Endothelium: The Next Frontier in Biocompatibility

Vascular endothelium plays a central role in two specific functional systems. It controls vascular tone, hemostasis, and substance transport. The endothelium is the “docking station” for trapping, deactivation, and re‐generation of activated blood compounds and provides the principal clearance mechanism for biologically active mediators released by different cell types. The second function is a regenerational one. During the period between insults (or between dialysis sessions), the endothelium has to restore the “first line of defense,” that is, to regenerate the injured athrombogenic surface of the vessel wall and its antioxidative potential, defoliate damaged endothelial cells, and interpolated new ones. These two important endothelial activities are required over and above its basic functions. Future research in artificial organs must take into account that continuous or intermittent blood‐membrane contact creates an altered endothelial response. These altered responses may result in adaptional reactions that may differ substantially in the acutely ill patient on continuous venovenous hemofiltration (CVVH) or in a stable patient on maintenance hemodialysis. By a reduction in such factors as immediate or delayed cell‐cell interactions (direct or indirect), it may be possible to influence the long‐term outcome of chronic hemodialysis patients. Other research should strive to enhance those factors of endothelial function that are essential in the defensive and restorative properties of endothelial tissue. This is especially important in such continuous therapies as CVVH, long‐term membrane oxygenation, and artificial heart and blood vessels. Currently, there are more unanswered questions than possible answers concerning endothelial functions in long‐term hemodialysis patients, but it is clear that excluding endothelial cell behavior from investigation of extracorporeal therapy in the future would be a substantial omission.