3. The right sided of the heart is completely separated from the left side by a wall of tissue called the .

Septum

Explanation:

In the normal heart, the right and left chambers are completely separated from each other by a wall of muscle called a septum. The right atrium is separated from the left atrium by the atrial septum and the right ventricle is separated from the left ventricle by the ventricular septum.

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