Extended
Kidney dialysis If the kidneys stop functioning, it will take only a few days before the concentration of urea and excess substances in the body increase to a level that damages the functioning of cells. It is essential that these substances are removed from the blood to keep them at a level that reduces the risk of cell damage. This is done using an artificial method of cleaning the blood, called dialysis. During dialysis, blood from an artery in the patient’s arm is pumped through the dialysis machine, checked for bubbles and then returned to the body through a vein. The dialysis machine mimics the effect of the kidney by passing the blood through a tube that is separated from the dialysis fluid by a partially permeable membrane. The dialysis fluid contains a balance of solutes that cause waste products such as urea and excess ions, protein and water to diffuse out of the blood into the fluid. The fluid in the machine is continually refreshed so that the concentration gradient is maintained between the dialysis fluid and the blood, and to make sure that the blood is restored to the correct balance of substances needed in the body. Dialysis may take three or four hours to complete and needs to be done several times a week, to prevent damage to the body by the build-up of substances in the blood. 1. blood containing waste substances taken from arm blood from artery fresh fluid in
artery dialysis tubing vein
5. patients ‘cleaner’ blood returned to arm
4. fluid removed 3. waste substances ( including carrying urea urea) diffuse out of blood into and other wastes dialysis fluid
2. useful substances (sugar and salts) remain in blood
∆∆Fig 13.6 How dialysis works.
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Excretion in humans
dialysis fluid containing sugars and salts
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