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What is the innermost layer of the heart that is smooth thin lining to reduce the friction of blood flow through the chambers?

The heart consist of three layers of tissue; outer wall of the heart wall is the epicardium, the second is the myocardium which is the middle layer and the third is the endocardium which is the inner layer. The function of the endocardium is that it helps in keeping the blood flow through heart separately from the myocardium.