Figure 111

Hydrostatic ultrafiltration also takes place during hemodialysis. Because the spent dialysate effluent pump (see Fig. 1-4) creates negative pressure on the dialysate compartment of the membrane unit and the blood pump creates positive pressure in the blood compartment, there is a net hydrostatic pressure gradient between the compartments. This causes a flow of water and dissolved substances from blood to the dialysate compartment. The process of solute transfer associated with this flow of water is called "convective transport." In hemodialysis, the amount of low-molecular weight solute (eg, urea) removed by convection is negligible. In the continuous renal replacement therapies, this is a major mechanism for solute transport.

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