AIM: To investigate microvascular injury quantitatively in the small bowel with respect to cardiopulmonary bypass (CPB) and related mechanisms.METHODS: In 10 male SD rats, normothermic CPB was established and continued with a flow rate of 100-150 mL/kg per minute for 60 min, while another 10 sham-operated animals served as controls. An approximate 10-cm loop of the terminal ileum was exteriorized for observation by means of intravital fluorescence microscopy. The small bowel microcirculatory network including arterioles, capillaries, and collecting venules was observed prior to CPB, CPB 30 min, CPB 60 min, post-CPB 60 min and post-CPB 120 min. The intestinal capillary perfusion, microvascular permeability and leukocyte adherence were also measured.RESULTS: The systemic hemodynamics remained stable throughout the experiment in both groups. In CPB animals, significant arteriolar vasoconstriction, blood velocity reduction and functional capillary density diminution were found. As concomitances, exaggerated albumin extravasation and increased leukocyte accumulation were also noted. These changes were more pronounced and there were no signs of restitution at the end of the observation period.CONCLUSION: CPB induces significant microcirculatoryinjury of the small bowel in rats. The major underlying mechanisms are blood flow redistribution and generalized inflammatory response associated with CPB.