the water-permeability of these membranes to be great. Therefore, large amounts of water are reabsorbed into the blood, and only a little water will be excreted in the urine. However, when the body has plenty of water, the level of ADH drops, causing this portion of the membrane to become relatively impermeable to water. In this case, a larger amount of water remains in the nephron (in the collecting duct) to be excreted. Urea Reabsorption (Using "Channels"; Movement of Particles "Down" (With) a Concentration Gradient) Urea is a waste product formed in the liver during the metabolic breakdown of proteins.

The body does not use urea, and so the kidney's aim is to remove this metabolite through the blood. As the glomerular filtrate enters the tubule, it is rich in urea, because the urea freely passes through the membranes of the glomerulus. Although it might seem as though all of this urea would thus be excreted in the urine, in fact only about half of it is. The tubular membranes are freely permeable to urea. Water reabsorption raises the concentration of urea inside the tubules, since the urea in the tubules is now diluted with less water. Hence, urea will flow down the concentration gradient, out of the tubules and into the surrounding blood-containing capillaries. As more urea exits the tubules, the difference in urea concentration between the tubules and the capillaries decreases, until the two solutions are at equilibrium. Then, although urea may continue to pass between the fluids, there is no longer any net flow of urea out of the tubules. The remaining urea in the tubules will be excreted from the body.

Questions on Membranes and Concentration Gradients in the Kidneys Suppose that the sodium concentration in your body is neither unusually high nor unusually low, and you drink a large quantity of pure water. Based on the changes in the concentrations of sodium and water in the blood that would result from drinking so much water, predict what will happen to the levels of aldosterone and ADH in the body. Briefly, explain your answer. What effect will these hormonal changes have on sodium and water reabsorption (i.e., how will the amount of these substances excreted through the bladder be affected by drinking a large amount of water)? Briefly, explain why drugs consisting of nonpolar molecules are more difficult for the kidneys to remove from the body than drugs consisting of polar molecules. Artificial Kidney Dialysis When Kidneys Do Not Work When the kidneys do not function properly, dialysis must be performed artificially. Without this artificial kidney dialysis, toxic wastes build up in the blood and tissues, and cannot be filtered out by the ailing kidneys. This condition is known as uremia, which means literally "urine in the blood." Eventually this waste build-up leads to death.

Artificial Membranes The artificial kidney uses cellulose membranes in place of the phospholipid-bilayer membranes used by real kidneys to separate the components of blood. This cellulose membrane is the same type of membrane that you used in this experiment. Cellulose is a polymer of glucose molecules that form long, straight chains (Figure 8). Parallel chains form linkages with one another by hydrogen bonding to make strong fibers. These fibers in turn interact to form the strong, sheet-like structure of the membrane. Figure 8 This is a two-dimensional ChemDraw representation of a cellulose chain (polymer strand). One of the glucose units is shown in blue. Note: To view the molecule interactively, please use Jmol , and click on the button to the left. The arrangement of the cellulose fibers may contain gaps, creating tunnels through the membrane (Figure 9). These form the pores through which particles may pass from one side of the membrane to the other. The size of the gaps determines the size of the particles that will be able to pass through the membrane (i.e., the molecular weight cut-off, as described in the Introduction to the experiment in the lab manual).

Figure 9 This is a CPK representation of a cellulose membrane. Each cellulose fiber is colored to show the interactions of the fibers to form a sheetlike structure. Note the gaps between fibers that form pores in the membrane. Note: The coordinates for this model were determined using molecular-mechanics calculations, and the image was rendered using the Insight II molecular-modeling system from Molecular Simulations, Inc. (see References ). Types of Artificial Kidney Dialysis Two types of artificial kidney dialysis are used clinically. Hemodialysis uses a cellulose-membrane tube that is immersed in a large volume of fluid. The blood is pumped through this tubing, and then back into the patient's vein. The membrane has a molecular-weight cut-off that will allow most solutes in the blood to pass out of the tubing but retain the proteins and cells. The external solution in which the tubing is immersed is a salt solution with ionic concentrations near or slightly lower than the desired concentrations in the blood. Recall that if the external concentration of a particular species is lower than the internal concentration, then that species will pass through the cellulose membrane by diffusion into the external solution. In this manner, excess substances in the blood are removed from the body.

