What prevents oxygen-rich blood from mixing with oxygen-poor blood in the heart?

A) The tricuspid valve
B) The fibrous pericardium
C) The aortic and pulmonary semilunar valves
D) The interatrial and interventricular septa
E) The intercalated discs

The interatrial and interventricular septa are walls that separate the atria and ventricles, respectively. Oxygen-rich blood enters the left atrium and is pumped through the mitral valve into the left ventricle, while oxygen-poor blood enters the right atrium and is pumped through the tricuspid valve into the right ventricle. The interventricular septum prevents the oxygen-rich blood in the left ventricle from mixing with the oxygen-poor blood in the right ventricle. Similarly, the interatrial septum prevents oxygen-rich blood in the left atrium from mixing with oxygen-poor blood in the right atrium. Thus, the correct choice is D.