3.11+Renal+dialysis

3.11 Background Information on Dialysis

The kidneys perform many functions essential for life. Loss of kidney function results in the retention of urea and acids in the blood, water and salt imbalance and altered blood pressure. Individuals with kidney failure will only survive about two weeks unless they are treated.

Kidney transplants are one of the most successful surgical transplants but some patients must wait for a kidney that is closely (tissue-typed) matched to theirs. Other patients do not present as suitable for a transplant for various reasons; hence the need for dialysis units.

Haemodialysis involves using a dialysis unit. In a dialysis unit the blood of a patient, removed from an artery, is circulated through a length of selectively permeable dialysis tubing that is bathed in a warm dialysis fluid. The membrane of this tubing allows small molecules and ions to diffuse from a region of high concentration to a region of low concentration. Dialysis can be defined as the diffusion of a solute (glucose, salt etc) across a selectively permeable membrane. In this case, the solute molecules move from the stronger concentration (hypertonic) to the weaker (hypotonic). In order to conserve certain molecules or ions in the blood e.g. amino acids and glucose, these must be found in both the blood and the dialysing fluid in equal concentration. The molecules or ions that need to be removed from the blood must be at a lower concentration in a dialysing fluid.

Drugs or hormones can be added to the fluid if required. The “filtered” blood is returned to the patient through a vein and the used dialysis fluid is discarded. The dialysis unit processes several hundred mL blood per minute. Patients have to be on the machine 4-6 hours, three times a week. More recently, portable dialysis has been available to selected patients, where patients can undertake dialysis daily at home (2 ½ hours).

Peritoneal dialysis by comparison involves placing a fluid of correct composition into the abdominal cavity and left in place for a time. It is then drained out and replaced. The lining of the cavity acts as the dialysis membrane.

__ Comparison of Kidney Function and Renal Dialysis __ Kidney Function || Renal Dialysis || Natural body process || Artificial process to replace damaged kidneys || Removes wastes continuously || Performed intermittently (2/3 times per week for several hours at a time). || Varies output automatically depending on concentrations of wastes in the blood || Concentration of substances in blood and dialysis fluid monitored by computers so that most wastes are removed. || Diffusion, osmosis and active transport are used to remove wastes. || Diffusion only used || Filtration and reabsorption involved || Filtration only || Rapid process || Slow process ||