What is the difference between ultrafiltration and dialysis

These factors cause inflammation of the peritoneal membrane. Skip to main content. You are here Home » A to Z » Ultrafiltration. What is Ultrafiltration? Ultrafiltration in Hemodialysis In hemodialysis, fluid is removed by ultrafiltration using the dialysis membrane.

What can you do to help with ultrafiltration? Do not skip or shorten hemodialysis treatments — adequate dialysis is important for overall health Manage sodium and fluid in your diet. Talk to your dietitian about ways to limit salt and fluid intake see Nutrition and Hemodialysis Ultrafiltration in Peritoneal Dialysis Peritoneal dialysis PD removes fluid by ultrafiltration using the lining of your belly called the peritoneal membrane.

What can cause ultrafiltration to fail? What can you do to keep good ultrafiltration? Figure 5: Summary of the diffusion of common substances between the dialysate and the serum. In hemodialysis water is removed by hydrostatic ultrafiltration which is a pressure phenomenon. The pressure on the dialysate side is lowered and water moves from a place of higher pressure to one of lower pressure, i.

This is how fluid gets removed every time a patient is dialyzed. The osmolality of the dialysate is actually lower than that of the plasma because of mainly the lack of urea in the dialysate. Sodium that is gained between dialysis treatments is ordinarily removed during dialysis by ultrafiltration not by diffusion.

As serum sodium concentration often does not change much between dialysis days while patients gain weight during the interdialytic period, they must have retained sodium and water in the same ratio as that present in plasma. The ultrafiltrate removed during dialysis and the corresponding plasma contains closely similar amounts of sodium and water.

In isolated ultrafiltration, no dialysate is used and no diffusion is involved. The pressure in the dialysate compartment is lowered in order to allow movement of fluid out of the blood space. With isolated ultrafiltration small molecular weight substances are also removed as the semipermeable membrane is still permeable to the same molecules as before. However they are removed at the same rate as water and the serum concentration of a permeant small molecule such as urea does not change.

In hemofiltration large amount of fluid i. To maintain a normal blood volume, an equal volume of a replacement fluid needs to be infused back to the patient. The replacement fluid resembles plasma and contains sodium, chloride, calcium, dextrose and bicarbonate.

Other molecules such as potassium may also be present. With hemofiltration the urea and other permeant small molecules concentration in the blood will drop as there is no urea in the replacement fluid and urea is removed with the fluid removal. When hemofiltration is performed in continuous slow fashion, it is called continuous veno-venous hemofiltration or CVVH , as opposed to continuous veno-venous hemodialysis CVVHD.

This is the most efficient way of doing renal replacement therapy. Rebound refers to the fact that some substances such as urea or potassium which are in large part intracellular have low concentrations in the blood as a result of renal replacement therapy but the levels rise sharply in the next few hours.

This phenomenon happens because the still high postdialysis intracellular level allows diffusion of the substance into the extracellular space. The degree of rebound depends on the rate of dialysis. Access recirculation only has to do with the vascular access and mainly results from decreased access flow which in turn is usually the result of either a narrowing in the outflow vein or in the incoming artery.

The low access flow will cause some of the just dialyzed blood to go in a loop and be dialyzed again Figure 6. The only way the following could happen is for some of the blood that was just dialyzed to flow backward in the arm and go be dialyzed again.

Dialyzer clearance is defined as the volume of plasma or blood from which a given substance has been removed completely in a given time period. The dialyzer clearance of a substance depends on four factors: 1. Surface area of the dialyzer. Blood flow rate. Dialysate flow rate. Permeability of that substance with respect to the dialyzer membrane. It is important to point out that dialyzer clearance is not dependent on the serum concentration of the substance. High efficiency dialyzers have large surface area for dialysis, whereas high flux dialyzers have larger holes to permit passage of bigger molecules.

The trend in recent years has been to use a dialyzer which is both high efficiency and high flux. The following three cases are in order to better understand clearance concept with dialysis which has lots of similarities with normal kidneys also.

Patient 1 and 2 are the same weight, total body water, diet and have same creatinine generation. They both have normal renal function and bilateral hypernephroma. On day zero, both patients undergo bilateral total nephrectomy. Patient 1 starts 3 times a week dialysis on day 0. Patient 2 starts 3 times a week dialysis on day 15, when the serum creatinine is Dialysis prescription is same in both patients. What is the serum creatinine on patient 1 and patient 2 three months after nephrectomy and why?

