The arterial filter is an integral part of bypass circuitry. When introduced, manufacturers suggested a bypass loop for retrograde priming and de-airing, and for uninterrupted blood flow in case of malfunction. Practice has shown antegrade priming and de-airing is possible. This questions the necessity of the loop and presents the question-what occurs in the loop during bypass? After obtaining Human Research Ethics Board approval, eight consecutive patients (n = 8) were chosen for this study. Exclusive exclusion criterion was receiving any transfusions during cardiopulmonary bypass, as this could possibly influence results. The choice of patient numbers was based simply on proof of concept. Investigation involved isolation and collection of loop contents after cardiopulmonary bypass was completed. Testing included complete blood count, prothrombin time, international normalized ratio, partial thromboplastin time, activated clotting time, plasma free hemoglobin, slide photography with analysis for platelet clumping, and debris detection. One perfusionist collected samples, providing uniform collection and isolation technique. Regular blood samples were collected from the bypass circuitry, and from patients' pre-operative blood work. Analysis of data revealed that platelet counts in the bypass loops were statistically lower than control. Evidence of platelet clumping was present in 3 of 8 bypass loop samples, representing 37.5% of the study population. There was no clumping detected in any of the controls. In patients where platelet clumping was present, a positive correlation was noted between mean bypass time and size of platelet clumps. Prothrombin time and international normalized ratio results were immeasurable. Hemoglobin levels were higher in the loop samples. There was no evidence of debris or fibrin monomer present in any of the samples analyzed. The study results indicate that during "normal" cardiopulmonary bypass with an arterial filter bypass loop, platelet aggregates can accumulate in the loop, therefore opening the arterial bypass loop in any case may subject the patient to micro/macro emboli.