When we receive fresh hearts for research, we attempt to perfusion fix each heart so that they are preserved in an end-diastolic state, which is the state when the heart is filled with blood immediately before contraction. To initiate the specimen preservation process, four of the great vessels are cannulated (tubes placed inside) so that there is a cannula connected to each chamber of the heart. The remaining vessels are plugged or closed. The heart is then placed in the formalin-filled lower chamber and its cannulated vessels are connected to the upper formalin-filled chamber as seen in the figure to the right. The upper chamber is set to elicit a head pressure of approximately 50 mmHg. In this set-up, formalin flows antegrade through the aorta into the coronary arteries. As a result, the formalin perfuses through the specimen's tissue. The 10% buffered formalin solution preserves these specimens by cross-linking proteins within the tissue. After at least 24 hours of perfusion fixation, the heart is fixed, removed, and stored in an airtight container within formalin.