In the heart, the mixing of oxygen-rich and oxygen-poor blood is prevented by the .
• septum.
• pericardium.

Septum the left and right sides of the heart are separated by a structure called the interventricular septum. The right side of the heart is oxygen poor, and the left side is oxygen rich. 

Answer - Septum

The right side of the heart brings the deoxygenated (oxygen-poor) blood from other parts of the body toward the heart and the left side of the heart carries oxygenated (oxygen rich) blood from heart toward the other organs. The mixing of these two types of blood is prevented by a septum that divides the heart into two chambers: one for the oxygenated blood and the other for the deoxygenated blood.
Pericardium is a layer of covering on the heart that prevents friction build-up during cardiac activity.