Typical Number in Hospital: Cost Bands: References: 3

Water may be encouraged to pass through a semipermeable membrane between two fluids of roughly equal concentration by applying a pressure differential. This is similar to reverse osmosis but describes the situation when the osmotic pressure is small, and is called ultrafiltration. The technique is employed in haemodialysis to bring about a shift of water from the patient via the dialysing membrane.

The removal of large quantities of water from the patient causes a reaction which can be avoided by the correct sequencing of the two operations of ultrafiltration and haemodialysis.

A haemodialysis machine normally has the facility for adjusting the pressure between the blood and the dialysate solution across the dialysing membrane. Excessive pressures may, however, cause rupture of the membranes. Pressures up to about 400 mmHg are used.

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