To maintain the blood's concentration of a species, the external solution is made to have the same concentration of that species as the blood. In such a case, the two solutions are in dynamic equilibrium, and so the blood's concentration does not change. Peritoneal dialysis does not use an artificial membrane, but rather uses the lining of the patient's abdominal cavity, known as the peritoneum, as a dialysis membrane. Fluid is injected into the abdominal cavity, and solutions diffuse from the blood into this fluid. After several hours, the fluid is removed with a needle and replaced with new fluid. The patient is free to perform normal activities between fluid changings. Thus, artificial kidney dialysis uses the same chemical principles that are used naturally in the kidneys to maintain the chemical composition of the blood. Diffusion across semipermeable membranes, polarity, and concentration gradients are central to the dialysis process for both natural and artificial kidneys. Questions on Artificial Kidney Dialysis Why is a salt solution, rather than pure water, used as the external solution for artificial kidney dialysis? Are the pores formed by the cellulose fibers best suited for allowing the passage of polar or nonpolar molecules?

Briefly, explain your answer in terms of the functional groups of the cellulose polymer. Summary Blood contains particles of many different sizes, shapes, and polarity. Some of these particles (e.g., proteins) are essential for the body; some (e.g., urea) must be removed from the blood and the body; others (e.g., many ions) must be maintained at certain concentrations. The body has many different methods for controlling the composition of the blood (including hemoglobin , ferritin , and buffers , which are discussed in other tutorials), but the kidneys have the primary responsibility for controlling the blood's chemical composition. By filtration, reabsorption, and secretion mechanisms, the kidneys separated and regulate the components of the blood. Some of these components (e.g., proteins) are filtered from the fluid entering the tubule at the glomerulus, and remain in the blood. Other particles (e.g., water, ions, and sugar) are reabsorbed by the blood or secreted from the blood to maintain the proper concentrations; these processes occur while the fluid is flowing through the tubule.

Any blood components that remain in the nephron when the fluid reaches the collecting duct (e.g., waste products such as urea) are excreted from the body. The reabsorption and secretion of the blood components depend on the ability of these blood components to cross the nonpolar interior of the membrane surrounding the nephron tubule. The polar blood components can only pass through the membrane via special protein channels. These protein channels in the membrane are polar on the inside, to allow the passage of polar or charged particles through the membrane, and nonpolar on the outside, to interact with the nonpolar membrane interior. The size of the channel can determine which polar or charged particles will be able to cross the membrane through the channel. The concentrations of the blood components are maintained by diffusion through the membrane (via the protein channels if the component is polar) and concentration gradients. Depending on the component, the concentration is maintained by passive diffusion (going "down" (with) the gradient), or by pumping (going "against" the gradient). Hence, the ability of the kidneys to remove harmful particles from the blood, and to regulate the concentration of other particles in the blood, depends on the chemical concepts of diffusion, polarity, and concentration gradients.

Additional Links: To simulate Haemodialysis Machine (Theory) : Biomedical and Signal processing Laboratory : Biotechnology and Biomedical Engineering : COE PUNE Virtual Lab Theory Haemo is the Greek word for blood. Dialysis means a filtering process. Haemodialysis (HD) means the process of filtering blood. In haemodialysis, the filtering process takes place in a machine outside the body. Haemodialysis is a method for removing waste products such as creatinine and urea, as well as free water from the blood when the kidneys are in renal failure. When the kidneys do not work well, dialysis is needed to remove extra fluid and waste products from the body. Hemodialysis is a type of dialysis that uses a machine with an artificial filter to remove wastes and extra fluids from the blood. This treatment also helps control the chemical balance in the body and blood pressure. Each treatment takes about 4 hours and is done three times each week. Kidney is one of the important organs to remove waste from the body such as potassium and urea, as well as free water from the blood. Under the healthy conditions, a kidney removes waste products from the blood and also removes excess fluid in the form of urine.

