(a) Test circuit diagram (blood circulation before measurement). The atmospheric release circuit of the blood circuit must be emptied. The filtrate level was set at the height of the three-way stopcock for blood circuit switching, the filtration circuit was closed. The blood circuit was cycled for more than 5 min. (b) Test circuit diagram (during colloid osmotic pressure measurement). The roller pump was stopped, the three-way stopcock was switched, and the blood circuit was switched to an open-air circuit. The clamp of the filtrate circuit was then opened to expose the circuit to the atmosphere. After waiting 3 min for the solvent migration in the blood and filtrate circuits to equilibrate, the water level difference created between the two circuits was measured.