In Vivo Experiment.

New Zealand rabbits (4 mo old) were purchased from laboratory animal research center of Tsinghua University. All the experiments were conducted in accordance with the Institute’s Guide for the Care and Use of Laboratory Animals and were approved by the ethical committee of Tsinghua University (Approval No. AP#18-CHS1). The rabbit was anesthetized with 3% pentobarbital sodium (1 mL/kg) via the ear vein. The abdominal cavity of the rabbit was opened, and a part of the small intestine was placed under a microscope (Olympus IX83) with connection of relevant mesentery vascular network. Before the injection, the root mesenteric artery was clamped; meanwhile, the blood was added with Ca²⁺ and mixed with an air bubble within 15 s. Then, the 0.1-mL blood and air mixture was injected within 10 s, and the clamp was released to allow the blood to flow into the vascular network downstream. A small vessel with a diameter of about 100 μm in the downstream of the injection point was observed continuously. The observation location was far enough from the injection point to avoid any observable change in the size of the vessel during the whole injection process. The motion of the bubbles and the formation of the air embolism were observed in the vascular network under the microscope.