Dysfunctioning kidney could loss these functions which is a dangerous situation because this may also prove fatal for a human being. Dialysis is a renal replacement for kidney dysfunction. Blood flow The blood pump takes and returns the blood from the patient via the arterial and venous needles respectively. The blood is confined to the disposable plastic tubing and doesn't come in contact with any part of the machine. The blood coming from the pump flows to the dialyzer and the blood that leaves the dialyzer returns to the patient through the venous needle. The pump speed, and the resulting blood flow rate, is adjustable from zero to about 600 cc/min. While pump speed is controlled, blood flow rate is displayed. These two usually go hand in hand, but not always. The blood flow rate is calculated based on the pump RPM (revolution per minute), and the diameter of the tube. The calculated blood flow is displayed. The control doesn't check the real blood flow. It displays blood flow even if the pump turns without the plastic tube installed. The real blood flow may differ from the display because of: · Internal reverse leak; the rollers never squeeze the tube completely so not all the blood is pushed forward, a little goes backward.

· High arterial and/or venous pressure reduces the ability of the pump to suck or to deliver. · Any speed above some low value reduces gradually the blood flow. For a display of 600 cc/min the actual blood flow is 500 or less. In addition to the inherent features of the pump, a human error that may reduce blood flow is the way the plastic tube is installed in the pump. If the plastic tube inside the pump is too tight it cannot expand to its full volume, so the suction is reduced and the pump delivers less blood flow than the display indicates. The blood pressure is measured at both when it taken out from the limb and also when it is returned. Arterial & Venous Pressure These pressures are the result of the blood forced to flow through the plastic tubing, the dialyzer, and the needles. Arterial pressure When the blood flow rate increases the arterial pressure becomes more negative. If it is too negative the red blood cells may break down. This is called hemolysis. To be on the safe side never let the arterial pressure go below -220 mmHg. If it goes below that the pump speed should be reduced.

If your blood count is low you may notice that the pump speed should be reduced as your blood count goes up. Blood flow rate The effect of blood flow is easy to follow - any change in pump speed is immediately reflected on the pressure displays. The higher the flow the higher the pressure. Dialysis Process Concerning the blood, dialysis performs 2 different functions that are normally done by healthy kidneys: 2. Removing waste like urea, and excess electrolytes (chemicals) like potassium, magnesium, sodium, etc. The dialysis is performed inside the dialyzer, which is a plastic cylinder, in which the blood enters from the top, flows through thousands of extremely thin hollow fibers and leaves from the bottom. At the same time the dialysate enters from the bottom, flows around and in between the fibers, and leaves from the top. The fibers are semi permeable membranes, that is, smaller molecules in the blood stream can pass through them into the dialysate and bigger molecules as well as blood cells cannot. The dialysate is a water-based solution and its purpose is to absorb from the blood all that should be removed and nothing else.

Wastes and electrolytes move from the blood into the dialysate because their concentration in the blood is higher. This process is called diffusion. The dialysate flow ensures that fresh dialysate is present at all times so that the dialysate doesn't become saturated and the process never ends. Fluid is removed from the blood in the same way the kidneys do it - by blood pressure. This process is called Ultra Filtration (UF) and is similar to Reverse Osmosis (RO). With RO, the membrane pore size is very small and allows only water to pass through the membranes. In UF, the membrane pore size is larger, allowing some bigger molecules to pass through the pores with the water. The rate at which fluid is removed from the blood is called UFR (UF Rate). There is higher pressure in the blood passing through the dialyzer and lower pressure in the dialysate. This pressure difference is called TMP (Trans Membrane Pressure). The higher the TMP the higher the UFR. As you can see, the dialyzer is doing the job of dialysis.

