How to catch urea? Considerations on urea removal from hemofiltrate

Hemofiltration imitates the first step in the natural function of the kidney. After separation from corpuscular and high molecular weight blood components, a filtrate remains which contains urea together with electrolytes and other low molecular weight metabolites. To use a hemofilter in a recirculating closed-loop system, a big quantity of urea must be eliminated. A survey of published attempts to solve this problem is presented. Reasons are given for the difficulty to eliminate urea directly from dilute aqueous solutions. Explanations for ambiguous results of some reactions proposed for urea removal are discussed. The concept of hard and soft acids and bases is used to develop demands to the structure of a reagent which reacts preferentially with urea in aqueous solution. On monomeric model substances-activated aldehydes-this hypothesis is proven in vitro. In spite of the given technical possibility of urea removal, the authors doubt whether solving the problem of urea removal will enable a closed-loop system for alternative simpler or more economic ways of treating renal failure.