 For millions of patients with heart disease, open heart surgery has been a life-saving advance. Beginning in the 50s, the use of heart-lung bypass machines allowed surgeons to complete these previously unthinkable operations. Unfortunately, 15-30% of people lose kidney function from being on bypass. This damage is thought to be related to a lack of blood flow to the kidneys, but little is known about how bypass affects renal blood flow. By taking detailed measurements during surgery, researchers at the University of Gothenburg identified a new potential mechanism for this type of kidney injury. In 18 patients undergoing bypass for at least an hour, the Swedish team inserted several catheters to track blood flow and oxygen delivery to the kidneys, along with kidney function and oxygen consumption of the organs, before, during, and after the procedure. Urine and blood samples also provided valuable snapshots at the same time points. The researchers found that even though blood flow and oxygen to the entire body increased during bypass, that was not the case for the kidneys. Instead, renal blood flow remained the same, meaning that proportionally, the kidneys were not getting as much blood as before. Somewhat surprisingly, kidney function also stayed constant and therefore demanded the same amount of oxygen as before bypass, but now the organs were getting less oxygen delivered. The team suspects this imbalance occurred because the fluid used during bypass diluted the blood. Consistent with damage due to a lack of oxygen, levels of a protein called NAG, a marker of tubule damage, began rising in urine within a half hour of starting bypass and peaked just after surgery. The findings suggest that bypass redistributes blood away from the kidneys and that in combination with dilution of blood oxygen with the bypass fluid, the procedure reduces oxygen delivery to the kidneys by 20%. Ideal conditions for damage. Future work investigating how heart-long bypass puts kidneys at risk may reveal new ways to protect the vital waste filtering organs.