The rest of the machine takes care of supplying the blood and the dialysate to the dialyzer, controls the flow and the pressures, and provides visual indication and alarm when something goes wrong. The dialysate is a water-based solution. With all contemporary machines, the dialysate concentrate is fed into the machine, water is fed from a water tank and a marvelous pump mixes them to the desired concentration.. · Calcium (Ca) can be 3.5, 2.5, 2 mEq/L, depending upon the patient's calcium. (US units; Ca 3.5 mEq/L equal 1.75 mmol/L). · Potassium (K) can be 4, 3, 2, 1 mmol/L, or zero (free-K). (US units, used internationally). 4 is for patients that make a significant amount of urine and should not clear any potassium from their system. 3 is for patients that make some urine, 2 is for patients w/o any kidney function, 1 is used occasionally for patients with high potassium when needed.

Free-K is rarely used and only in extreme cases of high potassium (like after surgery), and should be used only for an hour or so at the beginning of the treatment (to avoid removing too much potassium). Dialysate flow The dialysate pump, in addition to mixing the concentrate with water, moves the dialysate through the dialyzer. This pump has adjustable flow rate. For regular dialysis at blood flow up to 250 cc/min the dialysate flow should be 500 cc/min. For more efficient dialysis, called high flux, the blood flow is 400-500 cc/min and the dialysate flow is 800 cc/min. In addition to the controlling the flow, this multi-function pump controls the pressure of the dialysate as to achieve the desired TMP and hence the desired UFR. Blood Leak Blood may leak inside the dialyzer because of ruptured fibers. This may happen even with a new dialyzer. There could be a minor leak, which is invisible, or a major leak in which blood can be seen in the dialyzer. In both cases the machine stops and sets the alarm.

To resume dialysis the dialyzer should be replaced. Heparin Pump This pump delivers heparin to the blood tubing. The pump is a regular syringe that is pushed by a controlled motor. On all machines the rate can be adjusted. Usually an initial dose of heparin is given when the treatment begins, then the pump delivers heparin until one hour before the end. All patients need continuous heparin. In case of a patient after surgery or another kind of bleeding, heparin is used sparingly or not at all. To avoid clotting in the dialyzer it is rinsed with 100 cc saline every hour or so. In spite of that, the blood in the dialyzer may clot, but unless the patient has a very low blood count no treatment is required (of course the heparin dose should be increased when possible). Temperature To make it more convenient for the patient, the dialysate temperature can be set to a desired value, so the blood returning to the patient will not be too cold. For most patients a setting of 37 C (98.6 F) is ok. Treatment Time Each treatment takes about 4 hours and is done three times each week.

Moreover,depending upon the patient’s clinical history the treatment may vary. What is hemodialysis? - DaVita Email Page What is hemodialysis? While healthy kidneys have several functions in the body, the most well-known job is to produce urine. When kidney function goes below 10% to 15% kidneys are no longer able to filter the blood and make urine. This causes toxins to build up in the body along with excess fluid. Fortunately, we live in a time when there are treatments and medicines that can replace the functions of the kidneys and keep the body alive. One type of renal replacement therapy — meaning a treatment that replaces kidney function — is hemodialysis. Hemodialysis is a therapy that filters waste, removes extra fluid and balances electrolytes (sodium, potassium, bicarbonate, chloride, calcium, magnesium and phosphate). How is hemodialysis done? In hemodialysis, blood is removed from the body and filtered through a man-made membrane called a dialyzer , or artificial kidney, and then the filtered blood is returned to the body.

