Extracorporeal blood circulation (EBC) in the form of cardiopulmonary bypass is essential for most open-heart surgeries. EBC often triggers a systemic inflammatory response, which can lead to postoperative complications or death. Extracellular vesicles (EVs), heterogeneous populations of membrane vesicles released by the cells, are potential biomarkers of adverse events during EBC. There is a need for a more detailed understanding of the relation between EBC and EVs and the effect of additional measures such as extracorporeal cytokine adsorber CytoSorb® (CytoSorb®, CytoSorbents Inc., USA) and corticosteroid administration. The main purpose of the master’s thesis was to study the impact of extracorporeal blood circulation alone and in combination with extracorporeal immunoadsorption or methylprednisolone on EVs levels and their size in blood samples of patients who have undergone a complex heart surgery. We also examined potential correlation of the results with clinical data. A secondary goal was to optimize the protocols for EVs characterization and quantification with the nanoparticle tracking analysis (NTA) and flow cytometry. Blood samples of 23 patients who have undergone heart surgery on SPS Kirurška klinika, University Medical Centre Ljubljana, were included in the research. Patients were divided in the control group (9 patients, without CytoSorb® capsule and without corticosteroids during EBC), Cytosorb group (8 patients, a capsule CytoSorb® was used during EBC), and the corticosteroid group (6 patients receiving methylprednisolone during EBC). The blood samples were taken before the induction into anaesthesia (pre-EBC) and after EBC (post-EBC). After the preparation of the samples, the analysis with NTA and flow cytometry followed. Preoperative EV levels and their size in the pre-EBC samples varied greatly between patients. EV size increased significantly after the surgery with EBC (p < 0,001). EV concentration did not change significantly after EBC (p = 0,298). Although the total biological nanoparticle concentration decreased statistically significantly after the EBC (p <0,001), this decrease does not reflect changes in the level of EV. EV fluorescence decreased statistically significantly after the EBC (p = 0,011) which indicates a decrease in esterase activity within the EVs. The trend of change in the EV levels due to EBC was not the same in all patients, while there was an increase in the size of the EVs after EBC in all patients. Extracorporeal immunoadsorption did not additionally affect the increase in EV size due to EBC (p < 0,001; control p < 0,001); however, it did additionally affect the increase of EV concentration (p = 0,016; control p = 0,466). Extracorporeal immunoadsorption had an additional effect on the decrease in fluorescence (esterase activity) of EVs, which was statistically significant only in the Cytosorb group (p < 0,001). Administration of methylprednisolone did not additionally affect the increase in EV size caused by EBC (p < 0,001; control p < 0,001). Methylprednisolone had no additional effect on the change in EV concentration due to EBC (p = 0,566, control p = 0,466). Administration of methylprednisolone did not further affect EV fluorescence or esterase activity (p = 0,059, control p = 0,507). The EV levels, their size and fluorescence in the pre-EBC and post-EBC blood samples correlated with the clinical data of patients. Relative changes in parameters that characterize EV correlated with clinical data, as well. We have demonstrated that surgery with EBC causes changes in EVs, and supplementary measures such as extracorporeal immunoadsorption may have an additional effect. No additional effect on the EVs due to methylprednisolone administration was demonstrated. Further research is needed, ideally on a larger number of patients, to examine the impact of EBC and additional measures on EVs in more detail. We have also showed that EVs are associated with clinical data, which opens up the possibility of their use as biological markers.