Return, the Heart, and Demise

Still unable to find meaning in his work, Red Blood Cell (AA2153) is determined to continue delivering oxygen and works hard at his job. Contrary to his determination, the body's internal environment continues to decline. Plaque, which is made up of cholesterol and fat deposits, has accumulated on the inner walls of the blood vessels, causing them to deteriorate. The buildup of plaque has even affected the coronary arteries which wrap around the heart and supply oxygen and nutrients to the entire heart muscle...

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S01E13 - Cardiac Arrest, Revival, and a Change