The average person has about 10 to 12 pints of blood; during dialysis only one pint (about two cups) is outside of the body at a time. To perform hemodialysis there needs to be an access created to get the blood from the body to the dialyzer and back to the body. There are three access types for hemodialysis: arteriovenous (AV) fistula, AV graft and central venous catheter. The AV fistula is the vascular access most recommended by the dialysis community; however, you and your doctor will decide which access is best for you. When a patient goes to hemodialysis, a nurse or technician will check vital signs and get the patient’s weight. The weight gain will tell how much excess fluid the patient has to have removed during the treatment. The patient is then “put on the machine”. Patient with a vascular access (AV fistula or AV graft) will get two needle sticks in their access; one needle takes blood out of the body, the other needle puts it back. Patients with a central venous catheter will have the two tubes from their access connected to the blood tubes that lead to the dialyzer and back to the body.

Once the patient is “put on the machine”, the dialysis machine is programmed and then treatment begins. Blood never actually goes through the dialysis machine . The dialysis machine is like a big computer and a pump. It keeps track of blood flow, blood pressure, how much fluid is removed and other vital information. It mixes the dialysate , or dialysis solution, which is the fluid bath that goes into the dialyzer. This fluid helps pull toxins from the blood, and then the bath goes down the drain. The dialysis machine has a blood pump that keeps the blood flowing by creating a pumping action on the blood tubes that carry the blood from the body to the dialyzer and back to the body. The dialysis machine also has many safety detection features. If you visit a dialysis center, you will likely hear some of the warning sounds made by a dialysis machine. How does hemodialysis work? The dialyzer is the key to hemodialysis. The dialyzer is called the artificial kidney because it filters the blood — a job the kidneys used to do. The dialyzer is a hollow plastic tube about a foot long and three inches in diameter that contains many tiny filters.

(Dialyzers are made in different sizes so doctors can prescribe the best one for their patients.) There are two sections in the dialyzer; the section for dialysate and the section for the blood. The two sections are divided by a semipermeable membrane so that they don’t mix together. A semipermeable membrane has microscopic holes that allow only some substances to cross the membrane. Because it is semipermeable, the membrane allows water and waste to pass through, but does not allow blood cells to pass through. Dialysate, also called dialysis fluid, dialysis solution or bath, is a solution of pure water, electrolytes and salts, such as bicarbonate and sodium. The purpose of dialysate is to pull toxins from the blood into the dialysate. The way this works is through a process called diffusion. In the blood of the hemodialysis patient, there is a high concentration of waste, while the dialysate has a low concentration of waste. Due to the difference in concentration, the waste will move through the semipermeable membrane to create an equal amount on both sides. The dialysis solution is then flushed down the drain along with the waste.

The electrolytes in the dialysis solution are also used to balance the electrolytes in the patient’s blood. The extra fluid is removed through a process called filtration. The fluid is pushed off by higher pressure on the blood side than on the dialysate side. How often is hemodialysis done? Blood needs to flow through the dialyzer for several hours to adequately clean the blood and rid the body of excess fluid. Traditional, in-center hemodialysis is generally done three times a week for about four hours each session. Your doctor will prescribe how long your treatments will be, usually between 3 to 5 hours, but most common is 4 hours. Talk to your doctor about how long you should be on hemodialysis. Some people feel that dialysis lasts a long time; however, healthy kidneys work 24 hours a day, 7 days a week and dialysis must do the job in only 12 or so hours a week. Alternative hemodialysis schedules include nocturnal and short daily. Normally, these treatments are performed by people who do home hemodialysis. With nocturnal hemodialysis, the patient has dialysis for about eight hours overnight while sleeping.

This is a longer, gentler treatment so patients say they have fewer problems with cramping and the “washed out” feeling reported after traditional hemodialysis. More dialysis centers are beginning to offer in-center nocturnal hemodialysis based on reports of patients feeling better about their quality of life and having good lab results. Short daily hemodialysis is performed five or six times per week for about two to three hours each treatment. Talk to your doctor if you are interested in home hemodialysis or in-center nocturnal dialysis. You may want to ask your social worker if addition treatments, such as the longer nocturnal hemodialysis and short daily hemodialysis will be covered through your insurance. Advantages and disadvantages of hemodialysis Hemodialysis is an effective treatment for those with end stage renal disease. However, hemodialysis alone will not provide a complete treatment for those with kidney failure. Diet and fluid restrictions need to be followed, and medicines may need to be taken to replace other functions of the kidneys, such as regulating blood pressure and stimulating production of red blood cells to prevent anemia .

