Question

why there is no mixing of oxygenated and de-oxygenated blood in human heart normally

Answer 1

Human heart is a 4 chambered heart.
The left ventricles and auricles completely separate from right ventricles and auricles/atria by the means of septum. So there is no mixing of oxygenated and de oxygenated blood in our heart.
Hope that helps you :-)

Answer 2

Hey frnd
There is no mixing of oxygenated and deoxygenated blood becoz human heart us divided into two parts by septum one side oxygenated blood flows and on another side deoxygented blood flows
Hope u find helpful