Kidney failure can result from a variety of conditions and can be temporary or permanent. Hemodialysis is available as a replacement treatment to perform the work that the kidneys normally do. However, the dialysis procedure can be associated with rapid changes in the composition of the blood - this may lead to changes in blood pressure and in turn reduced blood supply to important parts of the body. We aim to investigate if giving a medicine (called mannitol) during dialysis may be able to reduce the frequency of these low blood pressure events.