For those who choose in-center hemodialysis, some of the benefits are that they will have their treatments performed in a dialysis center by trained professionals. They can spend their time in dialysis sleeping, reading, writing, watching television, listening to music or doing other quiet activities. There are four days a week when they will not have to go to dialysis. Some of the disadvantages are that they will have to travel to and from hemodialysis three times each week and it takes advanced notice to travel and arrange for dialysis in a visiting dialysis center. The diet restrictions include limiting foods that contain phosphorus , potassium and sodium and drinking a limited amount of fluid . Some people report a “washed out” feeling after hemodialysis and go home to take a nap. Those who perform nocturnal hemodialysis (in center or at home) report that this washed out feeling is not as common. Also, because nocturnal dialysis is performed during nonproductive sleeping hours, many people report they feel that their lives are more “normal” because they don’t have to take time out of their days for dialysis. People who choose to perform hemodialysis at home say they enjoy the feeling of control they have over their lives.

Instead of going to the dialysis center at a certain time, they can choose when to perform hemodialysis around their schedule. There is another type of dialysis called peritoneal dialysis (PD). PD is done by filling the peritoneum in the abdomen with dialysate and using the peritoneal membrane as a semipermeable membrane. There are diet and fluid restrictions with peritoneal dialysis; however, these are usually not as limited as hemodialysis because this therapy is performed every day. PD treatments are performed at home, and so do not require three visits to a treatment center each week. Peritoneal dialysis also has a nighttime treatment option that makes it easier for patients to work, attend school or travel. All dialysis treatments have their advantages and disadvantages. Based on your lifestyle and medical needs, you and your doctor can discuss your options and decide which one is right for you. How can I find out more about hemodialysis? Talk to your doctor about hemodialysis. DaVita’s EMPOWER professional educators can also provide information about hemodialysis. Call 1-888-MyKidney (1-888-695-4363) to find out more.

You may also schedule a visit to a DaVita dialysis center in your area. To arrange your tour, please call DaVita Guest Services at 1-800-244-0680. If you would like to find out if there is a DaVita center near you that offers in-center nocturnal dialysis, please call 1-866-994-SLEEP (1-866-994-7533). External links Kidney failure - Better Health Channel Kidney failure Kidney failure can be sudden or gradual. Acute kidney failure is often short-lived, but requires treatment until kidney function returns. Chronic kidney disease develops over time and is related to a number of risk factors, including diabetes and high blood pressure. If chronic kidney disease is detected early, medication and lifestyle changes can increase the life of your kidneys. Treatment options for kidney failure include dialysis, transplant or non dialysis supportive care. What is kidney failure? Kidney failure, also known as end-stage kidney disease, occurs when the kidneys are no longer able to adequately remove waste from your blood and control the level of fluid in the body. Kidney failure can happen suddenly or gradually.

People with kidney failure need dialysis or a transplant to stay alive. If you have kidney disease, it does not mean that you will develop kidney failure. One in ten Australians aged 18 and over (approximately 1.7 million people) has at least one sign of chronic kidney disease, and over 20,000 receive dialysis or a kidney transplant for kidney failure. You can lose up to 90 per cent of kidney function before feeling sick. In many cases, the signs of disease aren’t noticed until the kidneys are close to failure. Symptoms of kidney disease In the early stages of kidney disease, people can have no symptoms. In fact, some people have no symptoms until over 90 per cent of their kidney function has gone. This is unfortunate because early detection of kidney disease and treatment is the key to preventing kidney failure. Symptoms of kidney disease can include: tiredness changes in the amount and number of times urine is passed changes in the appearance of urine or blood in the urine puffiness in the legs and ankles pain in the kidney area. These symptoms can be caused by other conditions, but if you are in a high-risk group for kidney disease, you should speak with your doctor.

