 Due to blood loss and SIRS postoperative hypovolemia is a usual feature after CABG surgery. Substantial controversy persists regarding the best choice of fluid to be used for resuscitation in this setting regarding renal function and oxygen supply-demand matching. In our study we aim to investigate differences in functional renal parameters by dividing patients postoperatively in two groups, substituted either with crystalloid fluid, Ringer-Acetat, 20 mlkg or colloid, HES 6 1300,4 10 mlkg under 20 minutes. Central and renal hemodynamics will be measured before baseline and after fluid administration by blood sampling and thermodilution in both pulmonary artery catheter and renal vein catheter. Measurement endpoints will mainly consist of central hemodynamics, renal blood flow, GFR and renal oxygen extraction rate which we aim to compare between groups. 