In the human heart, there is no mixing of deoxygenated and oxygenated blood due to the presence of inter - atrial and inter - ventricular septum. These septa completely divide the atria and ventricles into right and left and thus avoid mixing of blood. As the heart muscle contracts and relaxes, the valves open and shut, letting blood flow into the ventricles and atria at alternate times. This heart valve minimises any obstruction and allows blood to flow smoothly and freely in one direction. It closes completely and quickly, not allowing any blood to flow back through the valve. The heart has 4 valves: the mitral valve and tricuspid valve, which control blood flow from the atria to the ventricles and the aortic valve and pulmonary valve, which control blood flow out of the ventricles.