Treatment for kidney failure The treatment choices for kidney failure include: dialysis kidney transplantation non-dialysis supportive care. Dialysis or kidney transplantation is needed when there is less than 10 per cent of kidney function left. These options are also known as kidney replacement therapy. Some people choose non-dialysis supportive care rather than dialysis or kidney transplantation. Dialysis for kidney failure Dialysis artificially removes waste from your blood. There are two forms of dialysis - haemodialysis and peritoneal dialysis. Peritoneal dialysis is further broken down into two main types, continuous ambulatory peritoneal dialysis (CAPD) and automated peritoneal dialysis (APD). The choice of dialysis method depends of factors such as your age, health and lifestyle. Over 2,000 Australian adults start kidney replacement therapy each year. Kidney transplant A kidney transplant is a treatment for kidney failure, but it is not a cure. A transplant offers a more active life, and freedom from dialysis and restrictions on fluid and dietary intake. It is important to remember that a new kidney requires a lifetime of management and care.

Non-dialysis supportive care for kidney failure Non-dialysis supportive care is the treatment choice for kidney failure when a person has decided that the options of dialysis and transplant are not appropriate for them. Non-dialysis supportive care means that your care continues to be supervised and supported by health professionals. You may have medications and a restricted diet to improve your quality of life. Non-dialysis supportive care will not artificially prolong your life when your kidneys fail completely. Causes of kidney failure Some of the causes of kidney failure include: diabetes (diabetic nephropathy) - even if it is well managed, diabetes can cause kidney damage glomerulonephritis - swelling or inflammation of the tiny filtering units (nephrons) in the kidney polycystic kidney disease - an inherited condition that causes hundreds of cysts to form in the kidneys urinary reflux (reflux nephropathy) - a bladder-valve problem that allows urine to flow back into the kidneys, causing scarring medications - some drugs such as lithium and cyclosporin can cause kidney failure. Continued misuse of compound analgesic preparations (now banned) was once a common cause of permanent kidney damage.

Non-steroidal anti-inflammatory drugs (NSAIDs), taken in normal therapeutic doses, may occasionally cause acute kidney failure. Chronic kidney disease (CKD) Often, the development of kidney disease is gradual and kidney function worsens over a number of years. If you permanently lose more than one third of your kidney function, it is called ‘chronic kidney disease’ (CKD). This can lead to kidney failure. You are more at risk of chronic kidney disease if you: have diabetes are obese are over 60 years of age have a family history of end-stage kidney disease or hereditary kidney disease in a first or second degree relative have established heart problems (heart failure or a past heart attack) or have had a stroke smoke have a history of acute kidney injury are of Maori or Pacific origin are of Aboriginal or Torres Strait Islander origin. The risk of CKD resulting in kidney failure depends on your level of kidney damage. If kidney disease is found early, medication, combined with diet and lifestyle changes, can prolong the life of your kidneys. If you have one or more of the risk factors for developing CKD, it is important to ask your doctor to check your kidney function.

Acute kidney failure A sudden drop in kidney function is called acute kidney failure. This can be life threatening and requires immediate treatment. Causes of acute kidney failure can be divided into three groups: pre-renal (decreases to the kidney’s blood supply) renal (damage to the kidney itself) post-renal (obstructions in other parts of the urinary tract). Acute kidney failure is often short-lived and many people with acute kidney failure need dialysis while they are waiting for their kidneys to recover. Sometimes, treatment is a matter of controlling blood pressure and blood chemistry, while waiting for kidney function to return. The outcome for people with acute kidney failure depends on the underlying cause and presence or absence of other medical conditions. In cases where the cause is pre-renal or post-renal, the kidneys often recover well once the factor causing it is removed. Acute kidney failure is less common than chronic (ongoing) kidney failure. Diagnosis of kidney failure A number of tests can be used to examine kidney function. If chronic kidney disease is found, tests may be used to determine: the cause of the kidney damage the amount of kidney damage treatment options.