Patient 1 and patient 2 have the same urea generation, total body water, weight and diet. Patient 1 has been getting dialyzed 8 hours 6 days a week for the last year. Patient 2 has been getting dialyzed 4 hours 3 times a week for the last year. Other than the time, the rest of the dialysis prescription is the same in both patients.

All the dialysate from both patients is collected over one week. Which patient would have higher urea excretion over the week? Patient 1 and patient 2 have the same urea generation, total body water, weight, and diet. Patient 1 starts dialysis 4 hours 3 times a week on day 0. Patient 2 starts dialysis 2 hours 6 times a week on day 0.

What would be the serum BUN on patient 1 and patient 2 on day 6? Some large molecules even though they are too big to go through the dialyzer membrane, still get removed a little bit because of adsorption.

Adsorption refers to the molecule sticking to the dialyzer membrane and being removed without actually going across the membrane.

URR is the urea reduction ratio and represents the percentage decrease in serum urea concentration with dialysis. K represents dialyzer clearance, T duration of dialysis and V urea distribution volume.

A few hours after dialysis the serum urea concentration rises rapidly. Hypertension is very common in dialysis patients. It is classically assumed that hypertension in dialysis patients is volume- mediated. Thus the treatment for this hypertension first involves removing more fluid with dialysis. Such removal is however not possible in many cases because of several problems. Most importantly many patients drop their blood pressures to very low levels with fluid removal thus limiting the amount of fluid that can be removed.

The reason that many patients cannot tolerate excess fluid removal well is because the removal rate has exceeded the refilling rate the rate of entry of extracellular fluid into the capillaries of the capillaries of the body.

Having patients take their blood pressure medicines after dialysis maybe helpful at times. More frequent dialysis or longer treatments may also be help to remove more fluid but often nursing staff shortage gets in the way. High sodium baths, sodium modeling with the dialysates, treatment with midodrine and carnitine infusions have been tried with varying success. Salt restriction is always an attractive option but is rarely followed through.

Apart from intradialytic hypotension, some patients are unable to tolerate more fluid removal because of the development of severe cramps with dialysis. Again the options for treating this problem are limited. Patients with intradialytic hypotension are often treated with intravenous saline administration, thus contributing to the fluid excess. Lastly some patients just refuse to remove more fluid as they perceive themselves to be thin already and fear any more weight loss. This is despite the fact the dialysis only removes the salt and the water and does not cause real non-excess-fluid weight loss.

In spite of various obstacles, salt restriction and more frequent and longer dialysis treatments are the best solutions to the problem of intradialytic hypotension. Anemia in dialysis patients is mainly because of a lack of erythropoietin. Erythropoietin administration has made a tremendous improvement in the lives of dialysis patients. Not uncommonly dialysis patients are still anemic despite adequate erythropoietin administration.

This is most commonly because of relative iron deficiency. Dialysis is a process that helps patients suffering from kidney failures. When the kidneys fail to functions naturally and filter blood in order to form urine and excrete waste, various harmful substances such as toxins, drugs, poisons, etc.

It can eventually lead to fatal conditions. In these situations, dialysis is one of the medical processes that can perform to purify blood and aid in the excretory process. Hence, in simple words, dialysis is an artificial way of replacing the kidney functions. Through dialysis, small solute molecules separate from the larger solutes due to the difference in their diffusion rates. It occurs via a semi-permeable membrane.

There are two main types of dialysis namely hemodialysis and peritoneal dialysis. In hemodialysis , an artificial kidney or a dialysis machine is used to purify the blood. On the other hand, peritoneal dialysis does not use a machine. Instead, it uses a dialysate and the membrane lining of our abdomen in order to clean our blood.

Ultrafiltration is one of the three processes occurring in our kidneys during the blood filtration. Then blood filters under high pressure. Accordingly, most of the substances in the blood except globular proteins, red blood cells, white blood cells and platelets enter into the nephron.

The afferent arteriole brings blood in while efferent arteriole takes blood out from the glomerulus. The pressure needed for ultrafiltration develops due to the diameter difference between the afferent incoming and efferent outgoing capillaries of the glomerulus. The diameter of the efferent arteriole is less than the afferent arteriole, increasing the blood pressure and causing it to be filtered. This phenomenon, where filtration is taking place under high hydrostatic pressure through a semi-permeable membrane is the process of ultrafiltration.

Not only in kidneys but this can also be simulated in external environments to separate substances from a mixture especially in industries in order to purify solution mixtures and concentrate them.