What is the purpose of the compliance loop in the Radnoti Working Heart System?

Having flowed from the left atrial cannula into the left atrium, the perfusion fluid is ejected via the mitral valve into the left ventricle from where it is ejected through the aortic cannula against a hydrostatic pressure via the compliance bubble trap chamber. 
The afterload is determined by the height of the compliance loop reservoir relative to the heart (see diagram below). In the compliance loop, the bubble trap compliance chamber is pre-filled with perfusate to the point where it contains approximately a 2mm diameter bubble of air for the working heart. The trapped air bubble mimics normal vascular elasticity. The air bubble may be adjusted by placing a syringe at the top vent outlet of the bubble trap compliance chamber. Once the size has been set, close the stopcock to trap the bubble.
Keeping the compliance air bubble relatively close to the heart is an essential component of the perfusion circuit and greatly improves successful long term function of the heart.