Extracorporeal life support (ECLS) patients are at risk for complications caused by gaseous microemboli (GME). GMEs can cause hypoxia, inflammation, coagulation, and end-organ damage. The objective of this in vitro study was to assess dynamics of GME formation during circulation of whole blood or a glycerol blood surrogate. We hypothesized that there is no difference in GME counts and sizes between whole blood and the glycerol blood surrogate and that the membrane lung reduces GME counts over time. A circulation platform was developed using the Cardiohelp ECLS system to run either donor blood or glycerol solution. We conducted 10 repetitions consisting of three phases of ultrasound GME detection using the EDAC™ Quantifier (Luna Innovations, Charlottesville, VA, USA) for each group. Phases were 3-minute recordings at the initiation of 2 L/min flow (Phase 1), post-injection of a GME suspension (Phase 2), and 10 minutes after injection (Phase 3). The number and size of GME pre- and post-ML were recorded separately and binned based on diameter ranges. In Phase 1, GME count in blood was higher than in glycerol. In Phase 2, there was a large increase in GME counts; however, most GME were reduced post-membrane in both groups. In Phase 3, there was a significant decrease in GME counts compared to Phase 2. GME > 100 μm in glycerol decreased post membrane. We demonstrated GME formation and decay dynamics during in vitro circulation in an ECLS system with blood and glycerol. GME counts were higher in blood, likely due to varying rheological properties. There were decreases in GME levels post membrane in both groups after GME injection, with the membrane lung effectively trapping the GME, and additional reduction 10 minutes after GME injection.