Tests can include: blood tests to establish the estimated glomerular filtration rate (eGFR), which measures how well the kidneys filter wastes from the blood urine tests for albumin, blood, glucose and red or white blood cells a blood pressure check ultrasound, computed tomography (CT scan), x-ray and other imaging techniques to take pictures of your kidneys a kidney biopsy, where a needle is used to remove a small piece of kidney tissue for examination under a microscope. Where to get help Hemodialysis Hemodialysis Treatment Overview Chronic kidney disease and acute kidney injury (also known as acute renal failure ) cause the kidneys to lose their ability to filter and remove waste and extra fluid from the body. Hemodialysis is a process that uses a man-made membrane (dialyzer) to: Remove wastes, such as urea , from the blood . Restore the proper balance of electrolytes in the blood . Eliminate extra fluid from the body. For hemodialysis, you are connected to a filter (dialyzer) by tubes attached to your blood vessels. Your blood is slowly pumped from your body into the dialyzer, where waste products and extra fluid are removed. The filtered blood is then pumped back into your body.

There are different types of hemodialysis. Talk about these with your doctor to decide which one might be best for you. In-center hemodialysis. You go to a hospital or a dialysis center. Hemodialysis usually is done 3 days a week and takes 3 to 5 hours a day. Home hemodialysis. After you are trained, you do your dialysis treatments at home. Hemodialysis is usually done 3 days a week (or every other day). Discuss with your doctor how long each session needs to be. A session could be as long as 6 hours, which may help you feel better. Daily home hemodialysis. After you are trained, you do your dialysis treatments at home. Hemodialysis is done 5 to 7 days a week. Each session takes about 3 hours. Nocturnal home hemodialysis. After you are trained, you do your dialysis treatments at home. Hemodialysis is done 3 to 7 nights a week. Each session is done overnight (about 6 to 8 hours). Before treatments can begin, your doctor will need to create a site where the blood can flow in and out of your body during the dialysis sessions.

This is called the dialysis access . The type of dialysis access you have will depend in part on how quickly you need to begin hemodialysis. There are different types of access for hemodialysis: Fistula. A fistula is created by connecting an artery to a vein in your lower arm. A fistula allows repeated access for each dialysis session. It may take several months for the fistula to form. A fistula may not clot as easily as other dialysis access methods. A fistula is the most effective dialysis access and the most durable. Complications include infection at the site of access and clot formation (thrombosis). Graft. A vascular access that uses a synthetic tube implanted under the skin in your arm (graft) may be used if you have very small veins. The tube becomes an artificial vein that can be used repeatedly for needle placement and blood access during hemodialysis. A graft does not need to develop as a fistula does, so a graft can sometimes be used as soon as 1 week after placement. Compared with fistulas, grafts tend to have more problems with clotting or infection and need to be replaced sooner.

A polytetrafluoroethylene (PTFE or Gore-Tex) graft is the most common type used for hemodialysis. Venous catheter. A tube, or catheter, may be used temporarily if you have not had time to get a permanent access. The catheter is usually placed in a vein in the neck, chest, or groin. Because it can clog and become infected, this type of catheter is not routinely used for permanent access. But if you need to start hemodialysis right away, a catheter may be used until your permanent access is ready. Hemodialysis for acute kidney injury may be done daily until kidney function returns. What To Expect After Treatment About once a month, you will have blood tests to make sure you are getting the right amount of hemodialysis. These tests are done to help find out how well hemodialysis is working . Your weight before and after each session will be recorded, as will the length of time it takes to complete the dialysis session. If you have hemodialysis at home, you will need to keep records of your weight before and after each session and the length of each session.

Why It Is Done Hemodialysis is often started after symptoms or complications of kidney failure develop. Symptoms or complications may include:

Question: What name is given to the process where machines are used to artificially simulate the function of the kidneys?
Answer: Dialysis