Answer: Transcardial perfusion is a procedure designed to replace all blood in the brain with a chosen solution.
For many experimental methods, the brain is required to be chemically fixed with a substance like paraformaldehyde or formalin. After fixation, the proteins in the brain are cross-linked, which causes them to stop changing over time. Additionally, fixative is effective at kiling bacteria and inactivating degradation enzymes. Fixing makes brains more resilient to being changed over time; they are “fixed” in time.
In order for the fixative (paraformaldehyde or formalin) to access and to be distributed to every part of the brain, an experimenter is able to modify and reuse the natural circulatory system. The blood vessels in the brain are very efficient at distributing liquids to every part of the brain. Therefore, if a scientist wanted to replace the blood with a chemical fixative, they would be able to use the heart to pump the fixative across the entire brain.
In the heart, the left ventricle is responsible for pumping the blood out into the systemic circulation. For a transcardial perfusion, the scientist would expose the heart by performing a thoracotomy (resection of the thorax region). The left ventricle would be exposed, and a syringe filled with physiological saline would be inserted into the left ventricle. From there, a small incision would be made into the right atrium, which is the chamber where the blood is returned into the heart after being circulated around the body. This must be done to relieve the elevated blood pressure in the circulatory system. If this pressure is not relieved, there is the added risk of a stroke-like event that could cause the thin blood vessels in the brain to explode, damaging the brain. Then, the saline would be delivered into the circulation, replacing the blood. After this has been completed, the fixative would then be injected.
Once the transcardial perfusion is completed, the blood would have been completely replaced by the fix. The brain can then be extracted and kept in a high sucrose cryoprotectant solution and stored in a refrigerator for later analysis.