Final answer:
Air is evacuated from the extracorporeal circuit to prevent air embolisms during hemodialysis, a process where blood is filtered to remove waste products such as urea.
Step-by-step explanation:
Air is removed from the extracorporeal circuit before initiating treatment because it may cause air embolisms when trapped in the dialyzer. During hemodialysis, a medical procedure for patients with kidney failure, harmful substances like urea are removed from the blood. In the dialyzer, blood flows through tubing made of a semipermeable membrane, which is surrounded by a dialysate. The concentration of solutes like urea is higher in the blood than in the fresh dialysate, facilitating diffusion of wastes from the blood into the dialysate. The membrane is permeable to small molecules like urea but impermeable to larger components such as red blood cells, ensuring that only waste products and excess water are filtered out.