Ultrafiltration involves two processes that are going on simultaneously.
While excess fluid is removed from blood vessels, more fluid simultaneously flows into the blood vessels to replace it. The blood flowing in comes from tiny spaces in tissues, where it has accumulated.
Ultrafiltration seeks to keep a constant volume of blood and fluid within the blood vessels, to avoid low blood pressure. The volume of fluid and blood in the blood vessels stays the same, while fluid overload in the tissues is relieved.
Using a semipermeable membrane (selective filter) is essential to ultrafiltration. Only some molecules can pass through the semipermeable membrane, based on a molecule's size and the difference in pressure on both sides of the filter.
In general, small molecules in the blood, including sodium, pass through the ultrafiltration membrane. The process, therefore, does not cause changes in the body's levels of certain molecules. Diuretic medicines, on the other hand, can cause an imbalance and that can have adverse clinical effects for a patient.