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How Does Dialysis Work?

written by: Dr Mike C • edited by: Emma Lloyd • updated: 8/18/2010

The kidneys perform the vital function of removing waste products and toxins from the bloodstream and excreting them from the body in urine. However, kidney disease can destroy this function and is fatal if untreated. This article explores how does dialysis work to do the job of the kidneys.

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    The Role Of Kidneys

    In order to understand just how does dialysis work, it is important to understand what the organ that the dialysis process is supposed to support does. In mammals, the function of the kidneys is to remove impurities from the bloodstream and excrete them from the body via the process of urination.

    In normal human beings there are two kidneys which are located just under the ribcage on either side of the spine (this explains why an early symptom of a kidney problem is back pain – it can be excruciating). These organs are approximately the size of a human fist and weigh about 150g each. The blood to be filtered passes into the kidney through the renal artery and purified blood flows back to the body via the renal vein. Urine is produced in the kidneys and carries away waste products through the ureter to the bladder where it is collected until the person urinates.

    Within the kidneys are tiny filters called nephrons. Everybody starts with about 1 million nephrons per kidney, but approximately 100,000 nephrons are lost with each decade of life. Each nephron contains a glomerulus, a tiny blood capillary, and a urine collecting tubule which intertwine within the nephron. The kidneys filter approximately 200 litres of blood each day and produce about 2 litres of waste. Since the human body contains approximately 5 litres of blood, the entire blood supply passes through the kidneys 40 times per day.

    The kidneys are responsible for ensuring that the correct balance of acids and salts is maintained in the body and they regulate the amount of water retained by the body. They also produce three important hormones which are involved in blood cell production, blood pressure regulation and control of the body’s calcium balance.

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    Causes of Kidney Disease

    If both kidneys fail, death is inevitable without dialysis or a kidney transplant. The most common causes of kidney disease are diabetes and high blood pressure. 11% of Americans over 20 years of age are estimated to show evidence of chronic kidney disease. In 2007, more than half a million Americans were being treated for end stage kidney disease at a cost of more than $32 billion.

    Kidney dialysis is used as an intervention for kidney failure and is designed to perform the same task of removing unwanted material from the bloodstream. Kidney dialysis is indicated when an individual’s kidney function falls below 15 to 10% of its normal activity. There are two main forms of dialysis; haemodialysis and peritoneal dialysis. Peritoneal dialysis involves the administration of a fluid which absorbs impurities into the stomach; after a certain time, the fluid is drained off and the process is repeated. This article deals with hemodialysis.

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    Hemodialysis

    The history of dialysis can be traced back to 1854 and experimentation conducted by Thomas Graham at Glasgow University in Scotland, in which he demonstrated a semi-permeable membrane. This experiment provides the explanation for just how does dialysis work. In a hemodialysis system, blood flows from a vein in the patient into the dialyser, where it is cleaned, and then back into the patient.

    The patient’s blood flows through tubes made of a semi-permeable membrane material in the dialysis machine. A fluid, known as a dialysate, is pumped outside the membrane, and flows in the opposite direction. Impurities in the blood are sufficiently small to pass through the membrane and into the dialysate. This mechanism allows impurities to be removed from the patient’s bloodstream and the cleaned blood to be returned to their body. The haemodialysis system, therefore, can be thought of as an artificial kidney. Since the patient will need to be connected to the dialysis machine so frequently, reliable vascular access is an issue. This topic is explored in a sister article.

    A patient will usually require three sessions of dialysis per week and each session typically lasts between three and four hours. In some cases, dialysis can be done overnight in the patient’s home, but it is still an invasive and time consuming treatment. However, without dialysis and in the absence of a kidney transplant, patients with acute renal failure would die.

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    References

    1. Kidney Health Australia: http://www.kidney.org.au/KidneyDisease/Howourkidneyswork/tabid/590/Default.aspx
    2. National Kidney and Urologic Diseases Information Clearinghouse: http://kidney.niddk.nih.gov/Kudiseases/pubs/yourkidneys/
    3. Advanced Renal Education Program: http://www.advancedrenaleducation.com/Hemodialysis/HistoryofHemodialysis/tabid/177/Default.aspx
    4. North West Kidney Centers, Dialysis: http://www.nwkidney.org/nkc/dialysis/startingOut/basic/howDialysisWork.html